egui/crates/egui/src/ui.rs

#![warn(missing_docs)] // Let's keep `Ui` well-documented.
#![allow(clippy::use_self)]

use std::{any::Any, hash::Hash, sync::Arc};

use emath::GuiRounding as _;
use epaint::mutex::RwLock;

use crate::{
    containers::{CollapsingHeader, CollapsingResponse, Frame},
    ecolor::Hsva,
    emath, epaint,
    epaint::text::Fonts,
    grid,
    layout::{Direction, Layout},
    menu,
    menu::MenuState,
    pass_state,
    placer::Placer,
    pos2, style,
    util::IdTypeMap,
    vec2, widgets,
    widgets::{
        color_picker, Button, Checkbox, DragValue, Hyperlink, Image, ImageSource, Label, Link,
        RadioButton, SelectableLabel, Separator, Spinner, TextEdit, Widget,
    },
    Align, Color32, Context, CursorIcon, DragAndDrop, Id, InnerResponse, InputState, LayerId,
    Memory, Order, Painter, PlatformOutput, Pos2, Rangef, Rect, Response, Rgba, RichText, Sense,
    Style, TextStyle, TextWrapMode, UiBuilder, UiStack, UiStackInfo, Vec2, WidgetRect, WidgetText,
};

#[cfg(debug_assertions)]
use crate::Stroke;
// ----------------------------------------------------------------------------

/// This is what you use to place widgets.
///
/// Represents a region of the screen with a type of layout (horizontal or vertical).
///
/// ```
/// # egui::__run_test_ui(|ui| {
/// ui.add(egui::Label::new("Hello World!"));
/// ui.label("A shorter and more convenient way to add a label.");
/// ui.horizontal(|ui| {
///     ui.label("Add widgets");
///     if ui.button("on the same row!").clicked() {
///         /* … */
///     }
/// });
/// # });
/// ```
pub struct Ui {
    /// Generated based on id of parent ui together with an optional id salt.
    ///
    /// This should be stable from one frame to next
    /// so it can be used as a source for storing state
    /// (e.g. window position, or if a collapsing header is open).
    ///
    /// However, it is not necessarily globally unique.
    /// For instance, sibling `Ui`s share the same [`Self::id`]
    /// unless they where explicitly given different id salts using
    /// [`UiBuilder::id_salt`].
    id: Id,

    /// This is a globally unique ID of this `Ui`,
    /// based on where in the hierarchy of widgets this Ui is in.
    ///
    /// This means it is not _stable_, as it can change if new widgets
    /// are added or removed prior to this one.
    /// It should therefore only be used for transient interactions (clicks etc),
    /// not for storing state over time.
    unique_id: Id,

    /// This is used to create a unique interact ID for some widgets.
    ///
    /// This value is based on where in the hierarchy of widgets this Ui is in,
    /// and the value is increment with each added child widget.
    /// This works as an Id source only as long as new widgets aren't added or removed.
    /// They are therefore only good for Id:s that has no state.
    next_auto_id_salt: u64,

    /// Specifies paint layer, clip rectangle and a reference to [`Context`].
    painter: Painter,

    /// The [`Style`] (visuals, spacing, etc) of this ui.
    /// Commonly many [`Ui`]:s share the same [`Style`].
    /// The [`Ui`] implements copy-on-write for this.
    style: Arc<Style>,

    /// Handles the [`Ui`] size and the placement of new widgets.
    placer: Placer,

    /// If false we are unresponsive to input,
    /// and all widgets will assume a gray style.
    enabled: bool,

    /// Set to true in special cases where we do one frame
    /// where we size up the contents of the Ui, without actually showing it.
    sizing_pass: bool,

    /// Indicates whether this Ui belongs to a Menu.
    menu_state: Option<Arc<RwLock<MenuState>>>,

    /// The [`UiStack`] for this [`Ui`].
    stack: Arc<UiStack>,

    /// The sense for the ui background.
    sense: Sense,

    /// Whether [`Ui::remember_min_rect`] should be called when the [`Ui`] is dropped.
    /// This is an optimization, so we don't call [`Ui::remember_min_rect`] multiple times at the
    /// end of a [`Ui::scope`].
    min_rect_already_remembered: bool,
}

impl Ui {
    // ------------------------------------------------------------------------
    // Creation:

    /// Create a new top-level [`Ui`].
    ///
    /// Normally you would not use this directly, but instead use
    /// [`crate::SidePanel`], [`crate::TopBottomPanel`], [`crate::CentralPanel`], [`crate::Window`] or [`crate::Area`].
    pub fn new(ctx: Context, id: Id, ui_builder: UiBuilder) -> Self {
        let UiBuilder {
            id_salt,
            ui_stack_info,
            layer_id,
            max_rect,
            layout,
            disabled,
            invisible,
            sizing_pass,
            style,
            sense,
        } = ui_builder;

        let layer_id = layer_id.unwrap_or(LayerId::background());

        debug_assert!(
            id_salt.is_none(),
            "Top-level Ui:s should not have an id_salt"
        );

        let max_rect = max_rect.unwrap_or_else(|| ctx.screen_rect());
        let clip_rect = max_rect;
        let layout = layout.unwrap_or_default();
        let disabled = disabled || invisible;
        let style = style.unwrap_or_else(|| ctx.style());
        let sense = sense.unwrap_or(Sense::hover());

        let placer = Placer::new(max_rect, layout);
        let ui_stack = UiStack {
            id,
            layout_direction: layout.main_dir,
            info: ui_stack_info,
            parent: None,
            min_rect: placer.min_rect(),
            max_rect: placer.max_rect(),
        };
        let mut ui = Ui {
            id,
            unique_id: id,
            next_auto_id_salt: id.with("auto").value(),
            painter: Painter::new(ctx, layer_id, clip_rect),
            style,
            placer,
            enabled: true,
            sizing_pass,
            menu_state: None,
            stack: Arc::new(ui_stack),
            sense,
            min_rect_already_remembered: false,
        };

        // Register in the widget stack early, to ensure we are behind all widgets we contain:
        let start_rect = Rect::NOTHING; // This will be overwritten when `remember_min_rect` is called
        ui.ctx().create_widget(
            WidgetRect {
                id: ui.unique_id,
                layer_id: ui.layer_id(),
                rect: start_rect,
                interact_rect: start_rect,
                sense,
                enabled: ui.enabled,
            },
            true,
        );

        if disabled {
            ui.disable();
        }
        if invisible {
            ui.set_invisible();
        }

        ui
    }

    /// Create a new [`Ui`] at a specific region.
    ///
    /// Note: calling this function twice from the same [`Ui`] will create a conflict of id. Use
    /// [`Self::scope`] if needed.
    ///
    /// When in doubt, use `None` for the `UiStackInfo` argument.
    #[deprecated = "Use ui.new_child() instead"]
    pub fn child_ui(
        &mut self,
        max_rect: Rect,
        layout: Layout,
        ui_stack_info: Option<UiStackInfo>,
    ) -> Self {
        self.new_child(
            UiBuilder::new()
                .max_rect(max_rect)
                .layout(layout)
                .ui_stack_info(ui_stack_info.unwrap_or_default()),
        )
    }

    /// Create a new [`Ui`] at a specific region with a specific id.
    ///
    /// When in doubt, use `None` for the `UiStackInfo` argument.
    #[deprecated = "Use ui.new_child() instead"]
    pub fn child_ui_with_id_source(
        &mut self,
        max_rect: Rect,
        layout: Layout,
        id_salt: impl Hash,
        ui_stack_info: Option<UiStackInfo>,
    ) -> Self {
        self.new_child(
            UiBuilder::new()
                .id_salt(id_salt)
                .max_rect(max_rect)
                .layout(layout)
                .ui_stack_info(ui_stack_info.unwrap_or_default()),
        )
    }

    /// Create a child `Ui` with the properties of the given builder.
    ///
    /// This is a very low-level function.
    /// Usually you are better off using [`Self::scope_builder`].
    ///
    /// Note that calling this does not allocate any space in the parent `Ui`,
    /// so after adding widgets to the child `Ui` you probably want to allocate
    /// the [`Ui::min_rect`] of the child in the parent `Ui` using e.g.
    /// [`Ui::advance_cursor_after_rect`].
    pub fn new_child(&mut self, ui_builder: UiBuilder) -> Self {
        let UiBuilder {
            id_salt,
            ui_stack_info,
            layer_id,
            max_rect,
            layout,
            disabled,
            invisible,
            sizing_pass,
            style,
            sense,
        } = ui_builder;

        let mut painter = self.painter.clone();

        let id_salt = id_salt.unwrap_or_else(|| Id::from("child"));
        let max_rect = max_rect.unwrap_or_else(|| self.available_rect_before_wrap());
        let mut layout = layout.unwrap_or(*self.layout());
        let enabled = self.enabled && !disabled && !invisible;
        if let Some(layer_id) = layer_id {
            painter.set_layer_id(layer_id);
        }
        if invisible {
            painter.set_invisible();
        }
        let sizing_pass = self.sizing_pass || sizing_pass;
        let style = style.unwrap_or_else(|| self.style.clone());
        let sense = sense.unwrap_or(Sense::hover());

        if sizing_pass {
            // During the sizing pass we want widgets to use up as little space as possible,
            // so that we measure the only the space we _need_.
            layout.cross_justify = false;
            if layout.cross_align == Align::Center {
                layout.cross_align = Align::Min;
            }
        }

        debug_assert!(!max_rect.any_nan());
        let stable_id = self.id.with(id_salt);
        let unique_id = stable_id.with(self.next_auto_id_salt);
        let next_auto_id_salt = unique_id.value().wrapping_add(1);

        self.next_auto_id_salt = self.next_auto_id_salt.wrapping_add(1);

        let placer = Placer::new(max_rect, layout);
        let ui_stack = UiStack {
            id: unique_id,
            layout_direction: layout.main_dir,
            info: ui_stack_info,
            parent: Some(self.stack.clone()),
            min_rect: placer.min_rect(),
            max_rect: placer.max_rect(),
        };
        let mut child_ui = Ui {
            id: stable_id,
            unique_id,
            next_auto_id_salt,
            painter,
            style,
            placer,
            enabled,
            sizing_pass,
            menu_state: self.menu_state.clone(),
            stack: Arc::new(ui_stack),
            sense,
            min_rect_already_remembered: false,
        };

        if disabled {
            child_ui.disable();
        }

        // Register in the widget stack early, to ensure we are behind all widgets we contain:
        let start_rect = Rect::NOTHING; // This will be overwritten when `remember_min_rect` is called
        child_ui.ctx().create_widget(
            WidgetRect {
                id: child_ui.unique_id,
                layer_id: child_ui.layer_id(),
                rect: start_rect,
                interact_rect: start_rect,
                sense,
                enabled: child_ui.enabled,
            },
            true,
        );

        child_ui
    }

    // -------------------------------------------------

    /// Set to true in special cases where we do one frame
    /// where we size up the contents of the Ui, without actually showing it.
    ///
    /// This will also turn the Ui invisible.
    /// Should be called right after [`Self::new`], if at all.
    #[inline]
    #[deprecated = "Use UiBuilder.sizing_pass().invisible()"]
    pub fn set_sizing_pass(&mut self) {
        self.sizing_pass = true;
        self.set_invisible();
    }

    /// Set to true in special cases where we do one frame
    /// where we size up the contents of the Ui, without actually showing it.
    #[inline]
    pub fn is_sizing_pass(&self) -> bool {
        self.sizing_pass
    }

    // -------------------------------------------------

    /// Generated based on id of parent ui together with an optional id salt.
    ///
    /// This should be stable from one frame to next
    /// so it can be used as a source for storing state
    /// (e.g. window position, or if a collapsing header is open).
    ///
    /// However, it is not necessarily globally unique.
    /// For instance, sibling `Ui`s share the same [`Self::id`]
    /// unless they where explicitly given different id salts using
    /// [`UiBuilder::id_salt`].
    #[inline]
    pub fn id(&self) -> Id {
        self.id
    }

    /// This is a globally unique ID of this `Ui`,
    /// based on where in the hierarchy of widgets this Ui is in.
    ///
    /// This means it is not _stable_, as it can change if new widgets
    /// are added or removed prior to this one.
    /// It should therefore only be used for transient interactions (clicks etc),
    /// not for storing state over time.
    #[inline]
    pub fn unique_id(&self) -> Id {
        self.unique_id
    }

    /// Style options for this [`Ui`] and its children.
    ///
    /// Note that this may be a different [`Style`] than that of [`Context::style`].
    #[inline]
    pub fn style(&self) -> &Arc<Style> {
        &self.style
    }

    /// Mutably borrow internal [`Style`].
    /// Changes apply to this [`Ui`] and its subsequent children.
    ///
    /// To set the style of all [`Ui`]:s, use [`Context::set_style_of`].
    ///
    /// Example:
    /// ```
    /// # egui::__run_test_ui(|ui| {
    /// ui.style_mut().override_text_style = Some(egui::TextStyle::Heading);
    /// # });
    /// ```
    pub fn style_mut(&mut self) -> &mut Style {
        Arc::make_mut(&mut self.style) // clone-on-write
    }

    /// Changes apply to this [`Ui`] and its subsequent children.
    ///
    /// To set the visuals of all [`Ui`]:s, use [`Context::set_visuals_of`].
    pub fn set_style(&mut self, style: impl Into<Arc<Style>>) {
        self.style = style.into();
    }

    /// Reset to the default style set in [`Context`].
    pub fn reset_style(&mut self) {
        self.style = self.ctx().style();
    }

    /// The current spacing options for this [`Ui`].
    /// Short for `ui.style().spacing`.
    #[inline]
    pub fn spacing(&self) -> &crate::style::Spacing {
        &self.style.spacing
    }

