example: Do not send Window to another thread (#287)

The threading implications of accessing a window from another thread are
unclear, so Winit chooses to block on the main thread on macOS and iOS.

This means that we have to send the surface width and height to the
other thread instead of trying to access it dynamically from the window.

Author: madsmtm

examples/winit_multithread.rs

@@ -18,42 +18,35 @@ pub mod ex {
     type Surface = softbuffer::Surface<OwnedDisplayHandle, Arc<Window>>;
 
     fn render_thread(
-        window: Arc<Window>,
-        do_render: mpsc::Receiver<Arc<Mutex<Surface>>>,
+        do_render: mpsc::Receiver<(Arc<Mutex<Surface>>, NonZeroU32, NonZeroU32)>,
         done: mpsc::Sender<()>,
     ) {
         loop {
             println!("waiting for render...");
-            let Ok(surface) = do_render.recv() else {
+            let Ok((surface, width, height)) = do_render.recv() else {
                 println!("main thread destroyed");
                 break;
             };
 
             // Perform the rendering.
             let mut surface = surface.lock().unwrap();
-            if let (Some(width), Some(height)) = {
-                let size = window.inner_size();
-                println!("got size: {size:?}");
-                (NonZeroU32::new(size.width), NonZeroU32::new(size.height))
-            } {
-                println!("resizing...");
-                surface.resize(width, height).unwrap();
-
-                let mut buffer = surface.buffer_mut().unwrap();
-                for y in 0..buffer.height().get() {
-                    for x in 0..buffer.width().get() {
-                        let red = x % 255;
-                        let green = y % 255;
-                        let blue = (x * y) % 255;
-                        let index = y * buffer.width().get() + x;
-                        buffer[index as usize] = blue | (green << 8) | (red << 16);
-                    }
+            println!("resizing...");
+            surface.resize(width, height).unwrap();
+
+            let mut buffer = surface.buffer_mut().unwrap();
+            for y in 0..buffer.height().get() {
+                for x in 0..buffer.width().get() {
+                    let red = x % 255;
+                    let green = y % 255;
+                    let blue = (x * y) % 255;
+                    let index = y * buffer.width().get() + x;
+                    buffer[index as usize] = blue | (green << 8) | (red << 16);
                 }
-
-                println!("presenting...");
-                buffer.present().unwrap();
             }
 
+            println!("presenting...");
+            buffer.present().unwrap();
+
             // We're done, tell the main thread to keep going.
             done.send(()).ok();
         }
@@ -77,10 +70,7 @@ pub mod ex {
                 let (start_render, do_render) = mpsc::channel();
                 let (render_done, finish_render) = mpsc::channel();
                 println!("starting thread...");
-                std::thread::spawn({
-                    let window = window.clone();
-                    move || render_thread(window, do_render, render_done)
-                });
+                std::thread::spawn(move || render_thread(do_render, render_done));
 
                 (window, start_render, finish_render)
             },
@@ -105,9 +95,16 @@ pub mod ex {
                         eprintln!("RedrawRequested fired before Resumed or after Suspended");
                         return;
                     };
-                    // Start the render and then finish it.
-                    start_render.send(surface.clone()).unwrap();
-                    finish_render.recv().unwrap();
+
+                    let size = window.inner_size();
+                    println!("got size: {size:?}");
+                    if let (Some(width), Some(height)) =
+                        (NonZeroU32::new(size.width), NonZeroU32::new(size.height))
+                    {
+                        // Start the render and then finish it.
+                        start_render.send((surface.clone(), width, height)).unwrap();
+                        finish_render.recv().unwrap();
+                    }
                 }
                 WindowEvent::CloseRequested
                 | WindowEvent::KeyboardInput {