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<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> {
    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<FnMut()>));
    }

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