Add undo/redo demo. (#4811)

* Closes https://github.com/emilk/egui/issues/1713 

I almost went to implement my own undo/redo system, and then found the
egui undoer.
Went to make a small demo to test for myself how it worked, and then
found the linked issue.
So here is a tweaked version of that :)

Co-authored-by: Wybe Westra <w.westra@kwantcontrols.nl>

crates/egui_demo_lib/src/demo/demo_app_windows.rs

@@ -44,6 +44,7 @@ impl Default for Demos {
             Box::<super::text_edit::TextEditDemo>::default(),
             Box::<super::text_layout::TextLayoutDemo>::default(),
             Box::<super::tooltips::Tooltips>::default(),
+            Box::<super::undo_redo::UndoRedoDemo>::default(),
             Box::<super::widget_gallery::WidgetGallery>::default(),
             Box::<super::window_options::WindowOptions>::default(),
         ])

crates/egui_demo_lib/src/demo/mod.rs

@@ -32,6 +32,7 @@ pub mod text_edit;
 pub mod text_layout;
 pub mod toggle_switch;
 pub mod tooltips;
+pub mod undo_redo;
 pub mod widget_gallery;
 pub mod window_options;
 

crates/egui_demo_lib/src/demo/undo_redo.rs

@@ -0,0 +1,78 @@
+use egui::{util::undoer::Undoer, Button};
+
+#[derive(Debug, PartialEq, Eq, Clone)]
+#[cfg_attr(feature = "serde", derive(serde::Deserialize, serde::Serialize))]
+#[cfg_attr(feature = "serde", serde(default))]
+pub struct State {
+    pub toggle_value: bool,
+    pub text: String,
+}
+
+impl Default for State {
+    fn default() -> Self {
+        Self {
+            toggle_value: Default::default(),
+            text: "Text with undo/redo".to_owned(),
+        }
+    }
+}
+
+/// Showcase [`egui::util::undoer::Undoer`]
+#[derive(Debug, Default)]
+#[cfg_attr(feature = "serde", derive(serde::Deserialize, serde::Serialize))]
+#[cfg_attr(feature = "serde", serde(default))]
+pub struct UndoRedoDemo {
+    pub state: State,
+    pub undoer: Undoer<State>,
+}
+
+impl crate::Demo for UndoRedoDemo {
+    fn name(&self) -> &'static str {
+        "⟲ Undo Redo"
+    }
+
+    fn show(&mut self, ctx: &egui::Context, open: &mut bool) {
+        egui::Window::new(self.name())
+            .open(open)
+            .resizable(false)
+            .show(ctx, |ui| {
+                use crate::View as _;
+                self.ui(ui);
+            });
+    }
+}
+
+impl crate::View for UndoRedoDemo {
+    fn ui(&mut self, ui: &mut egui::Ui) {
+        ui.vertical_centered(|ui| {
+            ui.add(crate::egui_github_link_file!());
+        });
+
+        ui.checkbox(&mut self.state.toggle_value, "Checkbox with undo/redo");
+        ui.text_edit_singleline(&mut self.state.text);
+
+        ui.separator();
+
+        let can_undo = self.undoer.has_undo(&self.state);
+        let can_redo = self.undoer.has_redo(&self.state);
+
+        ui.horizontal(|ui| {
+            let undo = ui.add_enabled(can_undo, Button::new("⟲ Undo")).clicked();
+            let redo = ui.add_enabled(can_redo, Button::new("⟳ Redo")).clicked();
+
+            if undo {
+                if let Some(undo_text) = self.undoer.undo(&self.state) {
+                    self.state = undo_text.clone();
+                }
+            }
+            if redo {
+                if let Some(redo_text) = self.undoer.redo(&self.state) {
+                    self.state = redo_text.clone();
+                }
+            }
+        });
+
+        self.undoer
+            .feed_state(ui.ctx().input(|input| input.time), &self.state);
+    }
+}