1use js_sys::Promise;
2use std::cell::{Cell, RefCell};
3use std::collections::VecDeque;
4use std::rc::Rc;
5use wasm_bindgen::prelude::*;
6
7#[wasm_bindgen]
8extern "C" {
9 #[wasm_bindgen]
10 fn queueMicrotask(closure: &Closure<dyn FnMut(JsValue)>);
11
12 type Global;
13
14 #[wasm_bindgen(method, getter, js_name = queueMicrotask)]
15 fn hasQueueMicrotask(this: &Global) -> JsValue;
16}
17
18struct QueueState {
19 // The queue of Tasks which are to be run in order. In practice this is all the
20 // synchronous work of futures, and each `Task` represents calling `poll` on
21 // a future "at the right time".
22 tasks: RefCell<VecDeque<Rc<crate::task::Task>>>,
23
24 // This flag indicates whether we've scheduled `run_all` to run in the future.
25 // This is used to ensure that it's only scheduled once.
26 is_scheduled: Cell<bool>,
27}
28
29impl QueueState {
30 fn run_all(&self) {
31 // "consume" the schedule
32 let _was_scheduled: bool = self.is_scheduled.replace(val:false);
33 debug_assert!(_was_scheduled);
34
35 // Stop when all tasks that have been scheduled before this tick have been run.
36 // Tasks that are scheduled while running tasks will run on the next tick.
37 let mut task_count_left: usize = self.tasks.borrow().len();
38 while task_count_left > 0 {
39 task_count_left -= 1;
40 let task: Rc = match self.tasks.borrow_mut().pop_front() {
41 Some(task: Rc) => task,
42 None => break,
43 };
44 task.run();
45 }
46
47 // All of the Tasks have been run, so it's now possible to schedule the
48 // next tick again
49 }
50}
51
52pub(crate) struct Queue {
53 state: Rc<QueueState>,
54 promise: Promise,
55 closure: Closure<dyn FnMut(JsValue)>,
56 has_queue_microtask: bool,
57}
58
59impl Queue {
60 // Schedule a task to run on the next tick
61 pub(crate) fn schedule_task(&self, task: Rc<crate::task::Task>) {
62 self.state.tasks.borrow_mut().push_back(task);
63 // Use queueMicrotask to execute as soon as possible. If it does not exist
64 // fall back to the promise resolution
65 if !self.state.is_scheduled.replace(val:true) {
66 if self.has_queue_microtask {
67 queueMicrotask(&self.closure);
68 } else {
69 let _ = self.promise.then(&self.closure);
70 }
71 }
72 }
73 // Append a task to the currently running queue, or schedule it
74 pub(crate) fn push_task(&self, task: Rc<crate::task::Task>) {
75 // It would make sense to run this task on the same tick. For now, we
76 // make the simplifying choice of always scheduling tasks for a future tick.
77 self.schedule_task(task)
78 }
79}
80
81impl Queue {
82 fn new() -> Self {
83 let state = Rc::new(QueueState {
84 is_scheduled: Cell::new(false),
85 tasks: RefCell::new(VecDeque::new()),
86 });
87
88 let has_queue_microtask = js_sys::global()
89 .unchecked_into::<Global>()
90 .hasQueueMicrotask()
91 .is_function();
92
93 Self {
94 promise: Promise::resolve(&JsValue::undefined()),
95
96 closure: {
97 let state = Rc::clone(&state);
98
99 // This closure will only be called on the next microtask event
100 // tick
101 Closure::new(move |_| state.run_all())
102 },
103
104 state,
105 has_queue_microtask,
106 }
107 }
108}
109
110thread_local! {
111 pub(crate) static QUEUE: Queue = Queue::new();
112}
113