wasm-bindgen/crates/futures/src/queue.rs

use js_sys::Promise;
use std::cell::{Cell, RefCell};
use std::collections::VecDeque;
use std::rc::Rc;
use wasm_bindgen::prelude::*;

struct QueueState {
    // The queue of Tasks which will be run in order. In practice this is all the
    // synchronous work of futures, and each `Task` represents calling `poll` on
    // a future "at the right time"
    tasks: RefCell<VecDeque<Rc<crate::task::Task>>>,

    // This flag indicates whether we're currently executing inside of
    // `run_all` or have scheduled `run_all` to run in the future. This is
    // used to ensure that it's only scheduled once.
    is_spinning: Cell<bool>,
}

impl QueueState {
    fn run_all(&self) {
        debug_assert!(self.is_spinning.get());

        // Runs all Tasks until empty. This blocks the event loop if a Future is
        // stuck in an infinite loop, so we may want to yield back to the main
        // event loop occasionally. For now though greedy execution should get
        // the job done.
        loop {
            let task = match self.tasks.borrow_mut().pop_front() {
                Some(task) => task,
                None => break,
            };
            task.run();
        }

        // All of the Tasks have been run, so it's now possible to schedule the
        // next tick again
        self.is_spinning.set(false);
    }
}

pub(crate) struct Queue {
    state: Rc<QueueState>,
    promise: Promise,
    closure: Closure<dyn FnMut(JsValue)>,
}

impl Queue {
    pub(crate) fn push_task(&self, task: Rc<crate::task::Task>) {
        self.state.tasks.borrow_mut().push_back(task);

        // If we're already inside the `run_all` loop then that'll pick up the
        // task we just enqueued. If we're not in `run_all`, though, then we need
        // to schedule a microtask.
        //
        // Note that we currently use a promise and a closure to do this, but
        // eventually we should probably use something like `queueMicrotask`:
        // https://developer.mozilla.org/en-US/docs/Web/API/WindowOrWorkerGlobalScope/queueMicrotask
        if !self.state.is_spinning.replace(true) {
            self.promise.then(&self.closure);
        }
    }
}

impl Queue {
    fn new() -> Self {
        let state = Rc::new(QueueState {
            is_spinning: Cell::new(false),
            tasks: RefCell::new(VecDeque::new()),
        });

        Self {
            promise: Promise::resolve(&JsValue::undefined()),

            closure: {
                let state = Rc::clone(&state);

                // This closure will only be called on the next microtask event
                // tick
                Closure::wrap(Box::new(move |_| state.run_all()))
            },

            state,
        }
    }
}

thread_local! {
    pub(crate) static QUEUE: Queue = Queue::new();
}