window_ondemand: wait for Destroyed event before exiting app

Considering the strict requirement that applications can't keep windows
across run_ondemand calls, this tries to make the window_ondemand example
explicitly wait for its Window to be destroyed before exiting each
run_ondemand iteration.

This updates the example to only `.set_exit()` after it gets a
`Destroyed` event after the Window has been dropped.

On Windows this works to ensure the Window is destroyed before the
example waits for 5 seconds.

Unfortunately though:
1. The Wayland backend doesn't emit `Destroyed` events for windows
2. The macOS backend emits `Destroyed` events before the window is
   really destroyed.

and so the example isn't currently portable.

examples/window_ondemand.rs

@@ -3,13 +3,16 @@
 // Limit this example to only compatible platforms.
 #[cfg(any(windows_platform, macos_platform, x11_platform, wayland_platform,))]
 fn main() -> Result<(), impl std::error::Error> {
+    use std::time::Duration;
+
     use simple_logger::SimpleLogger;
 
     use winit::{
+        error::RunLoopError,
         event::{Event, WindowEvent},
         event_loop::EventLoop,
         platform::run_ondemand::EventLoopExtRunOnDemand,
-        window::{Window, WindowBuilder},
+        window::{Window, WindowBuilder, WindowId},
     };
 
     #[path = "util/fill.rs"]
@@ -17,18 +20,19 @@ fn main() -> Result<(), impl std::error::Error> {
 
     #[derive(Default)]
     struct App {
+        window_id: Option<WindowId>,
         window: Option<Window>,
     }
 
     SimpleLogger::new().init().unwrap();
     let mut event_loop = EventLoop::new();
 
-    {
+    fn run_app(event_loop: &mut EventLoop<()>, idx: usize) -> Result<(), RunLoopError> {
         let mut app = App::default();
 
         event_loop.run_ondemand(move |event, event_loop, control_flow| {
             control_flow.set_wait();
-            println!("Run 1: {:?}", event);
+            println!("Run {idx}: {:?}", event);
 
             if let Some(window) = &app.window {
                 match event {
@@ -36,8 +40,8 @@ fn main() -> Result<(), impl std::error::Error> {
                         event: WindowEvent::CloseRequested,
                         window_id,
                     } if window.id() == window_id => {
+                        println!("--------------------------------------------------------- Window {idx} CloseRequested");
                         app.window = None;
-                        control_flow.set_exit();
                     }
                     Event::MainEventsCleared => window.request_redraw(),
                     Event::RedrawRequested(_) => {
@@ -45,56 +49,37 @@ fn main() -> Result<(), impl std::error::Error> {
                     }
                     _ => (),
                 }
+            } else if let Some(id) = app.window_id {
+                match event {
+                    Event::WindowEvent {
+                        event: WindowEvent::Destroyed,
+                        window_id,
+                    } if id == window_id => {
+                        println!("--------------------------------------------------------- Window {idx} Destroyed");
+                        app.window_id = None;
+                        control_flow.set_exit();
+                    }
+                    _ => (),
+                }
             } else if let Event::Resumed = event {
-                app.window = Some(
-                    WindowBuilder::new()
+                let window = WindowBuilder::new()
                         .with_title("Fantastic window number one!")
                         .with_inner_size(winit::dpi::LogicalSize::new(128.0, 128.0))
                         .build(event_loop)
-                        .unwrap(),
-                );
+                        .unwrap();
+                app.window_id = Some(window.id());
+                app.window = Some(window);
             }
-        })?;
+        })
     }
 
+    run_app(&mut event_loop, 1)?;
+
     println!("--------------------------------------------------------- Finished first loop");
     println!("--------------------------------------------------------- Waiting 5 seconds");
-    std::thread::sleep_ms(5000);
-
-    let ret = {
-        let mut app = App::default();
-
-        event_loop.run_ondemand(move |event, event_loop, control_flow| {
-            control_flow.set_wait();
-            println!("Run 2: {:?}", event);
-
-            if let Some(window) = &app.window {
-                match event {
-                    Event::WindowEvent {
-                        event: WindowEvent::CloseRequested,
-                        window_id,
-                    } if window.id() == window_id => {
-                        app.window = None;
-                        control_flow.set_exit();
-                    }
-                    Event::MainEventsCleared => window.request_redraw(),
-                    Event::RedrawRequested(_) => {
-                        fill::fill_window(window);
-                    }
-                    _ => (),
-                }
-            } else if let Event::Resumed = event {
-                app.window = Some(
-                    WindowBuilder::new()
-                        .with_title("Fantastic window number two!")
-                        .with_inner_size(winit::dpi::LogicalSize::new(128.0, 128.0))
-                        .build(event_loop)
-                        .unwrap(),
-                );
-            }
-        })
-    };
+    std::thread::sleep(Duration::from_secs(5));
 
+    let ret = run_app(&mut event_loop, 2);
     println!("--------------------------------------------------------- Finished second loop");
     ret
 }