eframe: allow aborting an exit event (#1038)

eframe/CHANGELOG.md

@@ -9,6 +9,7 @@ NOTE: [`egui_web`](egui_web/CHANGELOG.md), [`egui-winit`](egui-winit/CHANGELOG.m
 * The default native backend is now `egui_glow` (instead of `egui_glium`) ([#1020](https://github.com/emilk/egui/pull/1020)).
 * The default web painter is now `egui_glow` (instead of WebGL) ([#1020](https://github.com/emilk/egui/pull/1020)).
 * Fix horizontal scrolling direction on Linux.
+* Added `App::on_exit_event` ([#1038](https://github.com/emilk/egui/pull/1038))
 
 
 ## 0.16.0 - 2021-12-29

eframe/examples/confirm_exit.rs

@@ -0,0 +1,49 @@
+#![cfg_attr(not(debug_assertions), windows_subsystem = "windows")] // hide console window on Windows in release
+
+use eframe::{egui, epi};
+
+#[derive(Default)]
+struct MyApp {
+    can_exit: bool,
+    is_exiting: bool,
+}
+
+impl epi::App for MyApp {
+    fn name(&self) -> &str {
+        "Confirm exit"
+    }
+
+    fn on_exit_event(&mut self) -> bool {
+        self.is_exiting = true;
+        self.can_exit
+    }
+
+    fn update(&mut self, ctx: &egui::Context, frame: &epi::Frame) {
+        egui::CentralPanel::default().show(ctx, |ui| {
+            ui.heading("Try to close the window");
+        });
+
+        if self.is_exiting {
+            egui::Window::new("Do you want to quit?")
+                .collapsible(false)
+                .resizable(false)
+                .show(ctx, |ui| {
+                    ui.horizontal(|ui| {
+                        if ui.button("Yes!").clicked() {
+                            self.can_exit = true;
+                            frame.quit();
+                        }
+
+                        if ui.button("Not yet").clicked() {
+                            self.is_exiting = false;
+                        }
+                    });
+                });
+        }
+    }
+}
+
+fn main() {
+    let options = eframe::NativeOptions::default();
+    eframe::run_native(Box::new(MyApp::default()), options);
+}

egui-winit/src/epi.rs

@@ -240,7 +240,11 @@ impl EpiIntegration {
         self.app
             .setup(&self.egui_ctx, &self.frame, self.persistence.storage());
         let app_output = self.frame.take_app_output();
-        self.quit |= app_output.quit;
+
+        if app_output.quit {
+            self.quit = self.app.on_exit_event();
+        }
+
         crate::epi::handle_app_output(window, self.egui_ctx.pixels_per_point(), app_output);
     }
 
@@ -260,7 +264,12 @@ impl EpiIntegration {
 
     pub fn on_event(&mut self, event: &winit::event::WindowEvent<'_>) {
         use winit::event::WindowEvent;
-        self.quit |= matches!(event, WindowEvent::CloseRequested | WindowEvent::Destroyed);
+        if *event == WindowEvent::CloseRequested {
+            self.quit = self.app.on_exit_event();
+        } else if *event == WindowEvent::Destroyed {
+            self.quit = true;
+        }
+
         self.egui_winit.on_event(&self.egui_ctx, event);
     }
 
@@ -282,7 +291,10 @@ impl EpiIntegration {
             .handle_output(window, &self.egui_ctx, egui_output);
 
         let app_output = self.frame.take_app_output();
-        self.quit |= app_output.quit;
+
+        if app_output.quit {
+            self.quit = self.app.on_exit_event();
+        }
 
         crate::epi::handle_app_output(window, self.egui_ctx.pixels_per_point(), app_output);
 

epi/src/lib.rs

@@ -144,7 +144,20 @@ pub trait App {
     /// where `APPNAME` is what is returned by [`Self::name()`].
     fn save(&mut self, _storage: &mut dyn Storage) {}
 
-    /// Called once on shutdown (before or after [`Self::save`])
+    /// Called before an exit that can be aborted.
+    /// By returning `false` the exit will be aborted. To continue the exit return `true`.
+    ///
+    /// A scenario where this method will be run is after pressing the close button on a native
+    /// window, which allows you to ask the user whether they want to do something before exiting.
+    /// See the example `eframe/examples/confirm_exit.rs` for practical usage.
+    ///
+    /// It will _not_ be called on the web or when the window is forcefully closed.
+    fn on_exit_event(&mut self) -> bool {
+        true
+    }
+
+    /// Called once on shutdown (before or after [`Self::save`]). If you need to abort an exit use
+    /// [`Self::on_exit_event`]
     fn on_exit(&mut self) {}
 
     // ---------