    /// Mutably borrow internal [`Spacing`](crate::style::Spacing).
    /// Changes apply to this [`Ui`] and its subsequent children.
    ///
    /// Example:
    /// ```
    /// # egui::__run_test_ui(|ui| {
    /// ui.spacing_mut().item_spacing = egui::vec2(10.0, 2.0);
    /// # });
    /// ```
    pub fn spacing_mut(&mut self) -> &mut crate::style::Spacing {
        &mut self.style_mut().spacing
    }

    /// The current visuals settings of this [`Ui`].
    /// Short for `ui.style().visuals`.
    #[inline]
    pub fn visuals(&self) -> &crate::Visuals {
        &self.style.visuals
    }

    /// Mutably borrow internal `visuals`.
    /// Changes apply to this [`Ui`] and its subsequent children.
    ///
    /// To set the visuals of all [`Ui`]:s, use [`Context::set_visuals_of`].
    ///
    /// Example:
    /// ```
    /// # egui::__run_test_ui(|ui| {
    /// ui.visuals_mut().override_text_color = Some(egui::Color32::RED);
    /// # });
    /// ```
    pub fn visuals_mut(&mut self) -> &mut crate::Visuals {
        &mut self.style_mut().visuals
    }

    /// Get a reference to this [`Ui`]'s [`UiStack`].
    #[inline]
    pub fn stack(&self) -> &Arc<UiStack> {
        &self.stack
    }

    /// Get a reference to the parent [`Context`].
    #[inline]
    pub fn ctx(&self) -> &Context {
        self.painter.ctx()
    }

    /// Use this to paint stuff within this [`Ui`].
    #[inline]
    pub fn painter(&self) -> &Painter {
        &self.painter
    }

    /// Number of physical pixels for each logical UI point.
    #[inline]
    pub fn pixels_per_point(&self) -> f32 {
        self.painter.pixels_per_point()
    }

    /// If `false`, the [`Ui`] does not allow any interaction and
    /// the widgets in it will draw with a gray look.
    #[inline]
    pub fn is_enabled(&self) -> bool {
        self.enabled
    }

    /// Calling `disable()` will cause the [`Ui`] to deny all future interaction
    /// and all the widgets will draw with a gray look.
    ///
    /// Usually it is more convenient to use [`Self::add_enabled_ui`] or [`Self::add_enabled`].
    ///
    /// Note that once disabled, there is no way to re-enable the [`Ui`].
    ///
    /// ### Example
    /// ```
    /// # egui::__run_test_ui(|ui| {
    /// # let mut enabled = true;
    /// ui.group(|ui| {
    ///     ui.checkbox(&mut enabled, "Enable subsection");
    ///     if !enabled {
    ///         ui.disable();
    ///     }
    ///     if ui.button("Button that is not always clickable").clicked() {
    ///         /* … */
    ///     }
    /// });
    /// # });
    /// ```
    pub fn disable(&mut self) {
        self.enabled = false;
        if self.is_visible() {
            self.painter
                .set_fade_to_color(Some(self.visuals().fade_out_to_color()));
        }
    }

    /// Calling `set_enabled(false)` will cause the [`Ui`] to deny all future interaction
    /// and all the widgets will draw with a gray look.
    ///
    /// Usually it is more convenient to use [`Self::add_enabled_ui`] or [`Self::add_enabled`].
    ///
    /// Calling `set_enabled(true)` has no effect - it will NOT re-enable the [`Ui`] once disabled.
    ///
    /// ### Example
    /// ```
    /// # egui::__run_test_ui(|ui| {
    /// # let mut enabled = true;
    /// ui.group(|ui| {
    ///     ui.checkbox(&mut enabled, "Enable subsection");
    ///     ui.set_enabled(enabled);
    ///     if ui.button("Button that is not always clickable").clicked() {
    ///         /* … */
    ///     }
    /// });
    /// # });
    /// ```
    #[deprecated = "Use disable(), add_enabled_ui(), or add_enabled() instead"]
    pub fn set_enabled(&mut self, enabled: bool) {
        if !enabled {
            self.disable();
        }
    }

    /// If `false`, any widgets added to the [`Ui`] will be invisible and non-interactive.
    ///
    /// This is `false` if any parent had [`UiBuilder::invisible`]
    /// or if [`Context::will_discard`].
    #[inline]
    pub fn is_visible(&self) -> bool {
        self.painter.is_visible()
    }

    /// Calling `set_invisible()` will cause all further widgets to be invisible,
    /// yet still allocate space.
    ///
    /// The widgets will not be interactive (`set_invisible()` implies `disable()`).
    ///
    /// Once invisible, there is no way to make the [`Ui`] visible again.
    ///
    /// Usually it is more convenient to use [`Self::add_visible_ui`] or [`Self::add_visible`].
    ///
    /// ### Example
    /// ```
    /// # egui::__run_test_ui(|ui| {
    /// # let mut visible = true;
    /// ui.group(|ui| {
    ///     ui.checkbox(&mut visible, "Show subsection");
    ///     if !visible {
    ///         ui.set_invisible();
    ///     }
    ///     if ui.button("Button that is not always shown").clicked() {
    ///         /* … */
    ///     }
    /// });
    /// # });
    /// ```
    pub fn set_invisible(&mut self) {
        self.painter.set_invisible();
        self.disable();
    }

    /// Calling `set_visible(false)` will cause all further widgets to be invisible,
    /// yet still allocate space.
    ///
    /// The widgets will not be interactive (`set_visible(false)` implies `set_enabled(false)`).
    ///
    /// Calling `set_visible(true)` has no effect.
    ///
    /// ### Example
    /// ```
    /// # egui::__run_test_ui(|ui| {
    /// # let mut visible = true;
    /// ui.group(|ui| {
    ///     ui.checkbox(&mut visible, "Show subsection");
    ///     ui.set_visible(visible);
    ///     if ui.button("Button that is not always shown").clicked() {
    ///         /* … */
    ///     }
    /// });
    /// # });
    /// ```
    #[deprecated = "Use set_invisible(), add_visible_ui(), or add_visible() instead"]
    pub fn set_visible(&mut self, visible: bool) {
        if !visible {
            self.painter.set_invisible();
            self.disable();
        }
    }

    /// Make the widget in this [`Ui`] semi-transparent.
    ///
    /// `opacity` must be between 0.0 and 1.0, where 0.0 means fully transparent (i.e., invisible)
    /// and 1.0 means fully opaque.
    ///
    /// ### Example
    /// ```
    /// # egui::__run_test_ui(|ui| {
    /// ui.group(|ui| {
    ///     ui.set_opacity(0.5);
    ///     if ui.button("Half-transparent button").clicked() {
    ///         /* … */
    ///     }
    /// });
    /// # });
    /// ```
    ///
    /// See also: [`Self::opacity`] and [`Self::multiply_opacity`].
    pub fn set_opacity(&mut self, opacity: f32) {
        self.painter.set_opacity(opacity);
    }

    /// Like [`Self::set_opacity`], but multiplies the given value with the current opacity.
    ///
    /// See also: [`Self::set_opacity`] and [`Self::opacity`].
    pub fn multiply_opacity(&mut self, opacity: f32) {
        self.painter.multiply_opacity(opacity);
    }

    /// Read the current opacity of the underlying painter.
    ///
    /// See also: [`Self::set_opacity`] and [`Self::multiply_opacity`].
    #[inline]
    pub fn opacity(&self) -> f32 {
        self.painter.opacity()
    }

    /// Read the [`Layout`].
    #[inline]
    pub fn layout(&self) -> &Layout {
        self.placer.layout()
    }

    /// Which wrap mode should the text use in this [`Ui`]?
    ///
    /// This is determined first by [`Style::wrap_mode`], and then by the layout of this [`Ui`].
    pub fn wrap_mode(&self) -> TextWrapMode {
        #[allow(deprecated)]
        if let Some(wrap_mode) = self.style.wrap_mode {
            wrap_mode
        }
        // `wrap` handling for backward compatibility
        else if let Some(wrap) = self.style.wrap {
            if wrap {
                TextWrapMode::Wrap
            } else {
                TextWrapMode::Extend
            }
        } else if let Some(grid) = self.placer.grid() {
            if grid.wrap_text() {
                TextWrapMode::Wrap
            } else {
                TextWrapMode::Extend
            }
        } else {
            let layout = self.layout();
            if layout.is_vertical() || layout.is_horizontal() && layout.main_wrap() {
                TextWrapMode::Wrap
            } else {
                TextWrapMode::Extend
            }
        }
    }

    /// Should text wrap in this [`Ui`]?
    ///
    /// This is determined first by [`Style::wrap_mode`], and then by the layout of this [`Ui`].
    #[deprecated = "Use `wrap_mode` instead"]
    pub fn wrap_text(&self) -> bool {
        self.wrap_mode() == TextWrapMode::Wrap
    }

    /// How to vertically align text
    #[inline]
    pub fn text_valign(&self) -> Align {
        self.style()
            .override_text_valign
            .unwrap_or_else(|| self.layout().vertical_align())
    }

    /// Create a painter for a sub-region of this Ui.
    ///
    /// The clip-rect of the returned [`Painter`] will be the intersection
    /// of the given rectangle and the `clip_rect()` of this [`Ui`].
    pub fn painter_at(&self, rect: Rect) -> Painter {
        self.painter().with_clip_rect(rect)
    }

    /// Use this to paint stuff within this [`Ui`].
    #[inline]
    pub fn layer_id(&self) -> LayerId {
        self.painter().layer_id()
    }

    /// The height of text of this text style.
    ///
    /// Returns a value rounded to [`emath::GUI_ROUNDING`].
    pub fn text_style_height(&self, style: &TextStyle) -> f32 {
        self.fonts(|f| f.row_height(&style.resolve(self.style())))
    }

    /// Screen-space rectangle for clipping what we paint in this ui.
    /// This is used, for instance, to avoid painting outside a window that is smaller than its contents.
    #[inline]
    pub fn clip_rect(&self) -> Rect {
        self.painter.clip_rect()
    }

    /// Constrain the rectangle in which we can paint.
    ///
    /// Short for `ui.set_clip_rect(ui.clip_rect().intersect(new_clip_rect))`.
    ///
    /// See also: [`Self::clip_rect`] and [`Self::set_clip_rect`].
    #[inline]
    pub fn shrink_clip_rect(&mut self, new_clip_rect: Rect) {
        self.painter.shrink_clip_rect(new_clip_rect);
    }

    /// Screen-space rectangle for clipping what we paint in this ui.
    /// This is used, for instance, to avoid painting outside a window that is smaller than its contents.
    ///
    /// Warning: growing the clip rect might cause unexpected results!
    /// When in doubt, use [`Self::shrink_clip_rect`] instead.
    pub fn set_clip_rect(&mut self, clip_rect: Rect) {
        self.painter.set_clip_rect(clip_rect);
    }

    /// Can be used for culling: if `false`, then no part of `rect` will be visible on screen.
    ///
    /// This is false if the whole `Ui` is invisible (see [`UiBuilder::invisible`])
    /// or if [`Context::will_discard`] is true.
    pub fn is_rect_visible(&self, rect: Rect) -> bool {
        self.is_visible() && rect.intersects(self.clip_rect())
    }
}

/// # Helpers for accessing the underlying [`Context`].
/// These functions all lock the [`Context`] owned by this [`Ui`].
/// Please see the documentation of [`Context`] for how locking works!
impl Ui {
    /// Read-only access to the shared [`InputState`].
    ///
    /// ```
    /// # egui::__run_test_ui(|ui| {
    /// if ui.input(|i| i.key_pressed(egui::Key::A)) {
    ///     // …
    /// }
    /// # });
    /// ```
    #[inline]
    pub fn input<R>(&self, reader: impl FnOnce(&InputState) -> R) -> R {
        self.ctx().input(reader)
    }

    /// Read-write access to the shared [`InputState`].
    #[inline]
    pub fn input_mut<R>(&self, writer: impl FnOnce(&mut InputState) -> R) -> R {
        self.ctx().input_mut(writer)
    }

    /// Read-only access to the shared [`Memory`].
    #[inline]
    pub fn memory<R>(&self, reader: impl FnOnce(&Memory) -> R) -> R {
        self.ctx().memory(reader)
    }

    /// Read-write access to the shared [`Memory`].
    #[inline]
    pub fn memory_mut<R>(&self, writer: impl FnOnce(&mut Memory) -> R) -> R {
        self.ctx().memory_mut(writer)
    }

    /// Read-only access to the shared [`IdTypeMap`], which stores superficial widget state.
    #[inline]
    pub fn data<R>(&self, reader: impl FnOnce(&IdTypeMap) -> R) -> R {
        self.ctx().data(reader)
    }

    /// Read-write access to the shared [`IdTypeMap`], which stores superficial widget state.
    #[inline]
    pub fn data_mut<R>(&self, writer: impl FnOnce(&mut IdTypeMap) -> R) -> R {
        self.ctx().data_mut(writer)
    }

    /// Read-only access to the shared [`PlatformOutput`].
    ///
    /// This is what egui outputs each frame.
    ///
    /// ```
    /// # let mut ctx = egui::Context::default();
    /// ctx.output_mut(|o| o.cursor_icon = egui::CursorIcon::Progress);
    /// ```
    #[inline]
    pub fn output<R>(&self, reader: impl FnOnce(&PlatformOutput) -> R) -> R {
        self.ctx().output(reader)
    }

    /// Read-write access to the shared [`PlatformOutput`].
    ///
    /// This is what egui outputs each frame.
    ///
    /// ```
    /// # let mut ctx = egui::Context::default();
    /// ctx.output_mut(|o| o.cursor_icon = egui::CursorIcon::Progress);
    /// ```
    #[inline]
    pub fn output_mut<R>(&self, writer: impl FnOnce(&mut PlatformOutput) -> R) -> R {
        self.ctx().output_mut(writer)
    }

