Add examples/window_ondemand

A minimal example that shows an application running the event loop more
than once via `run_ondemand`

There is a 5 second delay between each run to help highlight problems
with destroying the window from the first loop.

examples/window_ondemand.rs

@@ -0,0 +1,105 @@
+#![allow(clippy::single_match)]
+
+// 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 simple_logger::SimpleLogger;
+
+    use winit::{
+        event::{Event, WindowEvent},
+        event_loop::EventLoop,
+        platform::run_ondemand::EventLoopExtRunOnDemand,
+        window::{Window, WindowBuilder},
+    };
+
+    #[path = "util/fill.rs"]
+    mod fill;
+
+    #[derive(Default)]
+    struct App {
+        window: Option<Window>,
+    }
+
+    SimpleLogger::new().init().unwrap();
+    let mut event_loop = EventLoop::new();
+
+    {
+        let mut app = App::default();
+
+        event_loop.run_ondemand(move |event, event_loop, control_flow| {
+            control_flow.set_wait();
+            println!("Run 1: {:?}", 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 one!")
+                        .with_inner_size(winit::dpi::LogicalSize::new(128.0, 128.0))
+                        .build(event_loop)
+                        .unwrap(),
+                );
+            }
+        })?;
+    }
+
+    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(),
+                );
+            }
+        })
+    };
+
+    println!("--------------------------------------------------------- Finished second loop");
+    ret
+}
+
+#[cfg(not(any(windows_platform, macos_platform, x11_platform, wayland_platform,)))]
+fn main() {
+    println!("This example is not supported on this platform");
+}