pub struct Waker { /* private fields */ }Expand description
A Waker is a handle for waking up a task by notifying its executor that it
is ready to be run.
This handle encapsulates a RawWaker instance, which defines the
executor-specific wakeup behavior.
The typical life of a Waker is that it is constructed by an executor, wrapped in a
Context, then passed to Future::poll(). Then, if the future chooses to return
Poll::Pending, it must also store the waker somehow and call Waker::wake() when
the future should be polled again.
Implements Clone, Send, and Sync; therefore, a waker may be invoked
from any thread, including ones not in any way managed by the executor. For example,
this might be done to wake a future when a blocking function call completes on another
thread.
Note that it is preferable to use waker.clone_from(&new_waker) instead
of *waker = new_waker.clone(), as the former will avoid cloning the waker
unnecessarily if the two wakers wake the same task.
Implementations§
source§impl Waker
impl Waker
sourcepub fn wake(self)
pub fn wake(self)
Wake 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.
sourcepub fn wake_by_ref(&self)
pub fn wake_by_ref(&self)
Wake 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().
sourcepub fn will_wake(&self, other: &Waker) -> bool
pub fn will_wake(&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.
const: unstable · sourcepub unsafe fn from_raw(waker: RawWaker) -> Waker
pub unsafe fn from_raw(waker: RawWaker) -> Waker
Creates a new Waker from RawWaker.
The behavior of the returned Waker is undefined if the contract defined
in RawWaker’s and RawWakerVTable’s documentation is not upheld.
Therefore this method is unsafe.
sourcepub const fn noop() -> Waker
🔬This is a nightly-only experimental API. (noop_waker)
pub const fn noop() -> Waker
noop_waker)Creates a new Waker that does nothing when wake is called.
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.
Examples
#![feature(noop_waker)]
use std::future::Future;
use std::task;
let waker = task::Waker::noop();
let mut cx = task::Context::from_waker(&waker);
let mut future = Box::pin(async { 10 });
assert_eq!(future.as_mut().poll(&mut cx), task::Poll::Ready(10));