    /// Read-only access to [`Fonts`].
    #[inline]
    pub fn fonts<R>(&self, reader: impl FnOnce(&Fonts) -> R) -> R {
        self.ctx().fonts(reader)
    }
}

// ------------------------------------------------------------------------

/// # Sizes etc
impl Ui {
    /// Where and how large the [`Ui`] is already.
    /// All widgets that have been added to this [`Ui`] fits within this rectangle.
    ///
    /// No matter what, the final Ui will be at least this large.
    ///
    /// This will grow as new widgets are added, but never shrink.
    pub fn min_rect(&self) -> Rect {
        self.placer.min_rect()
    }

    /// Size of content; same as `min_rect().size()`
    pub fn min_size(&self) -> Vec2 {
        self.min_rect().size()
    }

    /// New widgets will *try* to fit within this rectangle.
    ///
    /// Text labels will wrap to fit within `max_rect`.
    /// Separator lines will span the `max_rect`.
    ///
    /// If a new widget doesn't fit within the `max_rect` then the
    /// [`Ui`] will make room for it by expanding both `min_rect` and `max_rect`.
    pub fn max_rect(&self) -> Rect {
        self.placer.max_rect()
    }

    /// Used for animation, kind of hacky
    pub(crate) fn force_set_min_rect(&mut self, min_rect: Rect) {
        self.placer.force_set_min_rect(min_rect);
    }

    // ------------------------------------------------------------------------

    /// Set the maximum size of the ui.
    /// You won't be able to shrink it below the current minimum size.
    pub fn set_max_size(&mut self, size: Vec2) {
        self.set_max_width(size.x);
        self.set_max_height(size.y);
    }

    /// Set the maximum width of the ui.
    /// You won't be able to shrink it below the current minimum size.
    pub fn set_max_width(&mut self, width: f32) {
        self.placer.set_max_width(width);
    }

    /// Set the maximum height of the ui.
    /// You won't be able to shrink it below the current minimum size.
    pub fn set_max_height(&mut self, height: f32) {
        self.placer.set_max_height(height);
    }

    // ------------------------------------------------------------------------

    /// Set the minimum size of the ui.
    /// This can't shrink the ui, only make it larger.
    pub fn set_min_size(&mut self, size: Vec2) {
        self.set_min_width(size.x);
        self.set_min_height(size.y);
    }

    /// Set the minimum width of the ui.
    /// This can't shrink the ui, only make it larger.
    pub fn set_min_width(&mut self, width: f32) {
        debug_assert!(0.0 <= width);
        self.placer.set_min_width(width);
    }

    /// Set the minimum height of the ui.
    /// This can't shrink the ui, only make it larger.
    pub fn set_min_height(&mut self, height: f32) {
        debug_assert!(0.0 <= height);
        self.placer.set_min_height(height);
    }

    // ------------------------------------------------------------------------

    /// Helper: shrinks the max width to the current width,
    /// so further widgets will try not to be wider than previous widgets.
    /// Useful for normal vertical layouts.
    pub fn shrink_width_to_current(&mut self) {
        self.set_max_width(self.min_rect().width());
    }

    /// Helper: shrinks the max height to the current height,
    /// so further widgets will try not to be taller than previous widgets.
    pub fn shrink_height_to_current(&mut self) {
        self.set_max_height(self.min_rect().height());
    }

    /// Expand the `min_rect` and `max_rect` of this ui to include a child at the given rect.
    pub fn expand_to_include_rect(&mut self, rect: Rect) {
        self.placer.expand_to_include_rect(rect);
    }

    /// `ui.set_width_range(min..=max);` is equivalent to `ui.set_min_width(min); ui.set_max_width(max);`.
    pub fn set_width_range(&mut self, width: impl Into<Rangef>) {
        let width = width.into();
        self.set_min_width(width.min);
        self.set_max_width(width.max);
    }

    /// `ui.set_height_range(min..=max);` is equivalent to `ui.set_min_height(min); ui.set_max_height(max);`.
    pub fn set_height_range(&mut self, height: impl Into<Rangef>) {
        let height = height.into();
        self.set_min_height(height.min);
        self.set_max_height(height.max);
    }

    /// Set both the minimum and maximum width.
    pub fn set_width(&mut self, width: f32) {
        self.set_min_width(width);
        self.set_max_width(width);
    }

    /// Set both the minimum and maximum height.
    pub fn set_height(&mut self, height: f32) {
        self.set_min_height(height);
        self.set_max_height(height);
    }

    /// Ensure we are big enough to contain the given x-coordinate.
    /// This is sometimes useful to expand a ui to stretch to a certain place.
    pub fn expand_to_include_x(&mut self, x: f32) {
        self.placer.expand_to_include_x(x);
    }

    /// Ensure we are big enough to contain the given y-coordinate.
    /// This is sometimes useful to expand a ui to stretch to a certain place.
    pub fn expand_to_include_y(&mut self, y: f32) {
        self.placer.expand_to_include_y(y);
    }

    // ------------------------------------------------------------------------
    // Layout related measures:

    /// The available space at the moment, given the current cursor.
    ///
    /// This how much more space we can take up without overflowing our parent.
    /// Shrinks as widgets allocate space and the cursor moves.
    /// A small size should be interpreted as "as little as possible".
    /// An infinite size should be interpreted as "as much as you want".
    pub fn available_size(&self) -> Vec2 {
        self.placer.available_size()
    }

    /// The available width at the moment, given the current cursor.
    ///
    /// See [`Self::available_size`] for more information.
    pub fn available_width(&self) -> f32 {
        self.available_size().x
    }

    /// The available height at the moment, given the current cursor.
    ///
    /// See [`Self::available_size`] for more information.
    pub fn available_height(&self) -> f32 {
        self.available_size().y
    }

    /// In case of a wrapping layout, how much space is left on this row/column?
    ///
    /// If the layout does not wrap, this will return the same value as [`Self::available_size`].
    pub fn available_size_before_wrap(&self) -> Vec2 {
        self.placer.available_rect_before_wrap().size()
    }

    /// In case of a wrapping layout, how much space is left on this row/column?
    ///
    /// If the layout does not wrap, this will return the same value as [`Self::available_size`].
    pub fn available_rect_before_wrap(&self) -> Rect {
        self.placer.available_rect_before_wrap()
    }
}

/// # [`Id`] creation
impl Ui {
    /// Use this to generate widget ids for widgets that have persistent state in [`Memory`].
    pub fn make_persistent_id<IdSource>(&self, id_salt: IdSource) -> Id
    where
        IdSource: Hash,
    {
        self.id.with(&id_salt)
    }

    /// This is the `Id` that will be assigned to the next widget added to this `Ui`.
    pub fn next_auto_id(&self) -> Id {
        Id::new(self.next_auto_id_salt)
    }

    /// Same as `ui.next_auto_id().with(id_salt)`
    pub fn auto_id_with<IdSource>(&self, id_salt: IdSource) -> Id
    where
        IdSource: Hash,
    {
        Id::new(self.next_auto_id_salt).with(id_salt)
    }

    /// Pretend like `count` widgets have been allocated.
    pub fn skip_ahead_auto_ids(&mut self, count: usize) {
        self.next_auto_id_salt = self.next_auto_id_salt.wrapping_add(count as u64);
    }
}

/// # Interaction
impl Ui {
    /// Check for clicks, drags and/or hover on a specific region of this [`Ui`].
    pub fn interact(&self, rect: Rect, id: Id, sense: Sense) -> Response {
        self.ctx().create_widget(
            WidgetRect {
                id,
                layer_id: self.layer_id(),
                rect,
                interact_rect: self.clip_rect().intersect(rect),
                sense,
                enabled: self.enabled,
            },
            true,
        )
    }

    /// Deprecated: use [`Self::interact`] instead.
    #[deprecated = "The contains_pointer argument is ignored. Use `ui.interact` instead."]
    pub fn interact_with_hovered(
        &self,
        rect: Rect,
        _contains_pointer: bool,
        id: Id,
        sense: Sense,
    ) -> Response {
        self.interact(rect, id, sense)
    }

    /// Read the [`Ui`]s background [`Response`].
    /// It's [`Sense`] will be based on the [`UiBuilder::sense`] used to create this [`Ui`].
    ///
    /// The rectangle of the [`Response`] (and interactive area) will be [`Self::min_rect`]
    /// of the last pass.
    ///
    /// The very first time when the [`Ui`] is created, this will return a [`Response`] with a
    /// [`Rect`] of [`Rect::NOTHING`].
    pub fn response(&self) -> Response {
        // This is the inverse of Context::read_response. We prefer a response
        // based on last frame's widget rect since the one from this frame is Rect::NOTHING until
        // Ui::interact_bg is called or the Ui is dropped.
        self.ctx()
            .viewport(|viewport| {
                viewport
                    .prev_pass
                    .widgets
                    .get(self.unique_id)
                    .or_else(|| viewport.this_pass.widgets.get(self.unique_id))
                    .copied()
            })
            .map(|widget_rect| self.ctx().get_response(widget_rect))
            .expect(
                "Since we always call Context::create_widget in Ui::new, this should never be None",
            )
    }

    /// Update the [`WidgetRect`] created in [`Ui::new`] or [`Ui::new_child`] with the current
    /// [`Ui::min_rect`].
    fn remember_min_rect(&mut self) -> Response {
        self.min_rect_already_remembered = true;
        // We remove the id from used_ids to prevent a duplicate id warning from showing
        // when the ui was created with `UiBuilder::sense`.
        // This is a bit hacky, is there a better way?
        self.ctx().pass_state_mut(|fs| {
            fs.used_ids.remove(&self.unique_id);
        });
        // This will update the WidgetRect that was first created in `Ui::new`.
        self.ctx().create_widget(
            WidgetRect {
                id: self.unique_id,
                layer_id: self.layer_id(),
                rect: self.min_rect(),
                interact_rect: self.clip_rect().intersect(self.min_rect()),
                sense: self.sense,
                enabled: self.enabled,
            },
            false,
        )
    }

    /// Interact with the background of this [`Ui`],
    /// i.e. behind all the widgets.
    ///
    /// The rectangle of the [`Response`] (and interactive area) will be [`Self::min_rect`].
    #[deprecated = "Use UiBuilder::sense with Ui::response instead"]
    pub fn interact_bg(&self, sense: Sense) -> Response {
        // This will update the WidgetRect that was first created in `Ui::new`.
        self.interact(self.min_rect(), self.unique_id, sense)
    }

    /// Is the pointer (mouse/touch) above this rectangle in this [`Ui`]?
    ///
    /// The `clip_rect` and layer of this [`Ui`] will be respected, so, for instance,
    /// if this [`Ui`] is behind some other window, this will always return `false`.
    ///
    /// However, this will NOT check if any other _widget_ in the same layer is covering this widget. For that, use [`Response::contains_pointer`] instead.
    pub fn rect_contains_pointer(&self, rect: Rect) -> bool {
        self.ctx()
            .rect_contains_pointer(self.layer_id(), self.clip_rect().intersect(rect))
    }

    /// Is the pointer (mouse/touch) above the current [`Ui`]?
    ///
    /// Equivalent to `ui.rect_contains_pointer(ui.min_rect())`
    ///
    /// Note that this tests against the _current_ [`Ui::min_rect`].
    /// If you want to test against the final `min_rect`,
    /// use [`Self::response`] instead.
    pub fn ui_contains_pointer(&self) -> bool {
        self.rect_contains_pointer(self.min_rect())
    }
}

/// # Allocating space: where do I put my widgets?
impl Ui {
    /// Allocate space for a widget and check for interaction in the space.
    /// Returns a [`Response`] which contains a rectangle, id, and interaction info.
    ///
    /// ## How sizes are negotiated
    /// Each widget should have a *minimum desired size* and a *desired size*.
    /// When asking for space, ask AT LEAST for your minimum, and don't ask for more than you need.
    /// If you want to fill the space, ask about [`Ui::available_size`] and use that.
    ///
    /// You may get MORE space than you asked for, for instance
    /// for justified layouts, like in menus.
    ///
    /// You will never get a rectangle that is smaller than the amount of space you asked for.
    ///
    /// ```
    /// # egui::__run_test_ui(|ui| {
    /// let response = ui.allocate_response(egui::vec2(100.0, 200.0), egui::Sense::click());
    /// if response.clicked() { /* … */ }
    /// ui.painter().rect_stroke(response.rect, 0.0, (1.0, egui::Color32::WHITE));
    /// # });
    /// ```
    pub fn allocate_response(&mut self, desired_size: Vec2, sense: Sense) -> Response {
        let (id, rect) = self.allocate_space(desired_size);
        let mut response = self.interact(rect, id, sense);
        response.intrinsic_size = Some(desired_size);
        response
    }

    /// Returns a [`Rect`] with exactly what you asked for.
    ///
    /// The response rect will be larger if this is part of a justified layout or similar.
    /// This means that if this is a narrow widget in a wide justified layout, then
    /// the widget will react to interactions outside the returned [`Rect`].
    pub fn allocate_exact_size(&mut self, desired_size: Vec2, sense: Sense) -> (Rect, Response) {
        let response = self.allocate_response(desired_size, sense);
        let rect = self
            .placer
            .align_size_within_rect(desired_size, response.rect);
        (rect, response)
    }

    /// Allocate at least as much space as needed, and interact with that rect.
    ///
    /// The returned [`Rect`] will be the same size as `Response::rect`.
    pub fn allocate_at_least(&mut self, desired_size: Vec2, sense: Sense) -> (Rect, Response) {
        let response = self.allocate_response(desired_size, sense);
        (response.rect, response)
    }

