Trait LocalWake

pub trait LocalWake {
    // Required method
    fn wake(self: Rc<Self>);

    // Provided method
    fn wake_by_ref(self: &Rc<Self>) { ... }
}

🔬This is a nightly-only experimental API. (local_waker #118959)

An analogous trait to Wake but used to construct a LocalWaker. This API works in exactly the same way as Wake, except that it uses an Rc instead of an Arc, and the result is a LocalWaker instead of a Waker.

The benefits of using LocalWaker over Waker are that it allows the local waker to hold data that does not implement Send and Sync. Additionally, it saves calls to Arc::clone, which requires atomic synchronization.

Examples

This is a simplified example of a spawn and a block_on function. The spawn function is used to push new tasks onto the run queue, while the block on function will remove them and poll them. When a task is woken, it will put itself back on the run queue to be polled by the executor.

Note: This example trades correctness for simplicity. A real world example would interleave poll calls with calls to an io reactor to wait for events instead of spinning on a loop.

#![feature(local_waker)]
#![feature(noop_waker)]
use std::task::{LocalWake, ContextBuilder, LocalWaker, Waker};
use std::future::Future;
use std::pin::Pin;
use std::rc::Rc;
use std::cell::RefCell;
use std::collections::VecDeque;


thread_local! {
    // A queue containing all tasks ready to do progress
    static RUN_QUEUE: RefCell<VecDeque<Rc<Task>>> = RefCell::default();
}

type BoxedFuture = Pin<Box<dyn Future<Output = ()>>>;

struct Task(RefCell<BoxedFuture>);

impl LocalWake for Task {
    fn wake(self: Rc<Self>) {
        RUN_QUEUE.with_borrow_mut(|queue| {
            queue.push_back(self)
        })
    }
}

fn spawn<F>(future: F)
where
    F: Future<Output=()> + 'static + Send + Sync
{
    let task = RefCell::new(Box::pin(future));
    RUN_QUEUE.with_borrow_mut(|queue| {
        queue.push_back(Rc::new(Task(task)));
    });
}

fn block_on<F>(future: F)
where
    F: Future<Output=()> + 'static + Sync + Send
{
    spawn(future);
    loop {
        let Some(task) = RUN_QUEUE.with_borrow_mut(|queue| queue.pop_front()) else {
            // we exit, since there are no more tasks remaining on the queue
            return;
        };

        // cast the Rc<Task> into a `LocalWaker`
        let local_waker: LocalWaker = task.clone().into();
        // Build the context using `ContextBuilder`
        let mut cx = ContextBuilder::from_waker(Waker::noop())
            .local_waker(&local_waker)
            .build();

        // Poll the task
        let _ = task.0
            .borrow_mut()
            .as_mut()
            .poll(&mut cx);
    }
}

block_on(async {
    println!("hello world");
});

Required Methods

fn wake(self: Rc<Self>)

🔬This is a nightly-only experimental API. (local_waker #118959)

Wake this task.

Provided Methods

fn wake_by_ref(self: &Rc<Self>)

🔬This is a nightly-only experimental API. (local_waker #118959)

Wake this task without consuming the local waker.

If an executor supports a cheaper way to wake without consuming the waker, it should override this method. By default, it clones the Rc and calls wake on the clone.

Object Safety

This trait is not object safe.

Implementors