Add new example that demonstrates the different control flow schemes (#1460)

User can switch between Wait, WaitUntil and Poll modes with key '1', '2' and '3' respectivly.
User can toggle request_redraw calls with the 'R' key.

Helpful for testing all control flow modes and use of request_redraw.

examples/control_flow.rs

@@ -0,0 +1,113 @@
+use std::{thread, time};
+
+use winit::{
+    event::{Event, KeyboardInput, WindowEvent},
+    event_loop::{ControlFlow, EventLoop},
+    window::WindowBuilder,
+};
+
+#[derive(Debug, Clone, Copy, PartialEq, Eq)]
+enum Mode {
+    Wait,
+    WaitUntil,
+    Poll,
+}
+
+const WAIT_TIME: time::Duration = time::Duration::from_millis(100);
+const POLL_SLEEP_TIME: time::Duration = time::Duration::from_millis(100);
+
+fn main() {
+    simple_logger::init().unwrap();
+
+    println!("Press '1' to switch to Wait mode.");
+    println!("Press '2' to switch to WaitUntil mode.");
+    println!("Press '3' to switch to Poll mode.");
+    println!("Press 'R' to toggle request_redraw() calls.");
+    println!("Press 'Esc' to close the window.");
+
+    let event_loop = EventLoop::new();
+    let window = WindowBuilder::new()
+        .with_title("Press 1, 2, 3 to change control flow mode. Press R to toggle redraw requests.")
+        .build(&event_loop)
+        .unwrap();
+
+    let mut mode = Mode::Wait;
+    let mut request_redraw = false;
+    let mut wait_cancelled = false;
+    let mut close_requested = false;
+
+    event_loop.run(move |event, _, control_flow| {
+        use winit::event::{ElementState, StartCause, VirtualKeyCode};
+        println!("{:?}", event);
+        match event {
+            Event::NewEvents(start_cause) => {
+                wait_cancelled = mode == Mode::WaitUntil;
+                match start_cause {
+                    StartCause::ResumeTimeReached {
+                        start: _,
+                        requested_resume: _,
+                    } => {
+                        wait_cancelled = false;
+                    }
+                    _ => (),
+                }
+            }
+            Event::WindowEvent { event, .. } => match event {
+                WindowEvent::CloseRequested => {
+                    close_requested = true;
+                }
+                WindowEvent::KeyboardInput {
+                    input:
+                        KeyboardInput {
+                            virtual_keycode: Some(virtual_code),
+                            state: ElementState::Pressed,
+                            ..
+                        },
+                    ..
+                } => match virtual_code {
+                    VirtualKeyCode::Key1 => {
+                        mode = Mode::Wait;
+                        println!("\nmode: {:?}\n", mode);
+                    }
+                    VirtualKeyCode::Key2 => {
+                        mode = Mode::WaitUntil;
+                        println!("\nmode: {:?}\n", mode);
+                    }
+                    VirtualKeyCode::Key3 => {
+                        mode = Mode::Poll;
+                        println!("\nmode: {:?}\n", mode);
+                    }
+                    VirtualKeyCode::R => {
+                        request_redraw = !request_redraw;
+                        println!("\nrequest_redraw: {}\n", request_redraw);
+                    }
+                    VirtualKeyCode::Escape => {
+                        close_requested = true;
+                    }
+                    _ => (),
+                },
+                _ => (),
+            },
+            Event::MainEventsCleared => {
+                if request_redraw && !wait_cancelled && !close_requested {
+                    window.request_redraw();
+                }
+                if close_requested {
+                    *control_flow = ControlFlow::Exit;
+                }
+            }
+            Event::RedrawRequested(_window_id) => {}
+            Event::RedrawEventsCleared => {
+                *control_flow = match mode {
+                    Mode::Wait => ControlFlow::Wait,
+                    Mode::WaitUntil => ControlFlow::WaitUntil(time::Instant::now() + WAIT_TIME),
+                    Mode::Poll => {
+                        thread::sleep(POLL_SLEEP_TIME);
+                        ControlFlow::Poll
+                    }
+                };
+            }
+            _ => (),
+        }
+    });
+}