From ab770997818212fd84a6b858f45ae222e55edbe5 Mon Sep 17 00:00:00 2001
From: Erlend Walstad <96946613+lampsitter@users.noreply.github.com>
Date: Mon, 17 Jan 2022 18:45:09 +0100
Subject: [PATCH] eframe: allow aborting an exit event (#1038)

---
 eframe/CHANGELOG.md             |  1 +
 eframe/examples/confirm_exit.rs | 49 +++++++++++++++++++++++++++++++++
 egui-winit/src/epi.rs           | 18 ++++++++++--
 epi/src/lib.rs                  | 15 +++++++++-
 4 files changed, 79 insertions(+), 4 deletions(-)
 create mode 100644 eframe/examples/confirm_exit.rs

diff --git a/eframe/CHANGELOG.md b/eframe/CHANGELOG.md
index b704213e..d236aac5 100644
--- a/eframe/CHANGELOG.md
+++ b/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
diff --git a/eframe/examples/confirm_exit.rs b/eframe/examples/confirm_exit.rs
new file mode 100644
index 00000000..f1e8a359
--- /dev/null
+++ b/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);
+}
diff --git a/egui-winit/src/epi.rs b/egui-winit/src/epi.rs
index 64ce7a9d..869921ab 100644
--- a/egui-winit/src/epi.rs
+++ b/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);
 
diff --git a/epi/src/lib.rs b/epi/src/lib.rs
index 3b46494d..95fce001 100644
--- a/epi/src/lib.rs
+++ b/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) {}
 
     // ---------