Struct core::task::LocalWaker
source · pub struct LocalWaker { /* private fields */ }local_waker #118959)Expand description
A LocalWaker is analogous to a Waker, but it does not implement Send or Sync.
This handle encapsulates a RawWaker instance, which defines the
executor-specific wakeup behavior.
Local wakers can be requested from a Context with the local_waker method.
The typical life of a LocalWaker is that it is constructed by an executor, wrapped in a
Context using ContextBuilder, then passed to Future::poll(). Then, if the future chooses to return
Poll::Pending, it must also store the waker somehow and call LocalWaker::wake() when
the future should be polled again.
Implements Clone, but neither Send nor Sync; therefore, a local waker may
not be moved to other threads. In general, when deciding to use wakers or local wakers,
local wakers are preferable unless the waker needs to be sent across threads. This is because
wakers can incur in additional cost related to memory synchronization.
Note that it is preferable to use local_waker.clone_from(&new_waker) instead
of *local_waker = new_waker.clone(), as the former will avoid cloning the waker
unnecessarily if the two wakers wake the same task.
§Examples
Usage of a local waker to implement a future analogous to std::thread::yield_now().
#![feature(local_waker)]
use std::future::{Future, poll_fn};
use std::task::Poll;
// a future that returns pending once.
fn yield_now() -> impl Future<Output=()> + Unpin {
let mut yielded = false;
poll_fn(move |cx| {
if !yielded {
yielded = true;
cx.local_waker().wake_by_ref();
return Poll::Pending;
}
return Poll::Ready(())
})
}
yield_now().await;Implementations§
source§impl LocalWaker
impl LocalWaker
sourcepub fn wake(self)
🔬This is a nightly-only experimental API. (local_waker #118959)
pub fn wake(self)
local_waker #118959)Wakes up the task associated with this LocalWaker.
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 LocalWaker 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)
🔬This is a nightly-only experimental API. (local_waker #118959)
pub fn wake_by_ref(&self)
local_waker #118959)Wakes up the task associated with this LocalWaker without consuming the LocalWaker.
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: &LocalWaker) -> bool
🔬This is a nightly-only experimental API. (local_waker #118959)
pub fn will_wake(&self, other: &LocalWaker) -> bool
local_waker #118959)Returns true if this LocalWaker and another LocalWaker 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.
sourcepub const unsafe fn from_raw(waker: RawWaker) -> LocalWaker
🔬This is a nightly-only experimental API. (local_waker #118959)
pub const unsafe fn from_raw(waker: RawWaker) -> LocalWaker
local_waker #118959)Creates a new LocalWaker from RawWaker.
The behavior of the returned LocalWaker 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() -> &'static LocalWaker
🔬This is a nightly-only experimental API. (noop_waker #98286)
pub const fn noop() -> &'static LocalWaker
noop_waker #98286)Returns a reference to a LocalWaker 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 LocalWaker::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 LocalWaker is needed, clone() this one.
§Examples
#![feature(local_waker)]
#![feature(noop_waker)]
use std::future::Future;
use std::task::{ContextBuilder, LocalWaker, Waker, Poll};
let mut cx = ContextBuilder::from_waker(Waker::noop())
.local_waker(LocalWaker::noop())
.build();
let mut future = Box::pin(async { 10 });
assert_eq!(future.as_mut().poll(&mut cx), Poll::Ready(10));