    /// Reserve this much space and move the cursor.
    /// Returns where to put the widget.
    ///
    /// ## How sizes are negotiated
    /// Each widget should have a *minimum desired size* and a *desired size*.
    /// When asking for space, ask AT LEAST for your minimum, and don't ask for more than you need.
    /// If you want to fill the space, ask about [`Ui::available_size`] and use that.
    ///
    /// You may get MORE space than you asked for, for instance
    /// for justified layouts, like in menus.
    ///
    /// You will never get a rectangle that is smaller than the amount of space you asked for.
    ///
    /// Returns an automatic [`Id`] (which you can use for interaction) and the [`Rect`] of where to put your widget.
    ///
    /// ```
    /// # egui::__run_test_ui(|ui| {
    /// let (id, rect) = ui.allocate_space(egui::vec2(100.0, 200.0));
    /// let response = ui.interact(rect, id, egui::Sense::click());
    /// # });
    /// ```
    pub fn allocate_space(&mut self, desired_size: Vec2) -> (Id, Rect) {
        #[cfg(debug_assertions)]
        let original_available = self.available_size_before_wrap();

        let rect = self.allocate_space_impl(desired_size);

        #[cfg(debug_assertions)]
        {
            let too_wide = desired_size.x > original_available.x;
            let too_high = desired_size.y > original_available.y;

            let debug_expand_width = self.style().debug.show_expand_width;
            let debug_expand_height = self.style().debug.show_expand_height;

            if (debug_expand_width && too_wide) || (debug_expand_height && too_high) {
                self.painter
                    .rect_stroke(rect, 0.0, (1.0, Color32::LIGHT_BLUE));

                let stroke = Stroke::new(2.5, Color32::from_rgb(200, 0, 0));
                let paint_line_seg = |a, b| self.painter().line_segment([a, b], stroke);

                if debug_expand_width && too_wide {
                    paint_line_seg(rect.left_top(), rect.left_bottom());
                    paint_line_seg(rect.left_center(), rect.right_center());
                    paint_line_seg(
                        pos2(rect.left() + original_available.x, rect.top()),
                        pos2(rect.left() + original_available.x, rect.bottom()),
                    );
                    paint_line_seg(rect.right_top(), rect.right_bottom());
                }

                if debug_expand_height && too_high {
                    paint_line_seg(rect.left_top(), rect.right_top());
                    paint_line_seg(rect.center_top(), rect.center_bottom());
                    paint_line_seg(rect.left_bottom(), rect.right_bottom());
                }
            }
        }

        let id = Id::new(self.next_auto_id_salt);
        self.next_auto_id_salt = self.next_auto_id_salt.wrapping_add(1);

        (id, rect)
    }

    /// Reserve this much space and move the cursor.
    /// Returns where to put the widget.
    fn allocate_space_impl(&mut self, desired_size: Vec2) -> Rect {
        let item_spacing = self.spacing().item_spacing;
        let frame_rect = self.placer.next_space(desired_size, item_spacing);
        debug_assert!(!frame_rect.any_nan());
        let widget_rect = self.placer.justify_and_align(frame_rect, desired_size);

        self.placer
            .advance_after_rects(frame_rect, widget_rect, item_spacing);

        register_rect(self, widget_rect);

        widget_rect
    }

    /// Allocate a specific part of the [`Ui`].
    ///
    /// Ignore the layout of the [`Ui`]: just put my widget here!
    /// The layout cursor will advance to past this `rect`.
    pub fn allocate_rect(&mut self, rect: Rect, sense: Sense) -> Response {
        let rect = rect.round_ui();
        let id = self.advance_cursor_after_rect(rect);
        self.interact(rect, id, sense)
    }

    /// Allocate a rect without interacting with it.
    pub fn advance_cursor_after_rect(&mut self, rect: Rect) -> Id {
        debug_assert!(!rect.any_nan());
        let rect = rect.round_ui();

        let item_spacing = self.spacing().item_spacing;
        self.placer.advance_after_rects(rect, rect, item_spacing);
        register_rect(self, rect);

        let id = Id::new(self.next_auto_id_salt);
        self.next_auto_id_salt = self.next_auto_id_salt.wrapping_add(1);
        id
    }

    pub(crate) fn placer(&self) -> &Placer {
        &self.placer
    }

    /// Where the next widget will be put.
    ///
    /// One side of this will always be infinite: the direction in which new widgets will be added.
    /// The opposing side is what is incremented.
    /// The crossing sides are initialized to `max_rect`.
    ///
    /// So one can think of `cursor` as a constraint on the available region.
    ///
    /// If something has already been added, this will point to `style.spacing.item_spacing` beyond the latest child.
    /// The cursor can thus be `style.spacing.item_spacing` pixels outside of the `min_rect`.
    pub fn cursor(&self) -> Rect {
        self.placer.cursor()
    }

    pub(crate) fn set_cursor(&mut self, cursor: Rect) {
        self.placer.set_cursor(cursor);
    }

    /// Where do we expect a zero-sized widget to be placed?
    pub fn next_widget_position(&self) -> Pos2 {
        self.placer.next_widget_position()
    }

    /// Allocated the given space and then adds content to that space.
    /// If the contents overflow, more space will be allocated.
    /// When finished, the amount of space actually used (`min_rect`) will be allocated.
    /// So you can request a lot of space and then use less.
    #[inline]
    pub fn allocate_ui<R>(
        &mut self,
        desired_size: Vec2,
        add_contents: impl FnOnce(&mut Self) -> R,
    ) -> InnerResponse<R> {
        self.allocate_ui_with_layout(desired_size, *self.layout(), add_contents)
    }

    /// Allocated the given space and then adds content to that space.
    /// If the contents overflow, more space will be allocated.
    /// When finished, the amount of space actually used (`min_rect`) will be allocated.
    /// So you can request a lot of space and then use less.
    #[inline]
    pub fn allocate_ui_with_layout<R>(
        &mut self,
        desired_size: Vec2,
        layout: Layout,
        add_contents: impl FnOnce(&mut Self) -> R,
    ) -> InnerResponse<R> {
        self.allocate_ui_with_layout_dyn(desired_size, layout, Box::new(add_contents))
    }

    fn allocate_ui_with_layout_dyn<'c, R>(
        &mut self,
        desired_size: Vec2,
        layout: Layout,
        add_contents: Box<dyn FnOnce(&mut Self) -> R + 'c>,
    ) -> InnerResponse<R> {
        debug_assert!(desired_size.x >= 0.0 && desired_size.y >= 0.0);
        let item_spacing = self.spacing().item_spacing;
        let frame_rect = self.placer.next_space(desired_size, item_spacing);
        let child_rect = self.placer.justify_and_align(frame_rect, desired_size);
        self.allocate_new_ui(
            UiBuilder::new().max_rect(child_rect).layout(layout),
            add_contents,
        )
    }

    /// Allocated the given rectangle and then adds content to that rectangle.
    ///
    /// If the contents overflow, more space will be allocated.
    /// When finished, the amount of space actually used (`min_rect`) will be allocated.
    /// So you can request a lot of space and then use less.
    #[deprecated = "Use `allocate_new_ui` instead"]
    pub fn allocate_ui_at_rect<R>(
        &mut self,
        max_rect: Rect,
        add_contents: impl FnOnce(&mut Self) -> R,
    ) -> InnerResponse<R> {
        self.allocate_new_ui(UiBuilder::new().max_rect(max_rect), add_contents)
    }

    /// Allocated space (`UiBuilder::max_rect`) and then add content to it.
    ///
    /// If the contents overflow, more space will be allocated.
    /// When finished, the amount of space actually used (`min_rect`) will be allocated in the parent.
    /// So you can request a lot of space and then use less.
    pub fn allocate_new_ui<R>(
        &mut self,
        ui_builder: UiBuilder,
        add_contents: impl FnOnce(&mut Self) -> R,
    ) -> InnerResponse<R> {
        self.allocate_new_ui_dyn(ui_builder, Box::new(add_contents))
    }

    fn allocate_new_ui_dyn<'c, R>(
        &mut self,
        ui_builder: UiBuilder,
        add_contents: Box<dyn FnOnce(&mut Self) -> R + 'c>,
    ) -> InnerResponse<R> {
        let mut child_ui = self.new_child(ui_builder);
        let inner = add_contents(&mut child_ui);
        let rect = child_ui.min_rect();
        let item_spacing = self.spacing().item_spacing;
        self.placer.advance_after_rects(rect, rect, item_spacing);
        register_rect(self, rect);
        let response = self.interact(rect, child_ui.unique_id, Sense::hover());
        InnerResponse::new(inner, response)
    }

    /// Convenience function to get a region to paint on.
    ///
    /// Note that egui uses screen coordinates for everything.
    ///
    /// ```
    /// # use egui::*;
    /// # use std::f32::consts::TAU;
    /// # egui::__run_test_ui(|ui| {
    /// let size = Vec2::splat(16.0);
    /// let (response, painter) = ui.allocate_painter(size, Sense::hover());
    /// let rect = response.rect;
    /// let c = rect.center();
    /// let r = rect.width() / 2.0 - 1.0;
    /// let color = Color32::from_gray(128);
    /// let stroke = Stroke::new(1.0, color);
    /// painter.circle_stroke(c, r, stroke);
    /// painter.line_segment([c - vec2(0.0, r), c + vec2(0.0, r)], stroke);
    /// painter.line_segment([c, c + r * Vec2::angled(TAU * 1.0 / 8.0)], stroke);
    /// painter.line_segment([c, c + r * Vec2::angled(TAU * 3.0 / 8.0)], stroke);
    /// # });
    /// ```
    pub fn allocate_painter(&mut self, desired_size: Vec2, sense: Sense) -> (Response, Painter) {
        let response = self.allocate_response(desired_size, sense);
        let clip_rect = self.clip_rect().intersect(response.rect); // Make sure we don't paint out of bounds
        let painter = self.painter().with_clip_rect(clip_rect);
        (response, painter)
    }
}

/// # Scrolling
impl Ui {
    /// Adjust the scroll position of any parent [`crate::ScrollArea`] so that the given [`Rect`] becomes visible.
    ///
    /// If `align` is [`Align::TOP`] it means "put the top of the rect at the top of the scroll area", etc.
    /// If `align` is `None`, it'll scroll enough to bring the cursor into view.
    ///
    /// See also: [`Response::scroll_to_me`], [`Ui::scroll_to_cursor`]. [`Ui::scroll_with_delta`]..
    ///
    /// ```
    /// # use egui::Align;
    /// # egui::__run_test_ui(|ui| {
    /// egui::ScrollArea::vertical().show(ui, |ui| {
    ///     // …
    ///     let response = ui.button("Center on me.");
    ///     if response.clicked() {
    ///         ui.scroll_to_rect(response.rect, Some(Align::Center));
    ///     }
    /// });
    /// # });
    /// ```
    pub fn scroll_to_rect(&self, rect: Rect, align: Option<Align>) {
        self.scroll_to_rect_animation(rect, align, self.style.scroll_animation);
    }

    /// Same as [`Self::scroll_to_rect`], but allows you to specify the [`style::ScrollAnimation`].
    pub fn scroll_to_rect_animation(
        &self,
        rect: Rect,
        align: Option<Align>,
        animation: style::ScrollAnimation,
    ) {
        for d in 0..2 {
            let range = Rangef::new(rect.min[d], rect.max[d]);
            self.ctx().pass_state_mut(|state| {
                state.scroll_target[d] =
                    Some(pass_state::ScrollTarget::new(range, align, animation));
            });
        }
    }

    /// Adjust the scroll position of any parent [`crate::ScrollArea`] so that the cursor (where the next widget goes) becomes visible.
    ///
    /// If `align` is [`Align::TOP`] it means "put the top of the rect at the top of the scroll area", etc.
    /// If `align` is not provided, it'll scroll enough to bring the cursor into view.
    ///
    /// See also: [`Response::scroll_to_me`], [`Ui::scroll_to_rect`]. [`Ui::scroll_with_delta`].
    ///
    /// ```
    /// # use egui::Align;
    /// # egui::__run_test_ui(|ui| {
    /// egui::ScrollArea::vertical().show(ui, |ui| {
    ///     let scroll_bottom = ui.button("Scroll to bottom.").clicked();
    ///     for i in 0..1000 {
    ///         ui.label(format!("Item {}", i));
    ///     }
    ///
    ///     if scroll_bottom {
    ///         ui.scroll_to_cursor(Some(Align::BOTTOM));
    ///     }
    /// });
    /// # });
    /// ```
    pub fn scroll_to_cursor(&self, align: Option<Align>) {
        self.scroll_to_cursor_animation(align, self.style.scroll_animation);
    }

    /// Same as [`Self::scroll_to_cursor`], but allows you to specify the [`style::ScrollAnimation`].
    pub fn scroll_to_cursor_animation(
        &self,
        align: Option<Align>,
        animation: style::ScrollAnimation,
    ) {
        let target = self.next_widget_position();
        for d in 0..2 {
            let target = Rangef::point(target[d]);
            self.ctx().pass_state_mut(|state| {
                state.scroll_target[d] =
                    Some(pass_state::ScrollTarget::new(target, align, animation));
            });
        }
    }

