Improve "Reset everything" button in demo (#3255)

Closes https://github.com/emilk/egui/issues/3254

crates/egui_demo_app/src/wrap_app.rs

@@ -124,6 +124,15 @@ impl Default for Anchor {
 
 // ----------------------------------------------------------------------------
 
+#[derive(Clone, Copy, Debug)]
+#[must_use]
+enum Command {
+    Nothing,
+    ResetEverything,
+}
+
+// ----------------------------------------------------------------------------
+
 /// The state that we persist (serialize).
 #[derive(Default)]
 #[cfg_attr(feature = "serde", derive(serde::Deserialize, serde::Serialize))]
@@ -240,18 +249,19 @@ impl eframe::App for WrapApp {
             frame.set_fullscreen(!frame.info().window_info.fullscreen);
         }
 
+        let mut cmd = Command::Nothing;
         egui::TopBottomPanel::top("wrap_app_top_bar").show(ctx, |ui| {
             egui::trace!(ui);
             ui.horizontal_wrapped(|ui| {
                 ui.visuals_mut().button_frame = false;
-                self.bar_contents(ui, frame);
+                self.bar_contents(ui, frame, &mut cmd);
             });
         });
 
         self.state.backend_panel.update(ctx, frame);
 
         if !is_mobile(ctx) {
-            self.backend_panel(ctx, frame);
+            cmd = self.backend_panel(ctx, frame);
         }
 
         self.show_selected_app(ctx, frame);
@@ -264,6 +274,8 @@ impl eframe::App for WrapApp {
         if !frame.is_web() {
             egui::gui_zoom::zoom_with_keyboard_shortcuts(ctx, frame.info().native_pixels_per_point);
         }
+
+        self.run_cmd(ctx, cmd);
     }
 
     #[cfg(feature = "glow")]
@@ -280,12 +292,14 @@ impl eframe::App for WrapApp {
 }
 
 impl WrapApp {
-    fn backend_panel(&mut self, ctx: &egui::Context, frame: &mut eframe::Frame) {
+    fn backend_panel(&mut self, ctx: &egui::Context, frame: &mut eframe::Frame) -> Command {
         // The backend-panel can be toggled on/off.
         // We show a little animation when the user switches it.
         let is_open =
             self.state.backend_panel.open || ctx.memory(|mem| mem.everything_is_visible());
 
+        let mut cmd = Command::Nothing;
+
         egui::SidePanel::left("backend_panel")
             .resizable(false)
             .show_animated(ctx, is_open, |ui| {
@@ -294,11 +308,28 @@ impl WrapApp {
                 });
 
                 ui.separator();
-                self.backend_panel_contents(ui, frame);
+                self.backend_panel_contents(ui, frame, &mut cmd);
             });
+
+        cmd
     }
 
-    fn backend_panel_contents(&mut self, ui: &mut egui::Ui, frame: &mut eframe::Frame) {
+    fn run_cmd(&mut self, ctx: &egui::Context, cmd: Command) {
+        match cmd {
+            Command::Nothing => {}
+            Command::ResetEverything => {
+                self.state = Default::default();
+                ctx.memory_mut(|mem| *mem = Default::default());
+            }
+        }
+    }
+
+    fn backend_panel_contents(
+        &mut self,
+        ui: &mut egui::Ui,
+        frame: &mut eframe::Frame,
+        cmd: &mut Command,
+    ) {
         self.state.backend_panel.ui(ui, frame);
 
         ui.separator();
@@ -314,8 +345,7 @@ impl WrapApp {
             }
 
             if ui.button("Reset everything").clicked() {
-                self.state = Default::default();
+                *cmd = Command::ResetEverything;
-                ui.ctx().memory_mut(|mem| *mem = Default::default());
                 ui.close_menu();
             }
         });
@@ -330,7 +360,7 @@ impl WrapApp {
         }
     }
 
-    fn bar_contents(&mut self, ui: &mut egui::Ui, frame: &mut eframe::Frame) {
+    fn bar_contents(&mut self, ui: &mut egui::Ui, frame: &mut eframe::Frame, cmd: &mut Command) {
         egui::widgets::global_dark_light_mode_switch(ui);
 
         ui.separator();
@@ -338,7 +368,7 @@ impl WrapApp {
         if is_mobile(ui.ctx()) {
             ui.menu_button("💻 Backend", |ui| {
                 ui.set_style(ui.ctx().style()); // ignore the "menu" style set by `menu_button`.
-                self.backend_panel_contents(ui, frame);
+                self.backend_panel_contents(ui, frame, cmd);
             });
         } else {
             ui.toggle_value(&mut self.state.backend_panel.open, "💻 Backend");