impl Waker

fn wake(self: Self)

Wakes up the task associated with this Waker.

As long as the executor keeps running and the task is not finished, it is guaranteed that each invocation of wake() (or wake_by_ref()) will be followed by at least one poll() of the task to which this Waker belongs. This makes it possible to temporarily yield to other tasks while running potentially unbounded processing loops.

Note that the above implies that multiple wake-ups may be coalesced into a single poll() invocation by the runtime.

Also note that yielding to competing tasks is not guaranteed: it is the executor’s choice which task to run and the executor may choose to run the current task again.

fn wake_by_ref(self: &Self)

Wakes up the task associated with this Waker without consuming the Waker.

This is similar to wake(), but may be slightly less efficient in the case where an owned Waker is available. This method should be preferred to calling waker.clone().wake().

fn will_wake(self: &Self, other: &Waker) -> bool

Returns true if this Waker and another Waker would awake the same task.

This function works on a best-effort basis, and may return false even when the Wakers would awaken the same task. However, if this function returns true, it is guaranteed that the Wakers will awaken the same task.

This function is primarily used for optimization purposes — for example, this type's clone_from implementation uses it to avoid cloning the waker when they would wake the same task anyway.

unsafe const fn new(data: *const (), vtable: &'static RawWakerVTable) -> Self

Creates a new Waker from the provided data pointer and vtable.

The data pointer can be used to store arbitrary data as required by the executor. This could be e.g. a type-erased pointer to an Arc that is associated with the task. The value of this pointer will get passed to all functions that are part of the vtable as the first parameter.

It is important to consider that the data pointer must point to a thread safe type such as an Arc.

The vtable customizes the behavior of a Waker. For each operation on the Waker, the associated function in the vtable will be called.

Safety

The behavior of the returned Waker is undefined if the contract defined in RawWakerVTable's documentation is not upheld.

(Authors wishing to avoid unsafe code may implement the Wake trait instead, at the cost of a required heap allocation.)

unsafe const fn from_raw(waker: RawWaker) -> Waker

Creates a new Waker from RawWaker.

Safety

The behavior of the returned Waker is undefined if the contract defined in RawWaker's and RawWakerVTable's documentation is not upheld.

(Authors wishing to avoid unsafe code may implement the Wake trait instead, at the cost of a required heap allocation.)

const fn noop() -> &'static Waker

Returns a reference to a Waker that does nothing when used.

This is mostly useful for writing tests that need a Context to poll some futures, but are not expecting those futures to wake the waker or do not need to do anything specific if it happens.

More generally, using Waker::noop() to poll a future means discarding the notification of when the future should be polled again. So it should only be used when such a notification will not be needed to make progress.

If an owned Waker is needed, clone() this one.

Examples

use std::future::Future;
use std::task;

let mut cx = task::Context::from_waker(task::Waker::noop());

let mut future = Box::pin(async { 10 });
assert_eq!(future.as_mut().poll(&mut cx), task::Poll::Ready(10));

fn data(self: &Self) -> *const ()

Gets the data pointer used to create this Waker.

fn vtable(self: &Self) -> &'static RawWakerVTable

Gets the vtable pointer used to create this Waker.

const fn from_fn_ptr(f: fn()) -> Self

Constructs a Waker from a function pointer.

impl AsRef for Waker

fn as_ref(self: &Self) -> &LocalWaker

impl Clone for Waker

fn clone(self: &Self) -> Self

fn clone_from(self: &mut Self, source: &Self)

Assigns a clone of source to self, unless [self.will_wake(source)][Waker::will_wake] anyway.

This method is preferred over simply assigning source.clone() to self, as it avoids cloning the waker if self is already the same waker.

Examples

use std::future::Future;
use std::pin::Pin;
use std::sync::{Arc, Mutex};
use std::task::{Context, Poll, Waker};

struct Waiter {
    shared: Arc<Mutex<Shared>>,
}

struct Shared {
    waker: Waker,
    // ...
}

impl Future for Waiter {
    type Output = ();
    fn poll(self: Pin<&mut Self>, cx: &mut Context<'_>) -> Poll<()> {
        let mut shared = self.shared.lock().unwrap();

        // update the waker
        shared.waker.clone_from(cx.waker());

        // readiness logic ...
#       Poll::Ready(())
    }
}

impl Debug for Waker

fn fmt(self: &Self, f: &mut Formatter<'_>) -> Result

impl Drop for Waker

fn drop(self: &mut Self)

impl Freeze for Waker

impl RefUnwindSafe for Waker

impl Send for Waker

impl Sync for Waker

impl Unpin for Waker

impl UnsafeUnpin for Waker

impl UnwindSafe for Waker

impl<T> Any for Waker

fn type_id(self: &Self) -> TypeId

impl<T> Borrow for Waker

fn borrow(self: &Self) -> &T

impl<T> BorrowMut for Waker

fn borrow_mut(self: &mut Self) -> &mut T

impl<T> CloneToUninit for Waker

unsafe fn clone_to_uninit(self: &Self, dest: *mut u8)

impl<T> From for Waker

fn from(t: T) -> T

Returns the argument unchanged.

impl<T, U> Into for Waker

fn into(self: Self) -> U

Calls U::from(self).

That is, this conversion is whatever the implementation of [From]<T> for U chooses to do.

impl<T, U> TryFrom for Waker

fn try_from(value: U) -> Result<T, <T as TryFrom<U>>::Error>

impl<T, U> TryInto for Waker

fn try_into(self: Self) -> Result<U, <U as TryFrom<T>>::Error>