    /// Scroll this many points in the given direction, in the parent [`crate::ScrollArea`].
    ///
    /// The delta dictates how the _content_ (i.e. this UI) should move.
    ///
    /// A positive X-value indicates the content is being moved right,
    /// as when swiping right on a touch-screen or track-pad with natural scrolling.
    ///
    /// A positive Y-value indicates the content is being moved down,
    /// as when swiping down on a touch-screen or track-pad with natural scrolling.
    ///
    /// If this is called multiple times per frame for the same [`crate::ScrollArea`], the deltas will be summed.
    ///
    /// See also: [`Response::scroll_to_me`], [`Ui::scroll_to_rect`], [`Ui::scroll_to_cursor`]
    ///
    /// ```
    /// # use egui::{Align, Vec2};
    /// # egui::__run_test_ui(|ui| {
    /// let mut scroll_delta = Vec2::ZERO;
    /// if ui.button("Scroll down").clicked() {
    ///     scroll_delta.y -= 64.0; // move content up
    /// }
    /// egui::ScrollArea::vertical().show(ui, |ui| {
    ///     ui.scroll_with_delta(scroll_delta);
    ///     for i in 0..1000 {
    ///         ui.label(format!("Item {}", i));
    ///     }
    /// });
    /// # });
    /// ```
    pub fn scroll_with_delta(&self, delta: Vec2) {
        self.scroll_with_delta_animation(delta, self.style.scroll_animation);
    }

    /// Same as [`Self::scroll_with_delta`], but allows you to specify the [`style::ScrollAnimation`].
    pub fn scroll_with_delta_animation(&self, delta: Vec2, animation: style::ScrollAnimation) {
        self.ctx().pass_state_mut(|state| {
            state.scroll_delta.0 += delta;
            state.scroll_delta.1 = animation;
        });
    }
}

/// # Adding widgets
impl Ui {
    /// Add a [`Widget`] to this [`Ui`] at a location dependent on the current [`Layout`].
    ///
    /// The returned [`Response`] can be used to check for interactions,
    /// as well as adding tooltips using [`Response::on_hover_text`].
    ///
    /// See also [`Self::add_sized`] and [`Self::put`].
    ///
    /// ```
    /// # egui::__run_test_ui(|ui| {
    /// # let mut my_value = 42;
    /// let response = ui.add(egui::Slider::new(&mut my_value, 0..=100));
    /// response.on_hover_text("Drag me!");
    /// # });
    /// ```
    #[inline]
    pub fn add(&mut self, widget: impl Widget) -> Response {
        widget.ui(self)
    }

    /// Add a [`Widget`] to this [`Ui`] with a given size.
    /// The widget will attempt to fit within the given size, but some widgets may overflow.
    ///
    /// To fill all remaining area, use `ui.add_sized(ui.available_size(), widget);`
    ///
    /// See also [`Self::add`] and [`Self::put`].
    ///
    /// ```
    /// # egui::__run_test_ui(|ui| {
    /// # let mut my_value = 42;
    /// ui.add_sized([40.0, 20.0], egui::DragValue::new(&mut my_value));
    /// # });
    /// ```
    pub fn add_sized(&mut self, max_size: impl Into<Vec2>, widget: impl Widget) -> Response {
        // TODO(emilk): configure to overflow to main_dir instead of centered overflow
        // to handle the bug mentioned at https://github.com/emilk/egui/discussions/318#discussioncomment-627578
        // and fixed in https://github.com/emilk/egui/commit/035166276322b3f2324bd8b97ffcedc63fa8419f
        //
        // Make sure we keep the same main direction since it changes e.g. how text is wrapped:
        let layout = Layout::centered_and_justified(self.layout().main_dir());
        self.allocate_ui_with_layout(max_size.into(), layout, |ui| ui.add(widget))
            .inner
    }

    /// Add a [`Widget`] to this [`Ui`] at a specific location (manual layout).
    ///
    /// See also [`Self::add`] and [`Self::add_sized`].
    pub fn put(&mut self, max_rect: Rect, widget: impl Widget) -> Response {
        self.allocate_new_ui(
            UiBuilder::new()
                .max_rect(max_rect)
                .layout(Layout::centered_and_justified(Direction::TopDown)),
            |ui| ui.add(widget),
        )
        .inner
    }

    /// Add a single [`Widget`] that is possibly disabled, i.e. greyed out and non-interactive.
    ///
    /// If you call `add_enabled` from within an already disabled [`Ui`],
    /// the widget will always be disabled, even if the `enabled` argument is true.
    ///
    /// See also [`Self::add_enabled_ui`] and [`Self::is_enabled`].
    ///
    /// ```
    /// # egui::__run_test_ui(|ui| {
    /// ui.add_enabled(false, egui::Button::new("Can't click this"));
    /// # });
    /// ```
    pub fn add_enabled(&mut self, enabled: bool, widget: impl Widget) -> Response {
        if self.is_enabled() && !enabled {
            let old_painter = self.painter.clone();
            self.disable();
            let response = self.add(widget);
            self.enabled = true;
            self.painter = old_painter;
            response
        } else {
            self.add(widget)
        }
    }

    /// Add a section that is possibly disabled, i.e. greyed out and non-interactive.
    ///
    /// If you call `add_enabled_ui` from within an already disabled [`Ui`],
    /// the result will always be disabled, even if the `enabled` argument is true.
    ///
    /// See also [`Self::add_enabled`] and [`Self::is_enabled`].
    ///
    /// ### Example
    /// ```
    /// # egui::__run_test_ui(|ui| {
    /// # let mut enabled = true;
    /// ui.checkbox(&mut enabled, "Enable subsection");
    /// ui.add_enabled_ui(enabled, |ui| {
    ///     if ui.button("Button that is not always clickable").clicked() {
    ///         /* … */
    ///     }
    /// });
    /// # });
    /// ```
    pub fn add_enabled_ui<R>(
        &mut self,
        enabled: bool,
        add_contents: impl FnOnce(&mut Ui) -> R,
    ) -> InnerResponse<R> {
        self.scope(|ui| {
            if !enabled {
                ui.disable();
            }
            add_contents(ui)
        })
    }

    /// Add a single [`Widget`] that is possibly invisible.
    ///
    /// An invisible widget still takes up the same space as if it were visible.
    ///
    /// If you call `add_visible` from within an already invisible [`Ui`],
    /// the widget will always be invisible, even if the `visible` argument is true.
    ///
    /// See also [`Self::add_visible_ui`], [`Self::set_visible`] and [`Self::is_visible`].
    ///
    /// ```
    /// # egui::__run_test_ui(|ui| {
    /// ui.add_visible(false, egui::Label::new("You won't see me!"));
    /// # });
    /// ```
    pub fn add_visible(&mut self, visible: bool, widget: impl Widget) -> Response {
        if self.is_visible() && !visible {
            // temporary make us invisible:
            let old_painter = self.painter.clone();
            let old_enabled = self.enabled;

            self.set_invisible();

            let response = self.add(widget);

            self.painter = old_painter;
            self.enabled = old_enabled;
            response
        } else {
            self.add(widget)
        }
    }

    /// Add a section that is possibly invisible, i.e. greyed out and non-interactive.
    ///
    /// An invisible ui still takes up the same space as if it were visible.
    ///
    /// If you call `add_visible_ui` from within an already invisible [`Ui`],
    /// the result will always be invisible, even if the `visible` argument is true.
    ///
    /// See also [`Self::add_visible`], [`Self::set_visible`] and [`Self::is_visible`].
    ///
    /// ### Example
    /// ```
    /// # egui::__run_test_ui(|ui| {
    /// # let mut visible = true;
    /// ui.checkbox(&mut visible, "Show subsection");
    /// ui.add_visible_ui(visible, |ui| {
    ///     ui.label("Maybe you see this, maybe you don't!");
    /// });
    /// # });
    /// ```
    #[deprecated = "Use 'ui.scope_builder' instead"]
    pub fn add_visible_ui<R>(
        &mut self,
        visible: bool,
        add_contents: impl FnOnce(&mut Ui) -> R,
    ) -> InnerResponse<R> {
        let mut ui_builder = UiBuilder::new();
        if !visible {
            ui_builder = ui_builder.invisible();
        }
        self.scope_builder(ui_builder, add_contents)
    }

    /// Add extra space before the next widget.
    ///
    /// The direction is dependent on the layout.
    /// This will be in addition to the [`crate::style::Spacing::item_spacing`].
    ///
    /// [`Self::min_rect`] will expand to contain the space.
    #[inline]
    pub fn add_space(&mut self, amount: f32) {
        self.placer.advance_cursor(amount);
    }

    /// Show some text.
    ///
    /// Shortcut for `add(Label::new(text))`
    ///
    /// See also [`Label`].
    ///
    /// ### Example
    /// ```
    /// # egui::__run_test_ui(|ui| {
    /// use egui::{RichText, FontId, Color32};
    /// ui.label("Normal text");
    /// ui.label(RichText::new("Large text").font(FontId::proportional(40.0)));
    /// ui.label(RichText::new("Red text").color(Color32::RED));
    /// # });
    /// ```
    #[inline]
    pub fn label(&mut self, text: impl Into<WidgetText>) -> Response {
        Label::new(text).ui(self)
    }

    /// Show colored text.
    ///
    /// Shortcut for `ui.label(RichText::new(text).color(color))`
    pub fn colored_label(
        &mut self,
        color: impl Into<Color32>,
        text: impl Into<RichText>,
    ) -> Response {
        Label::new(text.into().color(color)).ui(self)
    }

    /// Show large text.
    ///
    /// Shortcut for `ui.label(RichText::new(text).heading())`
    pub fn heading(&mut self, text: impl Into<RichText>) -> Response {
        Label::new(text.into().heading()).ui(self)
    }

    /// Show monospace (fixed width) text.
    ///
    /// Shortcut for `ui.label(RichText::new(text).monospace())`
    pub fn monospace(&mut self, text: impl Into<RichText>) -> Response {
        Label::new(text.into().monospace()).ui(self)
    }

    /// Show text as monospace with a gray background.
    ///
    /// Shortcut for `ui.label(RichText::new(text).code())`
    pub fn code(&mut self, text: impl Into<RichText>) -> Response {
        Label::new(text.into().code()).ui(self)
    }

    /// Show small text.
    ///
    /// Shortcut for `ui.label(RichText::new(text).small())`
    pub fn small(&mut self, text: impl Into<RichText>) -> Response {
        Label::new(text.into().small()).ui(self)
    }

    /// Show text that stand out a bit (e.g. slightly brighter).
    ///
    /// Shortcut for `ui.label(RichText::new(text).strong())`
    pub fn strong(&mut self, text: impl Into<RichText>) -> Response {
        Label::new(text.into().strong()).ui(self)
    }

    /// Show text that is weaker (fainter color).
    ///
    /// Shortcut for `ui.label(RichText::new(text).weak())`
    pub fn weak(&mut self, text: impl Into<RichText>) -> Response {
        Label::new(text.into().weak()).ui(self)
    }

    /// Looks like a hyperlink.
    ///
    /// Shortcut for `add(Link::new(text))`.
    ///
    /// ```
    /// # egui::__run_test_ui(|ui| {
    /// if ui.link("Documentation").clicked() {
    ///     // …
    /// }
    /// # });
    /// ```
    ///
    /// See also [`Link`].
    #[must_use = "You should check if the user clicked this with `if ui.link(…).clicked() { … } "]
    pub fn link(&mut self, text: impl Into<WidgetText>) -> Response {
        Link::new(text).ui(self)
    }

    /// Link to a web page.
    ///
    /// Shortcut for `add(Hyperlink::new(url))`.
    ///
    /// ```
    /// # egui::__run_test_ui(|ui| {
    /// ui.hyperlink("https://www.egui.rs/");
    /// # });
    /// ```
    ///
    /// See also [`Hyperlink`].
    pub fn hyperlink(&mut self, url: impl ToString) -> Response {
        Hyperlink::new(url).ui(self)
    }

    /// Shortcut for `add(Hyperlink::from_label_and_url(label, url))`.
    ///
    /// ```
    /// # egui::__run_test_ui(|ui| {
    /// ui.hyperlink_to("egui on GitHub", "https://www.github.com/emilk/egui/");
    /// # });
    /// ```
    ///
    /// See also [`Hyperlink`].
    pub fn hyperlink_to(&mut self, label: impl Into<WidgetText>, url: impl ToString) -> Response {
        Hyperlink::from_label_and_url(label, url).ui(self)
    }

    /// No newlines (`\n`) allowed. Pressing enter key will result in the [`TextEdit`] losing focus (`response.lost_focus`).
    ///
    /// See also [`TextEdit`].
    pub fn text_edit_singleline<S: widgets::text_edit::TextBuffer>(
        &mut self,
        text: &mut S,
    ) -> Response {
        TextEdit::singleline(text).ui(self)
    }

    /// A [`TextEdit`] for multiple lines. Pressing enter key will create a new line.
    ///
    /// See also [`TextEdit`].
    pub fn text_edit_multiline<S: widgets::text_edit::TextBuffer>(
        &mut self,
        text: &mut S,
    ) -> Response {
        TextEdit::multiline(text).ui(self)
    }

    /// A [`TextEdit`] for code editing.
    ///
    /// This will be multiline, monospace, and will insert tabs instead of moving focus.
    ///
    /// See also [`TextEdit::code_editor`].
    pub fn code_editor<S: widgets::text_edit::TextBuffer>(&mut self, text: &mut S) -> Response {
        self.add(TextEdit::multiline(text).code_editor())
    }

    /// Usage: `if ui.button("Click me").clicked() { … }`
    ///
    /// Shortcut for `add(Button::new(text))`
    ///
    /// See also [`Button`].
    ///
    /// ```
    /// # egui::__run_test_ui(|ui| {
    /// if ui.button("Click me!").clicked() {
    ///     // …
    /// }
    ///
    /// # use egui::{RichText, Color32};
    /// if ui.button(RichText::new("delete").color(Color32::RED)).clicked() {
    ///     // …
    /// }
    /// # });
    /// ```
    #[must_use = "You should check if the user clicked this with `if ui.button(…).clicked() { … } "]
    #[inline]
    pub fn button(&mut self, text: impl Into<WidgetText>) -> Response {
        Button::new(text).ui(self)
    }

