Add `ComboBox::icon()` (#1405)

egui/src/containers/combo_box.rs

@@ -1,6 +1,9 @@
 use crate::{style::WidgetVisuals, *};
 use epaint::Shape;
 
+/// A function that paints the `ComboBox` icon
+pub type IconPainter = Box<dyn FnOnce(&Ui, Rect, &WidgetVisuals, bool)>;
+
 /// A drop-down selection menu with a descriptive label.
 ///
 /// ```
@@ -24,6 +27,7 @@ pub struct ComboBox {
     label: Option<WidgetText>,
     selected_text: WidgetText,
     width: Option<f32>,
+    icon: Option<IconPainter>,
 }
 
 impl ComboBox {
@@ -34,6 +38,7 @@ impl ComboBox {
             label: Some(label.into()),
             selected_text: Default::default(),
             width: None,
+            icon: None,
         }
     }
 
@@ -45,6 +50,7 @@ impl ComboBox {
             label: Some(label),
             selected_text: Default::default(),
             width: None,
+            icon: None,
         }
     }
 
@@ -55,6 +61,7 @@ impl ComboBox {
             label: Default::default(),
             selected_text: Default::default(),
             width: None,
+            icon: None,
         }
     }
 
@@ -70,6 +77,41 @@ impl ComboBox {
         self
     }
 
+    /// Use the provided function to render a different `ComboBox` icon.
+    /// Defaults to a triangle that expands when the cursor is hovering over the `ComboBox`.
+    ///
+    /// For example:
+    /// ```
+    /// # egui::__run_test_ui(|ui| {
+    /// # let text = "Selected text";
+    /// pub fn filled_triangle(
+    ///     ui: &egui::Ui,
+    ///     rect: egui::Rect,
+    ///     visuals: &egui::style::WidgetVisuals,
+    ///     _is_open: bool,
+    /// ) {
+    ///     let rect = egui::Rect::from_center_size(
+    ///         rect.center(),
+    ///         egui::Vec2::new(rect.width() * 0.6, rect.height() * 0.4),
+    ///     );
+    ///     ui.painter().add(egui::Shape::convex_polygon(
+    ///         vec![rect.left_top(), rect.right_top(), rect.center_bottom()],
+    ///         visuals.fg_stroke.color,
+    ///         visuals.fg_stroke,
+    ///     ));
+    /// }
+    ///
+    /// egui::ComboBox::from_id_source("my-combobox")
+    ///     .selected_text(text)
+    ///     .icon(filled_triangle)
+    ///     .show_ui(ui, |_ui| {});
+    /// # });
+    /// ```
+    pub fn icon(mut self, icon_fn: impl FnOnce(&Ui, Rect, &WidgetVisuals, bool) + 'static) -> Self {
+        self.icon = Some(Box::new(icon_fn));
+        self
+    }
+
     /// Show the combo box, with the given ui code for the menu contents.
     ///
     /// Returns `InnerResponse { inner: None }` if the combo box is closed.
@@ -91,6 +133,7 @@ impl ComboBox {
             label,
             selected_text,
             width,
+            icon,
         } = self;
 
         let button_id = ui.make_persistent_id(id_source);
@@ -99,7 +142,7 @@ impl ComboBox {
             if let Some(width) = width {
                 ui.spacing_mut().slider_width = width; // yes, this is ugly. Will remove later.
             }
-            let mut ir = combo_box_dyn(ui, button_id, selected_text, menu_contents);
+            let mut ir = combo_box_dyn(ui, button_id, selected_text, menu_contents, icon);
             if let Some(label) = label {
                 ir.response
                     .widget_info(|| WidgetInfo::labeled(WidgetType::ComboBox, label.text()));
@@ -165,6 +208,7 @@ fn combo_box_dyn<'c, R>(
     button_id: Id,
     selected_text: WidgetText,
     menu_contents: Box<dyn FnOnce(&mut Ui) -> R + 'c>,
+    icon: Option<IconPainter>,
 ) -> InnerResponse<Option<R>> {
     let popup_id = button_id.with("popup");
 
@@ -192,7 +236,17 @@ fn combo_box_dyn<'c, R>(
             } else {
                 ui.style().interact(&response)
             };
-            paint_icon(ui.painter(), icon_rect.expand(visuals.expansion), visuals);
+
+            if let Some(icon) = icon {
+                icon(
+                    ui,
+                    icon_rect.expand(visuals.expansion),
+                    visuals,
+                    is_popup_open,
+                );
+            } else {
+                paint_default_icon(ui.painter(), icon_rect.expand(visuals.expansion), visuals);
+            }
 
             let text_rect = Align2::LEFT_CENTER.align_size_within_rect(galley.size(), rect);
             galley.paint_with_visuals(ui.painter(), text_rect.min, visuals);
@@ -262,7 +316,7 @@ fn button_frame(
     response
 }
 
-fn paint_icon(painter: &Painter, rect: Rect, visuals: &WidgetVisuals) {
+fn paint_default_icon(painter: &Painter, rect: Rect, visuals: &WidgetVisuals) {
     let rect = Rect::from_center_size(
         rect.center(),
         vec2(rect.width() * 0.7, rect.height() * 0.45),