Trait Wake

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

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

The implementation of waking a task on an executor.

This trait can be used to create a Waker. An executor can define an implementation of this trait, and use that to construct a Waker to pass to the tasks that are executed on that executor.

This trait is a memory-safe and ergonomic alternative to constructing a RawWaker. It supports the common executor design in which the data used to wake up a task is stored in an Arc. Some executors (especially those for embedded systems) cannot use this API, which is why RawWaker exists as an alternative for those systems.

To construct a Waker from some type W implementing this trait, wrap it in an Arc<W> and call Waker::from() on that. It is also possible to convert to RawWaker in the same way.

Examples

A basic block_on function that takes a future and runs it to completion on the current thread.

Note: This example trades correctness for simplicity. In order to prevent deadlocks, production-grade implementations will also need to handle intermediate calls to thread::unpark as well as nested invocations.

use std::future::Future;
use std::sync::Arc;
use std::task::{Context, Poll, Wake};
use std::thread::{self, Thread};
use core::pin::pin;

/// A waker that wakes up the current thread when called.
struct ThreadWaker(Thread);

impl Wake for ThreadWaker {
    fn wake(self: Arc<Self>) {
        self.0.unpark();
    }
}

/// Run a future to completion on the current thread.
fn block_on<T>(fut: impl Future<Output = T>) -> T {
    // Pin the future so it can be polled.
    let mut fut = pin!(fut);

    // Create a new context to be passed to the future.
    let t = thread::current();
    let waker = Arc::new(ThreadWaker(t)).into();
    let mut cx = Context::from_waker(&waker);

    // Run the future to completion.
    loop {
        match fut.as_mut().poll(&mut cx) {
            Poll::Ready(res) => return res,
            Poll::Pending => thread::park(),
        }
    }
}

block_on(async {
    println!("Hi from inside a future!");
});

Required Methods

fn wake(self: Arc<Self>)

Wake this task.

Provided Methods

fn wake_by_ref(self: &Arc<Self>)

Wake this task without consuming the waker.

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

Dyn Compatibility

This trait is not dyn compatible.

In older versions of Rust, dyn compatibility was called "object safety", so this trait is not object safe.

Implementors