    /// A button as small as normal body text.
    ///
    /// Usage: `if ui.small_button("Click me").clicked() { … }`
    ///
    /// Shortcut for `add(Button::new(text).small())`
    #[must_use = "You should check if the user clicked this with `if ui.small_button(…).clicked() { … } "]
    pub fn small_button(&mut self, text: impl Into<WidgetText>) -> Response {
        Button::new(text).small().ui(self)
    }

    /// Show a checkbox.
    ///
    /// See also [`Self::toggle_value`].
    #[inline]
    pub fn checkbox(&mut self, checked: &mut bool, text: impl Into<WidgetText>) -> Response {
        Checkbox::new(checked, text).ui(self)
    }

    /// Acts like a checkbox, but looks like a [`SelectableLabel`].
    ///
    /// Click to toggle to bool.
    ///
    /// See also [`Self::checkbox`].
    pub fn toggle_value(&mut self, selected: &mut bool, text: impl Into<WidgetText>) -> Response {
        let mut response = self.selectable_label(*selected, text);
        if response.clicked() {
            *selected = !*selected;
            response.mark_changed();
        }
        response
    }

    /// Show a [`RadioButton`].
    /// Often you want to use [`Self::radio_value`] instead.
    #[must_use = "You should check if the user clicked this with `if ui.radio(…).clicked() { … } "]
    #[inline]
    pub fn radio(&mut self, selected: bool, text: impl Into<WidgetText>) -> Response {
        RadioButton::new(selected, text).ui(self)
    }

    /// Show a [`RadioButton`]. It is selected if `*current_value == selected_value`.
    /// If clicked, `selected_value` is assigned to `*current_value`.
    ///
    /// ```
    /// # egui::__run_test_ui(|ui| {
    ///
    /// #[derive(PartialEq)]
    /// enum Enum { First, Second, Third }
    /// let mut my_enum = Enum::First;
    ///
    /// ui.radio_value(&mut my_enum, Enum::First, "First");
    ///
    /// // is equivalent to:
    ///
    /// if ui.add(egui::RadioButton::new(my_enum == Enum::First, "First")).clicked() {
    ///     my_enum = Enum::First
    /// }
    /// # });
    /// ```
    pub fn radio_value<Value: PartialEq>(
        &mut self,
        current_value: &mut Value,
        alternative: Value,
        text: impl Into<WidgetText>,
    ) -> Response {
        let mut response = self.radio(*current_value == alternative, text);
        if response.clicked() && *current_value != alternative {
            *current_value = alternative;
            response.mark_changed();
        }
        response
    }

    /// Show a label which can be selected or not.
    ///
    /// See also [`SelectableLabel`] and [`Self::toggle_value`].
    #[must_use = "You should check if the user clicked this with `if ui.selectable_label(…).clicked() { … } "]
    pub fn selectable_label(&mut self, checked: bool, text: impl Into<WidgetText>) -> Response {
        SelectableLabel::new(checked, text).ui(self)
    }

    /// Show selectable text. It is selected if `*current_value == selected_value`.
    /// If clicked, `selected_value` is assigned to `*current_value`.
    ///
    /// Example: `ui.selectable_value(&mut my_enum, Enum::Alternative, "Alternative")`.
    ///
    /// See also [`SelectableLabel`] and [`Self::toggle_value`].
    pub fn selectable_value<Value: PartialEq>(
        &mut self,
        current_value: &mut Value,
        selected_value: Value,
        text: impl Into<WidgetText>,
    ) -> Response {
        let mut response = self.selectable_label(*current_value == selected_value, text);
        if response.clicked() && *current_value != selected_value {
            *current_value = selected_value;
            response.mark_changed();
        }
        response
    }

    /// Shortcut for `add(Separator::default())`
    ///
    /// See also [`Separator`].
    #[inline]
    pub fn separator(&mut self) -> Response {
        Separator::default().ui(self)
    }

    /// Shortcut for `add(Spinner::new())`
    ///
    /// See also [`Spinner`].
    #[inline]
    pub fn spinner(&mut self) -> Response {
        Spinner::new().ui(self)
    }

    /// Modify an angle. The given angle should be in radians, but is shown to the user in degrees.
    /// The angle is NOT wrapped, so the user may select, for instance 720° = 2𝞃 = 4π
    pub fn drag_angle(&mut self, radians: &mut f32) -> Response {
        let mut degrees = radians.to_degrees();
        let mut response = self.add(DragValue::new(&mut degrees).speed(1.0).suffix("°"));

        // only touch `*radians` if we actually changed the degree value
        if degrees != radians.to_degrees() {
            *radians = degrees.to_radians();
            response.changed = true;
        }

        response
    }

    /// Modify an angle. The given angle should be in radians,
    /// but is shown to the user in fractions of one Tau (i.e. fractions of one turn).
    /// The angle is NOT wrapped, so the user may select, for instance 2𝞃 (720°)
    pub fn drag_angle_tau(&mut self, radians: &mut f32) -> Response {
        use std::f32::consts::TAU;

        let mut taus = *radians / TAU;
        let mut response = self.add(DragValue::new(&mut taus).speed(0.01).suffix("τ"));

        if self.style().explanation_tooltips {
            response =
                response.on_hover_text("1τ = one turn, 0.5τ = half a turn, etc. 0.25τ = 90°");
        }

        // only touch `*radians` if we actually changed the value
        if taus != *radians / TAU {
            *radians = taus * TAU;
            response.changed = true;
        }

        response
    }

    /// Show an image available at the given `uri`.
    ///
    /// ⚠ This will do nothing unless you install some image loaders first!
    /// The easiest way to do this is via [`egui_extras::install_image_loaders`](https://docs.rs/egui_extras/latest/egui_extras/fn.install_image_loaders.html).
    ///
    /// The loaders handle caching image data, sampled textures, etc. across frames, so calling this is immediate-mode safe.
    ///
    /// ```
    /// # egui::__run_test_ui(|ui| {
    /// ui.image("https://picsum.photos/480");
    /// ui.image("file://assets/ferris.png");
    /// ui.image(egui::include_image!("../assets/ferris.png"));
    /// ui.add(
    ///     egui::Image::new(egui::include_image!("../assets/ferris.png"))
    ///         .max_width(200.0)
    ///         .rounding(10.0),
    /// );
    /// # });
    /// ```
    ///
    /// Using [`crate::include_image`] is often the most ergonomic, and the path
    /// will be resolved at compile-time and embedded in the binary.
    /// When using a "file://" url on the other hand, you need to make sure
    /// the files can be found in the right spot at runtime!
    ///
    /// See also [`crate::Image`], [`crate::ImageSource`].
    #[inline]
    pub fn image<'a>(&mut self, source: impl Into<ImageSource<'a>>) -> Response {
        Image::new(source).ui(self)
    }
}

/// # Colors
impl Ui {
    /// Shows a button with the given color.
    /// If the user clicks the button, a full color picker is shown.
    pub fn color_edit_button_srgba(&mut self, srgba: &mut Color32) -> Response {
        color_picker::color_edit_button_srgba(self, srgba, color_picker::Alpha::BlendOrAdditive)
    }

    /// Shows a button with the given color.
    /// If the user clicks the button, a full color picker is shown.
    pub fn color_edit_button_hsva(&mut self, hsva: &mut Hsva) -> Response {
        color_picker::color_edit_button_hsva(self, hsva, color_picker::Alpha::BlendOrAdditive)
    }

    /// Shows a button with the given color.
    /// If the user clicks the button, a full color picker is shown.
    /// The given color is in `sRGB` space.
    pub fn color_edit_button_srgb(&mut self, srgb: &mut [u8; 3]) -> Response {
        color_picker::color_edit_button_srgb(self, srgb)
    }

    /// Shows a button with the given color.
    /// If the user clicks the button, a full color picker is shown.
    /// The given color is in linear RGB space.
    pub fn color_edit_button_rgb(&mut self, rgb: &mut [f32; 3]) -> Response {
        color_picker::color_edit_button_rgb(self, rgb)
    }

    /// Shows a button with the given color.
    /// If the user clicks the button, a full color picker is shown.
    /// The given color is in `sRGBA` space with premultiplied alpha
    pub fn color_edit_button_srgba_premultiplied(&mut self, srgba: &mut [u8; 4]) -> Response {
        let mut color = Color32::from_rgba_premultiplied(srgba[0], srgba[1], srgba[2], srgba[3]);
        let response = self.color_edit_button_srgba(&mut color);
        *srgba = color.to_array();
        response
    }

    /// Shows a button with the given color.
    /// If the user clicks the button, a full color picker is shown.
    /// The given color is in `sRGBA` space without premultiplied alpha.
    /// If unsure, what "premultiplied alpha" is, then this is probably the function you want to use.
    pub fn color_edit_button_srgba_unmultiplied(&mut self, srgba: &mut [u8; 4]) -> Response {
        let mut rgba = Rgba::from_srgba_unmultiplied(srgba[0], srgba[1], srgba[2], srgba[3]);
        let response =
            color_picker::color_edit_button_rgba(self, &mut rgba, color_picker::Alpha::OnlyBlend);
        *srgba = rgba.to_srgba_unmultiplied();
        response
    }

    /// Shows a button with the given color.
    /// If the user clicks the button, a full color picker is shown.
    /// The given color is in linear RGBA space with premultiplied alpha
    pub fn color_edit_button_rgba_premultiplied(&mut self, rgba_premul: &mut [f32; 4]) -> Response {
        let mut rgba = Rgba::from_rgba_premultiplied(
            rgba_premul[0],
            rgba_premul[1],
            rgba_premul[2],
            rgba_premul[3],
        );
        let response = color_picker::color_edit_button_rgba(
            self,
            &mut rgba,
            color_picker::Alpha::BlendOrAdditive,
        );
        *rgba_premul = rgba.to_array();
        response
    }

    /// Shows a button with the given color.
    /// If the user clicks the button, a full color picker is shown.
    /// The given color is in linear RGBA space without premultiplied alpha.
    /// If unsure, what "premultiplied alpha" is, then this is probably the function you want to use.
    pub fn color_edit_button_rgba_unmultiplied(&mut self, rgba_unmul: &mut [f32; 4]) -> Response {
        let mut rgba = Rgba::from_rgba_unmultiplied(
            rgba_unmul[0],
            rgba_unmul[1],
            rgba_unmul[2],
            rgba_unmul[3],
        );
        let response =
            color_picker::color_edit_button_rgba(self, &mut rgba, color_picker::Alpha::OnlyBlend);
        *rgba_unmul = rgba.to_rgba_unmultiplied();
        response
    }
}

/// # Adding Containers / Sub-uis:
impl Ui {
    /// Put into a [`Frame::group`], visually grouping the contents together
    ///
    /// ```
    /// # egui::__run_test_ui(|ui| {
    /// ui.group(|ui| {
    ///     ui.label("Within a frame");
    /// });
    /// # });
    /// ```
    ///
    /// See also [`Self::scope`].
    pub fn group<R>(&mut self, add_contents: impl FnOnce(&mut Ui) -> R) -> InnerResponse<R> {
        crate::Frame::group(self.style()).show(self, add_contents)
    }

    /// Create a child Ui with an explicit [`Id`].
    ///
    /// ```
    /// # egui::__run_test_ui(|ui| {
    /// for i in 0..10 {
    ///     // ui.collapsing("Same header", |ui| { }); // this will cause an ID clash because of the same title!
    ///
    ///     ui.push_id(i, |ui| {
    ///         ui.collapsing("Same header", |ui| { }); // this is fine!
    ///     });
    /// }
    /// # });
    /// ```
    pub fn push_id<R>(
        &mut self,
        id_salt: impl Hash,
        add_contents: impl FnOnce(&mut Ui) -> R,
    ) -> InnerResponse<R> {
        self.scope_dyn(UiBuilder::new().id_salt(id_salt), Box::new(add_contents))
    }

    /// Push another level onto the [`UiStack`].
    ///
    /// You can use this, for instance, to tag a group of widgets.
    #[deprecated = "Use 'ui.scope_builder' instead"]
    pub fn push_stack_info<R>(
        &mut self,
        ui_stack_info: UiStackInfo,
        add_contents: impl FnOnce(&mut Ui) -> R,
    ) -> InnerResponse<R> {
        self.scope_dyn(
            UiBuilder::new().ui_stack_info(ui_stack_info),
            Box::new(add_contents),
        )
    }

    /// Create a scoped child ui.
    ///
    /// You can use this to temporarily change the [`Style`] of a sub-region, for instance:
    ///
    /// ```
    /// # egui::__run_test_ui(|ui| {
    /// ui.scope(|ui| {
    ///     ui.spacing_mut().slider_width = 200.0; // Temporary change
    ///     // …
    /// });
    /// # });
    /// ```
    pub fn scope<R>(&mut self, add_contents: impl FnOnce(&mut Ui) -> R) -> InnerResponse<R> {
        self.scope_dyn(UiBuilder::new(), Box::new(add_contents))
    }

    /// Create a child, add content to it, and then allocate only what was used in the parent `Ui`.
    pub fn scope_builder<R>(
        &mut self,
        ui_builder: UiBuilder,
        add_contents: impl FnOnce(&mut Ui) -> R,
    ) -> InnerResponse<R> {
        self.scope_dyn(ui_builder, Box::new(add_contents))
    }

