Event Loop

Takeaways:

• An event loop is a loop that polls for an event.
• When an event is received, some code is run. The code is the event_handler.
• The application provides the event_handler.
• Native: The application controls the event loop – winit.
• WASM: The browser controls the event loop, but consistent API is still exposed by winit.
• Github issue(s): amethyst#2191

// Not real code
impl EventLoop {
    pub fn run<F>(self, event_handler: F) -> !
    where
        F: ..
    {
        loop {
            let event = poll_event(); // Blocks until an event arrives.
            event_handler(event, ..);
        }
    }
}

// Not real code, but close 🤏.
let event_handler = move |event, _, control_flow| {
    match event {
        // tick the game
        Event::MainEventsCleared => self.run_game_logic(),

        // input
        Event::DeviceEvent(device_event) => self.notify_input(device_event),
        _ => {},
    }
};

event_loop.run(event_handler);

// Never reached, as EventLoop::run(_) returns `!`.

loop {
    let event = poll_event(); // Blocks until an event arrives.
    event_handler(event, ..);
}

Native

In a native application, the event loop receives events from the operating system.

WASM

In a browser, the event loop is not controlled by winit, but the browser.

To surrender control to the browser, winit sends the event handler to the browser, and panics.

The Window::requestAnimationFrame API is used to receive events.

// While `pixel_shift`` is less than 100, call `requestAnimationFrame`.
var pixel_shift = 0;

function move(_timestamp) {
  element.style.transform = 'translateX(' + pixel_shift + 'px)';

  if (pixel_shift < 100) {
    pixel_shift += 1;
    window.requestAnimationFrame(move);
  }
}

window.requestAnimationFrame(move);

To ensure requestAnimationFrame is called, in the event handler, the control_flow parameter must be set to ControlFlow::Poll.

let event_handler = move |event, _, control_flow| {
    // ..

    // Ensure the browser calls `requestAnimationFrame`
    // This will ensure this event handler is run, even if no events arrive.
    *control_flow = ControlFlow::Poll;
};

event_loop.run(event_handler);