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.
Constructing a Waker
from a RawWaker
is unsafe.
Implementing the Wake
trait is a safe alternative that requires memory allocation.
Implementations§
source§impl Waker
impl Waker
1.36.0 · sourcepub fn wake(self)
pub fn wake(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.
1.36.0 · sourcepub fn wake_by_ref(&self)
pub fn wake_by_ref(&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()
.
1.36.0 · 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 Waker
s would awaken the same task. However, if this function
returns true
, it is guaranteed that the Waker
s 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.
1.36.0 (const: 1.83.0) · sourcepub const unsafe fn from_raw(waker: RawWaker) -> Waker
pub const unsafe 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.)
sourcepub const fn noop() -> &'static Waker
🔬This is a nightly-only experimental API. (noop_waker
#98286)
pub const fn noop() -> &'static Waker
noop_waker
#98286)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
Trait Implementations§
source§impl AsRef<LocalWaker> for Waker
impl AsRef<LocalWaker> for Waker
source§fn as_ref(&self) -> &LocalWaker
fn as_ref(&self) -> &LocalWaker
1.36.0 · source§impl Clone for Waker
impl Clone for Waker
source§fn clone_from(&mut self, source: &Self)
fn clone_from(&mut self, source: &Self)
Assigns a clone of source
to self
, unless self.will_wake(source)
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 ...
}
}