    /// Create a child, add content to it, and then allocate only what was used in the parent `Ui`.
    pub fn scope_dyn<'c, R>(
        &mut self,
        ui_builder: UiBuilder,
        add_contents: Box<dyn FnOnce(&mut Ui) -> R + 'c>,
    ) -> InnerResponse<R> {
        let next_auto_id_salt = self.next_auto_id_salt;
        let mut child_ui = self.new_child(ui_builder);
        self.next_auto_id_salt = next_auto_id_salt; // HACK: we want `scope` to only increment this once, so that `ui.scope` is equivalent to `ui.allocate_space`.
        let ret = add_contents(&mut child_ui);
        let response = child_ui.remember_min_rect();
        self.advance_cursor_after_rect(child_ui.min_rect());
        InnerResponse::new(ret, response)
    }

    /// Redirect shapes to another paint layer.
    ///
    /// ```
    /// # use egui::{LayerId, Order, Id};
    /// # egui::__run_test_ui(|ui| {
    /// let layer_id = LayerId::new(Order::Tooltip, Id::new("my_floating_ui"));
    /// ui.with_layer_id(layer_id, |ui| {
    ///     ui.label("This is now in a different layer");
    /// });
    /// # });
    /// ```
    #[deprecated = "Use ui.scope_builder(UiBuilder::new().layer_id(…), …) instead"]
    pub fn with_layer_id<R>(
        &mut self,
        layer_id: LayerId,
        add_contents: impl FnOnce(&mut Self) -> R,
    ) -> InnerResponse<R> {
        self.scope_builder(UiBuilder::new().layer_id(layer_id), add_contents)
    }

    /// A [`CollapsingHeader`] that starts out collapsed.
    ///
    /// The name must be unique within the current parent,
    /// or you need to use [`CollapsingHeader::id_salt`].
    pub fn collapsing<R>(
        &mut self,
        heading: impl Into<WidgetText>,
        add_contents: impl FnOnce(&mut Ui) -> R,
    ) -> CollapsingResponse<R> {
        CollapsingHeader::new(heading).show(self, add_contents)
    }

    /// Create a child ui which is indented to the right.
    ///
    /// The `id_salt` here be anything at all.
    // TODO(emilk): remove `id_salt` argument?
    #[inline]
    pub fn indent<R>(
        &mut self,
        id_salt: impl Hash,
        add_contents: impl FnOnce(&mut Ui) -> R,
    ) -> InnerResponse<R> {
        self.indent_dyn(id_salt, Box::new(add_contents))
    }

    fn indent_dyn<'c, R>(
        &mut self,
        id_salt: impl Hash,
        add_contents: Box<dyn FnOnce(&mut Ui) -> R + 'c>,
    ) -> InnerResponse<R> {
        assert!(
            self.layout().is_vertical(),
            "You can only indent vertical layouts, found {:?}",
            self.layout()
        );

        let indent = self.spacing().indent;
        let mut child_rect = self.placer.available_rect_before_wrap();
        child_rect.min.x += indent;

        let mut child_ui = self.new_child(UiBuilder::new().id_salt(id_salt).max_rect(child_rect));
        let ret = add_contents(&mut child_ui);

        let left_vline = self.visuals().indent_has_left_vline;
        let end_with_horizontal_line = self.spacing().indent_ends_with_horizontal_line;

        if left_vline || end_with_horizontal_line {
            if end_with_horizontal_line {
                child_ui.add_space(4.0);
            }

            let stroke = self.visuals().widgets.noninteractive.bg_stroke;
            let left_top = child_rect.min - 0.5 * indent * Vec2::X;
            let left_bottom = pos2(left_top.x, child_ui.min_rect().bottom() - 2.0);

            if left_vline {
                // draw a faint line on the left to mark the indented section
                self.painter.line_segment([left_top, left_bottom], stroke);
            }

            if end_with_horizontal_line {
                let fudge = 2.0; // looks nicer with button rounding in collapsing headers
                let right_bottom = pos2(child_ui.min_rect().right() - fudge, left_bottom.y);
                self.painter
                    .line_segment([left_bottom, right_bottom], stroke);
            }
        }

        let response = self.allocate_rect(child_ui.min_rect(), Sense::hover());
        InnerResponse::new(ret, response)
    }

    /// Start a ui with horizontal layout.
    /// After you have called this, the function registers the contents as any other widget.
    ///
    /// Elements will be centered on the Y axis, i.e.
    /// adjusted up and down to lie in the center of the horizontal layout.
    /// The initial height is `style.spacing.interact_size.y`.
    /// Centering is almost always what you want if you are
    /// planning to mix widgets or use different types of text.
    ///
    /// If you don't want the contents to be centered, use [`Self::horizontal_top`] instead.
    ///
    /// The returned [`Response`] will only have checked for mouse hover
    /// but can be used for tooltips (`on_hover_text`).
    /// It also contains the [`Rect`] used by the horizontal layout.
    ///
    /// ```
    /// # egui::__run_test_ui(|ui| {
    /// ui.horizontal(|ui| {
    ///     ui.label("Same");
    ///     ui.label("row");
    /// });
    /// # });
    /// ```
    ///
    /// See also [`Self::with_layout`] for more options.
    #[inline]
    pub fn horizontal<R>(&mut self, add_contents: impl FnOnce(&mut Ui) -> R) -> InnerResponse<R> {
        self.horizontal_with_main_wrap_dyn(false, Box::new(add_contents))
    }

    /// Like [`Self::horizontal`], but allocates the full vertical height and then centers elements vertically.
    pub fn horizontal_centered<R>(
        &mut self,
        add_contents: impl FnOnce(&mut Ui) -> R,
    ) -> InnerResponse<R> {
        let initial_size = self.available_size_before_wrap();
        let layout = if self.placer.prefer_right_to_left() {
            Layout::right_to_left(Align::Center)
        } else {
            Layout::left_to_right(Align::Center)
        }
        .with_cross_align(Align::Center);
        self.allocate_ui_with_layout_dyn(initial_size, layout, Box::new(add_contents))
    }

    /// Like [`Self::horizontal`], but aligns content with top.
    pub fn horizontal_top<R>(
        &mut self,
        add_contents: impl FnOnce(&mut Ui) -> R,
    ) -> InnerResponse<R> {
        let initial_size = self.available_size_before_wrap();
        let layout = if self.placer.prefer_right_to_left() {
            Layout::right_to_left(Align::Center)
        } else {
            Layout::left_to_right(Align::Center)
        }
        .with_cross_align(Align::Min);
        self.allocate_ui_with_layout_dyn(initial_size, layout, Box::new(add_contents))
    }

    /// Start a ui with horizontal layout that wraps to a new row
    /// when it reaches the right edge of the `max_size`.
    /// After you have called this, the function registers the contents as any other widget.
    ///
    /// Elements will be centered on the Y axis, i.e.
    /// adjusted up and down to lie in the center of the horizontal layout.
    /// The initial height is `style.spacing.interact_size.y`.
    /// Centering is almost always what you want if you are
    /// planning to mix widgets or use different types of text.
    ///
    /// The returned [`Response`] will only have checked for mouse hover
    /// but can be used for tooltips (`on_hover_text`).
    /// It also contains the [`Rect`] used by the horizontal layout.
    ///
    /// See also [`Self::with_layout`] for more options.
    pub fn horizontal_wrapped<R>(
        &mut self,
        add_contents: impl FnOnce(&mut Ui) -> R,
    ) -> InnerResponse<R> {
        self.horizontal_with_main_wrap_dyn(true, Box::new(add_contents))
    }

    fn horizontal_with_main_wrap_dyn<'c, R>(
        &mut self,
        main_wrap: bool,
        add_contents: Box<dyn FnOnce(&mut Ui) -> R + 'c>,
    ) -> InnerResponse<R> {
        let initial_size = vec2(
            self.available_size_before_wrap().x,
            self.spacing().interact_size.y, // Assume there will be something interactive on the horizontal layout
        );

        let layout = if self.placer.prefer_right_to_left() {
            Layout::right_to_left(Align::Center)
        } else {
            Layout::left_to_right(Align::Center)
        }
        .with_main_wrap(main_wrap);

        self.allocate_ui_with_layout_dyn(initial_size, layout, add_contents)
    }

    /// Start a ui with vertical layout.
    /// Widgets will be left-justified.
    ///
    /// ```
    /// # egui::__run_test_ui(|ui| {
    /// ui.vertical(|ui| {
    ///     ui.label("over");
    ///     ui.label("under");
    /// });
    /// # });
    /// ```
    ///
    /// See also [`Self::with_layout`] for more options.
    #[inline]
    pub fn vertical<R>(&mut self, add_contents: impl FnOnce(&mut Ui) -> R) -> InnerResponse<R> {
        self.allocate_new_ui(
            UiBuilder::new().layout(Layout::top_down(Align::Min)),
            add_contents,
        )
    }

    /// Start a ui with vertical layout.
    /// Widgets will be horizontally centered.
    ///
    /// ```
    /// # egui::__run_test_ui(|ui| {
    /// ui.vertical_centered(|ui| {
    ///     ui.label("over");
    ///     ui.label("under");
    /// });
    /// # });
    /// ```
    #[inline]
    pub fn vertical_centered<R>(
        &mut self,
        add_contents: impl FnOnce(&mut Ui) -> R,
    ) -> InnerResponse<R> {
        self.allocate_new_ui(
            UiBuilder::new().layout(Layout::top_down(Align::Center)),
            add_contents,
        )
    }

    /// Start a ui with vertical layout.
    /// Widgets will be horizontally centered and justified (fill full width).
    ///
    /// ```
    /// # egui::__run_test_ui(|ui| {
    /// ui.vertical_centered_justified(|ui| {
    ///     ui.label("over");
    ///     ui.label("under");
    /// });
    /// # });
    /// ```
    pub fn vertical_centered_justified<R>(
        &mut self,
        add_contents: impl FnOnce(&mut Ui) -> R,
    ) -> InnerResponse<R> {
        self.allocate_new_ui(
            UiBuilder::new().layout(Layout::top_down(Align::Center).with_cross_justify(true)),
            add_contents,
        )
    }

    /// The new layout will take up all available space.
    ///
    /// ```
    /// # egui::__run_test_ui(|ui| {
    /// ui.with_layout(egui::Layout::right_to_left(egui::Align::TOP), |ui| {
    ///     ui.label("world!");
    ///     ui.label("Hello");
    /// });
    /// # });
    /// ```
    ///
    /// If you don't want to use up all available space, use [`Self::allocate_ui_with_layout`].
    ///
    /// See also the helpers [`Self::horizontal`], [`Self::vertical`], etc.
    #[inline]
    pub fn with_layout<R>(
        &mut self,
        layout: Layout,
        add_contents: impl FnOnce(&mut Self) -> R,
    ) -> InnerResponse<R> {
        self.allocate_new_ui(UiBuilder::new().layout(layout), add_contents)
    }

    /// This will make the next added widget centered and justified in the available space.
    ///
    /// Only one widget may be added to the inner `Ui`!
    pub fn centered_and_justified<R>(
        &mut self,
        add_contents: impl FnOnce(&mut Self) -> R,
    ) -> InnerResponse<R> {
        self.allocate_new_ui(
            UiBuilder::new().layout(Layout::centered_and_justified(Direction::TopDown)),
            add_contents,
        )
    }

    pub(crate) fn set_grid(&mut self, grid: grid::GridLayout) {
        self.placer.set_grid(grid);
    }

    pub(crate) fn save_grid(&mut self) {
        self.placer.save_grid();
    }

    pub(crate) fn is_grid(&self) -> bool {
        self.placer.is_grid()
    }

    /// Move to the next row in a grid layout or wrapping layout.
    /// Otherwise does nothing.
    pub fn end_row(&mut self) {
        self.placer
            .end_row(self.spacing().item_spacing, &self.painter().clone());
    }

    /// Set row height in horizontal wrapping layout.
    pub fn set_row_height(&mut self, height: f32) {
        self.placer.set_row_height(height);
    }

    /// Temporarily split a [`Ui`] into several columns.
    ///
    /// ```
    /// # egui::__run_test_ui(|ui| {
    /// ui.columns(2, |columns| {
    ///     columns[0].label("First column");
    ///     columns[1].label("Second column");
    /// });
    /// # });
    /// ```
    #[inline]
    pub fn columns<R>(
        &mut self,
        num_columns: usize,
        add_contents: impl FnOnce(&mut [Self]) -> R,
    ) -> R {
        self.columns_dyn(num_columns, Box::new(add_contents))
    }

    fn columns_dyn<'c, R>(
        &mut self,
        num_columns: usize,
        add_contents: Box<dyn FnOnce(&mut [Self]) -> R + 'c>,
    ) -> R {
        // TODO(emilk): ensure there is space
        let spacing = self.spacing().item_spacing.x;
        let total_spacing = spacing * (num_columns as f32 - 1.0);
        let column_width = (self.available_width() - total_spacing) / (num_columns as f32);
        let top_left = self.cursor().min;

        let mut columns: Vec<Self> = (0..num_columns)
            .map(|col_idx| {
                let pos = top_left + vec2((col_idx as f32) * (column_width + spacing), 0.0);
                let child_rect = Rect::from_min_max(
                    pos,
                    pos2(pos.x + column_width, self.max_rect().right_bottom().y),
                );
                let mut column_ui = self.new_child(
                    UiBuilder::new()
                        .max_rect(child_rect)
                        .layout(Layout::top_down_justified(Align::LEFT)),
                );
                column_ui.set_width(column_width);
                column_ui
            })
            .collect();

        let result = add_contents(&mut columns[..]);

        let mut max_column_width = column_width;
        let mut max_height = 0.0;
        for column in &columns {
            max_column_width = max_column_width.max(column.min_rect().width());
            max_height = column.min_size().y.max(max_height);
        }

        // Make sure we fit everything next frame:
        let total_required_width = total_spacing + max_column_width * (num_columns as f32);

        let size = vec2(self.available_width().max(total_required_width), max_height);
        self.advance_cursor_after_rect(Rect::from_min_size(top_left, size));
        result
    }

