lib.rs

use std::cell::RefCell;
use std::rc::Rc;
use wasm_bindgen::prelude::*;
use wasm_bindgen::JsCast;

fn window() -> web_sys::Window {
    web_sys::window().expect("no global `window` exists")
}

fn request_animation_frame(f: &Closure<dyn FnMut()>) {
    window()
        .request_animation_frame(f.as_ref().unchecked_ref())
        .expect("should register `requestAnimationFrame` OK");
}

fn document() -> web_sys::Document {
    window()
        .document()
        .expect("should have a document on window")
}

fn body() -> web_sys::HtmlElement {
    document().body().expect("document should have a body")
}

// This function is automatically invoked after the wasm module is instantiated.
#[wasm_bindgen(start)]
pub fn run() -> Result<(), JsValue> {
    // Here we want to call `requestAnimationFrame` in a loop, but only a fixed
    // number of times. After it's done we want all our resources cleaned up. To
    // achieve this we're using an `Rc`. The `Rc` will eventually store the
    // closure we want to execute on each frame, but to start out it contains
    // `None`.
    //
    // After the `Rc` is made we'll actually create the closure, and the closure
    // will reference one of the `Rc` instances. The other `Rc` reference is
    // used to store the closure, request the first frame, and then is dropped
    // by this function.
    //
    // Inside the closure we've got a persistent `Rc` reference, which we use
    // for all future iterations of the loop
    let f = Rc::new(RefCell::new(None));
    let g = f.clone();

    let mut i = 0;
    *g.borrow_mut() = Some(Closure::wrap(Box::new(move || {
        if i > 300 {
            body().set_text_content(Some("All done!"));

            // Drop our handle to this closure so that it will get cleaned
            // up once we return.
            let _ = f.borrow_mut().take();
            return;
        }

        // Set the body's text content to how many times this
        // requestAnimationFrame callback has fired.
        i += 1;
        let text = format!("requestAnimationFrame has been called {} times.", i);
        body().set_text_content(Some(&text));

        // Schedule ourself for another requestAnimationFrame callback.
        request_animation_frame(f.borrow().as_ref().unwrap());
    }) as Box<dyn FnMut()>));

    request_animation_frame(g.borrow().as_ref().unwrap());
    Ok(())
}