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74 lines
1.9 KiB
74 lines
1.9 KiB
//! A simple single-threaded executor that can spawn non-`Send` futures.
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use std::cell::Cell;
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use std::future::Future;
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use std::rc::Rc;
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use async_task::{Runnable, Task};
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thread_local! {
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// A queue that holds scheduled tasks.
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static QUEUE: (flume::Sender<Runnable>, flume::Receiver<Runnable>) = flume::unbounded();
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}
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/// Spawns a future on the executor.
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fn spawn<F, T>(future: F) -> Task<T>
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where
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F: Future<Output = T> + 'static,
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T: 'static,
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{
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// Create a task that is scheduled by pushing itself into the queue.
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let schedule = |runnable| QUEUE.with(|(s, _)| s.send(runnable).unwrap());
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let (runnable, task) = async_task::spawn_local(future, schedule);
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// Schedule the task by pushing it into the queue.
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runnable.schedule();
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task
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}
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/// Runs a future to completion.
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fn run<F, T>(future: F) -> T
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where
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F: Future<Output = T> + 'static,
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T: 'static,
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{
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// Spawn a task that sends its result through a channel.
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let (s, r) = flume::unbounded();
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spawn(async move { drop(s.send(future.await)) }).detach();
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loop {
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// If the original task has completed, return its result.
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if let Ok(val) = r.try_recv() {
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return val;
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}
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// Otherwise, take a task from the queue and run it.
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QUEUE.with(|(_, r)| r.recv().unwrap().run());
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}
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}
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fn main() {
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let val = Rc::new(Cell::new(0));
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// Run a future that increments a non-`Send` value.
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run({
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let val = val.clone();
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async move {
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// Spawn a future that increments the value.
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let task = spawn({
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let val = val.clone();
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async move {
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val.set(dbg!(val.get()) + 1);
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}
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});
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val.set(dbg!(val.get()) + 1);
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task.await;
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}
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});
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// The value should be 2 at the end of the program.
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dbg!(val.get());
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}
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