    /// Temporarily split a [`Ui`] into several columns.
    ///
    /// The same as [`Self::columns()`], but uses a constant for the column count.
    /// This allows for compile-time bounds checking, and makes the compiler happy.
    ///
    /// ```
    /// # egui::__run_test_ui(|ui| {
    /// ui.columns_const(|[col_1, col_2]| {
    ///     col_1.label("First column");
    ///     col_2.label("Second column");
    /// });
    /// # });
    /// ```
    #[inline]
    pub fn columns_const<const NUM_COL: usize, R>(
        &mut self,
        add_contents: impl FnOnce(&mut [Self; NUM_COL]) -> R,
    ) -> R {
        // TODO(emilk): ensure there is space
        let spacing = self.spacing().item_spacing.x;
        let total_spacing = spacing * (NUM_COL as f32 - 1.0);
        let column_width = (self.available_width() - total_spacing) / (NUM_COL as f32);
        let top_left = self.cursor().min;

        let mut columns = std::array::from_fn(|col_idx| {
            let pos = top_left + vec2((col_idx as f32) * (column_width + spacing), 0.0);
            let child_rect = Rect::from_min_max(
                pos,
                pos2(pos.x + column_width, self.max_rect().right_bottom().y),
            );
            let mut column_ui = self.new_child(
                UiBuilder::new()
                    .max_rect(child_rect)
                    .layout(Layout::top_down_justified(Align::LEFT)),
            );
            column_ui.set_width(column_width);
            column_ui
        });
        let result = add_contents(&mut columns);

        let mut max_column_width = column_width;
        let mut max_height = 0.0;
        for column in &columns {
            max_column_width = max_column_width.max(column.min_rect().width());
            max_height = column.min_size().y.max(max_height);
        }

        // Make sure we fit everything next frame:
        let total_required_width = total_spacing + max_column_width * (NUM_COL as f32);

        let size = vec2(self.available_width().max(total_required_width), max_height);
        self.advance_cursor_after_rect(Rect::from_min_size(top_left, size));
        result
    }

    /// Create something that can be drag-and-dropped.
    ///
    /// The `id` needs to be globally unique.
    /// The payload is what will be dropped if the user starts dragging.
    ///
    /// In contrast to [`Response::dnd_set_drag_payload`],
    /// this function will paint the widget at the mouse cursor while the user is dragging.
    #[doc(alias = "drag and drop")]
    pub fn dnd_drag_source<Payload, R>(
        &mut self,
        id: Id,
        payload: Payload,
        add_contents: impl FnOnce(&mut Self) -> R,
    ) -> InnerResponse<R>
    where
        Payload: Any + Send + Sync,
    {
        let is_being_dragged = self.ctx().is_being_dragged(id);

        if is_being_dragged {
            crate::DragAndDrop::set_payload(self.ctx(), payload);

            // Paint the body to a new layer:
            let layer_id = LayerId::new(Order::Tooltip, id);
            let InnerResponse { inner, response } =
                self.scope_builder(UiBuilder::new().layer_id(layer_id), add_contents);

            // Now we move the visuals of the body to where the mouse is.
            // Normally you need to decide a location for a widget first,
            // because otherwise that widget cannot interact with the mouse.
            // However, a dragged component cannot be interacted with anyway
            // (anything with `Order::Tooltip` always gets an empty [`Response`])
            // So this is fine!

            if let Some(pointer_pos) = self.ctx().pointer_interact_pos() {
                let delta = pointer_pos - response.rect.center();
                self.ctx()
                    .transform_layer_shapes(layer_id, emath::TSTransform::from_translation(delta));
            }

            InnerResponse::new(inner, response)
        } else {
            let InnerResponse { inner, response } = self.scope(add_contents);

            // Check for drags:
            let dnd_response = self
                .interact(response.rect, id, Sense::drag())
                .on_hover_cursor(CursorIcon::Grab);

            InnerResponse::new(inner, dnd_response | response)
        }
    }

    /// Surround the given ui with a frame which
    /// changes colors when you can drop something onto it.
    ///
    /// Returns the dropped item, if it was released this frame.
    ///
    /// The given frame is used for its margins, but it color is ignored.
    #[doc(alias = "drag and drop")]
    pub fn dnd_drop_zone<Payload, R>(
        &mut self,
        frame: Frame,
        add_contents: impl FnOnce(&mut Ui) -> R,
    ) -> (InnerResponse<R>, Option<Arc<Payload>>)
    where
        Payload: Any + Send + Sync,
    {
        let is_anything_being_dragged = DragAndDrop::has_any_payload(self.ctx());
        let can_accept_what_is_being_dragged =
            DragAndDrop::has_payload_of_type::<Payload>(self.ctx());

        let mut frame = frame.begin(self);
        let inner = add_contents(&mut frame.content_ui);
        let response = frame.allocate_space(self);

        // NOTE: we use `response.contains_pointer` here instead of `hovered`, because
        // `hovered` is always false when another widget is being dragged.
        let style = if is_anything_being_dragged
            && can_accept_what_is_being_dragged
            && response.contains_pointer()
        {
            self.visuals().widgets.active
        } else {
            self.visuals().widgets.inactive
        };

        let mut fill = style.bg_fill;
        let mut stroke = style.bg_stroke;

        if is_anything_being_dragged && !can_accept_what_is_being_dragged {
            // When dragging something else, show that it can't be dropped here:
            fill = self.visuals().gray_out(fill);
            stroke.color = self.visuals().gray_out(stroke.color);
        }

        frame.frame.fill = fill;
        frame.frame.stroke = stroke;

        frame.paint(self);

        let payload = response.dnd_release_payload::<Payload>();

        (InnerResponse { inner, response }, payload)
    }

    /// Create a new Scope and transform its contents via a [`emath::TSTransform`].
    /// This only affects visuals, inputs will not be transformed. So this is mostly useful
    /// to create visual effects on interactions, e.g. scaling a button on hover / click.
    ///
    /// Check out [`Context::set_transform_layer`] for a persistent transform that also affects
    /// inputs.
    pub fn with_visual_transform<R>(
        &mut self,
        transform: emath::TSTransform,
        add_contents: impl FnOnce(&mut Self) -> R,
    ) -> InnerResponse<R> {
        let start_idx = self.ctx().graphics(|gx| {
            gx.get(self.layer_id())
                .map_or(crate::layers::ShapeIdx(0), |l| l.next_idx())
        });

        let r = self.scope_dyn(UiBuilder::new(), Box::new(add_contents));

        self.ctx().graphics_mut(|g| {
            let list = g.entry(self.layer_id());
            let end_idx = list.next_idx();
            list.transform_range(start_idx, end_idx, transform);
        });

        r
    }
}

/// # Menus
impl Ui {
    /// Close the menu we are in (including submenus), if any.
    ///
    /// See also: [`Self::menu_button`] and [`Response::context_menu`].
    pub fn close_menu(&mut self) {
        if let Some(menu_state) = &mut self.menu_state {
            menu_state.write().close();
        }
        self.menu_state = None;
    }

    pub(crate) fn set_menu_state(&mut self, menu_state: Option<Arc<RwLock<MenuState>>>) {
        self.menu_state = menu_state;
    }

    #[inline]
    /// Create a menu button that when clicked will show the given menu.
    ///
    /// If called from within a menu this will instead create a button for a sub-menu.
    ///
    /// ```
    /// # egui::__run_test_ui(|ui| {
    /// ui.menu_button("My menu", |ui| {
    ///     ui.menu_button("My sub-menu", |ui| {
    ///         if ui.button("Close the menu").clicked() {
    ///             ui.close_menu();
    ///         }
    ///     });
    /// });
    /// # });
    /// ```
    ///
    /// See also: [`Self::close_menu`] and [`Response::context_menu`].
    pub fn menu_button<R>(
        &mut self,
        title: impl Into<WidgetText>,
        add_contents: impl FnOnce(&mut Ui) -> R,
    ) -> InnerResponse<Option<R>> {
        if let Some(menu_state) = self.menu_state.clone() {
            menu::submenu_button(self, menu_state, title, add_contents)
        } else {
            menu::menu_button(self, title, add_contents)
        }
    }

    /// Create a menu button with an image that when clicked will show the given menu.
    ///
    /// If called from within a menu this will instead create a button for a sub-menu.
    ///
    /// ```ignore
    /// # egui::__run_test_ui(|ui| {
    /// let img = egui::include_image!("../assets/ferris.png");
    ///
    /// ui.menu_image_button(title, img, |ui| {
    ///     ui.menu_button("My sub-menu", |ui| {
    ///         if ui.button("Close the menu").clicked() {
    ///             ui.close_menu();
    ///         }
    ///     });
    /// });
    /// # });
    /// ```
    ///
    ///
    /// See also: [`Self::close_menu`] and [`Response::context_menu`].
    #[inline]
    pub fn menu_image_button<'a, R>(
        &mut self,
        image: impl Into<Image<'a>>,
        add_contents: impl FnOnce(&mut Ui) -> R,
    ) -> InnerResponse<Option<R>> {
        if let Some(menu_state) = self.menu_state.clone() {
            menu::submenu_button(self, menu_state, String::new(), add_contents)
        } else {
            menu::menu_custom_button(self, Button::image(image), add_contents)
        }
    }

    /// Create a menu button with an image and a text that when clicked will show the given menu.
    ///
    /// If called from within a menu this will instead create a button for a sub-menu.
    ///
    /// ```
    /// # egui::__run_test_ui(|ui| {
    /// let img = egui::include_image!("../assets/ferris.png");
    /// let title = "My Menu";
    ///
    /// ui.menu_image_text_button(img, title, |ui| {
    ///     ui.menu_button("My sub-menu", |ui| {
    ///         if ui.button("Close the menu").clicked() {
    ///             ui.close_menu();
    ///         }
    ///     });
    /// });
    /// # });
    /// ```
    ///
    /// See also: [`Self::close_menu`] and [`Response::context_menu`].
    #[inline]
    pub fn menu_image_text_button<'a, R>(
        &mut self,
        image: impl Into<Image<'a>>,
        title: impl Into<WidgetText>,
        add_contents: impl FnOnce(&mut Ui) -> R,
    ) -> InnerResponse<Option<R>> {
        if let Some(menu_state) = self.menu_state.clone() {
            menu::submenu_button(self, menu_state, title, add_contents)
        } else {
            menu::menu_custom_button(self, Button::image_and_text(image, title), add_contents)
        }
    }
}

// ----------------------------------------------------------------------------

/// # Debug stuff
impl Ui {
    /// Shows where the next widget is going to be placed
    #[cfg(debug_assertions)]
    pub fn debug_paint_cursor(&self) {
        self.placer.debug_paint_cursor(&self.painter, "next");
    }
}

impl Drop for Ui {
    fn drop(&mut self) {
        if !self.min_rect_already_remembered {
            // Register our final `min_rect`
            self.remember_min_rect();
        }
        #[cfg(debug_assertions)]
        register_rect(self, self.min_rect());
    }
}

/// Show this rectangle to the user if certain debug options are set.
#[cfg(debug_assertions)]
fn register_rect(ui: &Ui, rect: Rect) {
    use emath::{Align2, GuiRounding};

    let debug = ui.style().debug;

    if debug.show_unaligned {
        let unaligned_line = |p0: Pos2, p1: Pos2| {
            let color = Color32::ORANGE;
            let font_id = TextStyle::Monospace.resolve(ui.style());
            ui.painter().line_segment([p0, p1], (1.0, color));
            ui.painter()
                .text(p0, Align2::LEFT_TOP, "Unaligned", font_id, color);
        };

        if rect.left() != rect.left().round_ui() {
            unaligned_line(rect.left_top(), rect.left_bottom());
        }
        if rect.right() != rect.right().round_ui() {
            unaligned_line(rect.right_top(), rect.right_bottom());
        }
        if rect.top() != rect.top().round_ui() {
            unaligned_line(rect.left_top(), rect.right_top());
        }
        if rect.bottom() != rect.bottom().round_ui() {
            unaligned_line(rect.left_bottom(), rect.right_bottom());
        }
    }

    let show_callstacks = debug.debug_on_hover
        || debug.debug_on_hover_with_all_modifiers && ui.input(|i| i.modifiers.all());

    if !show_callstacks {
        return;
    }

    if !ui.rect_contains_pointer(rect) {
        return;
    }

    let is_clicking = ui.input(|i| i.pointer.could_any_button_be_click());

    #[cfg(feature = "callstack")]
    let callstack = crate::callstack::capture();

    #[cfg(not(feature = "callstack"))]
    let callstack = String::default();

    // We only show one debug rectangle, or things get confusing:
    let debug_rect = pass_state::DebugRect {
        rect,
        callstack,
        is_clicking,
    };

    let mut kept = false;
    ui.ctx().pass_state_mut(|fs| {
        if let Some(final_debug_rect) = &mut fs.debug_rect {
            // or maybe pick the one with deepest callstack?
            if final_debug_rect.rect.contains_rect(rect) {
                *final_debug_rect = debug_rect;
                kept = true;
            }
        } else {
            fs.debug_rect = Some(debug_rect);
            kept = true;
        }
    });
    if !kept {
        return;
    }

    // ----------------------------------------------

    // Use the debug-painter to avoid clip rect,
    // otherwise the content of the widget may cover what we paint here!
    let painter = ui.ctx().debug_painter();

    if debug.hover_shows_next {
        ui.placer.debug_paint_cursor(&painter, "next");
    }
}

#[cfg(not(debug_assertions))]
fn register_rect(_ui: &Ui, _rect: Rect) {}

#[test]
fn ui_impl_send_sync() {
    fn assert_send_sync<T: Send + Sync>() {}
    assert_send_sync::<Ui>();
}