Save state on suspend on Android and iOS (#5601)

<!--
Please read the "Making a PR" section of
[`CONTRIBUTING.md`](https://github.com/emilk/egui/blob/master/CONTRIBUTING.md)
before opening a Pull Request!

* Keep your PR:s small and focused.
* The PR title is what ends up in the changelog, so make it descriptive!
* If applicable, add a screenshot or gif.
* If it is a non-trivial addition, consider adding a demo for it to
`egui_demo_lib`, or a new example.
* Do NOT open PR:s from your `master` branch, as that makes it hard for
maintainers to test and add commits to your PR.
* Remember to run `cargo fmt` and `cargo clippy`.
* Open the PR as a draft until you have self-reviewed it and run
`./scripts/check.sh`.
* When you have addressed a PR comment, mark it as resolved.

Please be patient! I will review your PR, but my time is limited!
-->

This pull request fixes a subset of #5492 by saving the application
state when the `suspended` event is received on Android. This way, even
if the user exits the app and closes it manually right after changing
some state, it will be saved since `suspended` gets fired when the app
is exited. It does not fix the `on_exit` function not being fired - this
seems to be a winit bug (the `exiting` function in the winit application
handler trait is not called on exit). Once it gets fixed, it may be
possible to remove logic introduced by this PR (however, I am not sure
how it would handle the app being killed by the system when in the
background, that would have to be tested).

I've tested the logic by:
* Leaving from the app to the home screen, then killing it from the
"recent apps" menu
 * Leaving from the app to the "recent apps" menu and killing it
 * Restarting the device while the app was running

In all of these instances, the state was saved (the last one being a
pleasant surprise). It was tested on the repository mentioned in #5492
with my forked repository as the source for eframe (I unfortunately am
not able to test it in a larger project of mine due to dependence on
"3rd party" egui libraries (like egui_notify) which do not compile along
with the master branch of eframe (different versions of egui), but I
believe it should work in the same manner in all scenarios). Tests were
conducted on a Galaxy Tab S8 running Android 14, One UI 6.1.1.

CI passed on my fork.

* [x] I have followed the instructions in the PR template

crates/eframe/src/native/glow_integration.rs

@@ -366,6 +366,20 @@ impl WinitApp for GlowWinitApp<'_> {
             .and_then(|r| r.glutin.borrow().window_from_viewport.get(&id).copied())
     }
 
+    fn save(&mut self) {
+        log::debug!("WinitApp::save called");
+        if let Some(running) = self.running.as_mut() {
+            profiling::function_scope!();
+
+            // This is used because of the "save on suspend" logic on Android. Once the application is suspended, there is no window associated to it, which was causing panics when `.window().expect()` was used.
+            let window_opt = running.glutin.borrow().window_opt(ViewportId::ROOT);
+
+            running
+                .integration
+                .save(running.app.as_mut(), window_opt.as_deref());
+        }
+    }
+
     fn save_and_destroy(&mut self) {
         if let Some(mut running) = self.running.take() {
             profiling::function_scope!();
@@ -413,7 +427,7 @@ impl WinitApp for GlowWinitApp<'_> {
         if let Some(running) = &mut self.running {
             running.glutin.borrow_mut().on_suspend()?;
         }
-        Ok(EventResult::Wait)
+        Ok(EventResult::Save)
     }
 
     fn device_event(
@@ -1214,10 +1228,12 @@ impl GlutinWindowContext {
             .expect("viewport doesn't exist")
     }
 
+    fn window_opt(&self, viewport_id: ViewportId) -> Option<Arc<Window>> {
+        self.viewport(viewport_id).window.clone()
+    }
+
     fn window(&self, viewport_id: ViewportId) -> Arc<Window> {
-        self.viewport(viewport_id)
-            .window
-            .clone()
+        self.window_opt(viewport_id)
             .expect("winit window doesn't exist")
     }
 

crates/eframe/src/native/run.rs

@@ -89,6 +89,7 @@ impl<T: WinitApp> WinitAppWrapper<T> {
         event_result: Result<EventResult>,
     ) {
         let mut exit = false;
+        let mut save = false;
 
         log::trace!("event_result: {event_result:?}");
 
@@ -126,6 +127,10 @@ impl<T: WinitApp> WinitAppWrapper<T> {
                     );
                     Ok(event_result)
                 }
+                EventResult::Save => {
+                    save = true;
+                    Ok(event_result)
+                }
                 EventResult::Exit => {
                     exit = true;
                     Ok(event_result)
@@ -139,6 +144,11 @@ impl<T: WinitApp> WinitAppWrapper<T> {
             self.return_result = Err(err);
         };
 
+        if save {
+            log::debug!("Received an EventResult::Save - saving app state");
+            self.winit_app.save();
+        }
+
         if exit {
             if self.run_and_return {
                 log::debug!("Asking to exit event loop…");

crates/eframe/src/native/wgpu_integration.rs

@@ -355,6 +355,13 @@ impl WinitApp for WgpuWinitApp<'_> {
         )
     }
 
+    fn save(&mut self) {
+        log::debug!("WinitApp::save called");
+        if let Some(running) = self.running.as_mut() {
+            running.save();
+        }
+    }
+
     fn save_and_destroy(&mut self) {
         if let Some(mut running) = self.running.take() {
             running.save_and_destroy();
@@ -415,7 +422,7 @@ impl WinitApp for WgpuWinitApp<'_> {
     fn suspended(&mut self, _: &ActiveEventLoop) -> crate::Result<EventResult> {
         #[cfg(target_os = "android")]
         self.drop_window()?;
-        Ok(EventResult::Wait)
+        Ok(EventResult::Save)
     }
 
     fn device_event(
@@ -488,13 +495,23 @@ impl WinitApp for WgpuWinitApp<'_> {
 }
 
 impl WgpuWinitRunning<'_> {
+    /// Saves the application state
+    fn save(&mut self) {
+        let shared = self.shared.borrow();
+        // This is done because of the "save on suspend" logic on Android. Once the application is suspended, there is no window associated to it.
+        let window = if let Some(Viewport { window, .. }) = shared.viewports.get(&ViewportId::ROOT)
+        {
+            window.as_deref()
+        } else {
+            None
+        };
+        self.integration.save(self.app.as_mut(), window);
+    }
+
     fn save_and_destroy(&mut self) {
         profiling::function_scope!();
 
-        let mut shared = self.shared.borrow_mut();
-        if let Some(Viewport { window, .. }) = shared.viewports.get(&ViewportId::ROOT) {
-            self.integration.save(self.app.as_mut(), window.as_deref());
-        }
+        self.save();
 
         #[cfg(feature = "glow")]
         self.app.on_exit(None);
@@ -502,6 +519,7 @@ impl WgpuWinitRunning<'_> {
         #[cfg(not(feature = "glow"))]
         self.app.on_exit();
 
+        let mut shared = self.shared.borrow_mut();
         shared.painter.destroy();
     }
 

crates/eframe/src/native/winit_integration.rs

@@ -70,6 +70,8 @@ pub trait WinitApp {
 
     fn window_id_from_viewport_id(&self, id: ViewportId) -> Option<WindowId>;
 
+    fn save(&mut self);
+
     fn save_and_destroy(&mut self);
 
     fn run_ui_and_paint(
@@ -119,6 +121,9 @@ pub enum EventResult {
 
     RepaintAt(WindowId, Instant),
 
+    /// Causes a save of the client state when the persistence feature is enabled.
+    Save,
+
     Exit,
 }