Struct serde::lib::core::task::LocalWaker

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#[repr(transparent)]
pub struct LocalWaker { waker: RawWaker, }
🔬This is a nightly-only experimental API. (local_waker)
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;

Fields§

§waker: RawWaker
🔬This is a nightly-only experimental API. (local_waker)

Implementations§

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impl LocalWaker

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pub fn wake(self)

🔬This is a nightly-only experimental API. (local_waker)

Wake 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.

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pub fn wake_by_ref(&self)

🔬This is a nightly-only experimental API. (local_waker)

Wake 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().

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pub fn will_wake(&self, other: &LocalWaker) -> bool

🔬This is a nightly-only experimental API. (local_waker)

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.

const: unstable · source

pub unsafe fn from_raw(waker: RawWaker) -> LocalWaker

🔬This is a nightly-only experimental API. (local_waker)

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.

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pub const fn noop() -> &'static LocalWaker

🔬This is a nightly-only experimental API. (noop_waker)

Creates a new LocalWaker 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(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));
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pub fn as_raw(&self) -> &RawWaker

🔬This is a nightly-only experimental API. (waker_getters)

Get a reference to the underlying RawWaker.

Trait Implementations§

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impl AsRef<LocalWaker> for Waker

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fn as_ref(&self) -> &LocalWaker

Converts this type into a shared reference of the (usually inferred) input type.
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impl Clone for LocalWaker

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fn clone(&self) -> LocalWaker

Returns a copy of the value. Read more
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fn clone_from(&mut self, source: &LocalWaker)

Performs copy-assignment from source. Read more
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impl Debug for LocalWaker

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

Formats the value using the given formatter. Read more
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impl Drop for LocalWaker

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fn drop(&mut self)

Executes the destructor for this type. Read more
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impl<W> From<Rc<W>> for LocalWaker
where W: LocalWake + 'static,

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fn from(waker: Rc<W>) -> LocalWaker

Use a Wake-able type as a LocalWaker.

No heap allocations or atomic operations are used for this conversion.

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impl !Send for LocalWaker

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impl !Sync for LocalWaker

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impl Unpin for LocalWaker

Auto Trait Implementations§

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impl<T> Any for T
where T: 'static + ?Sized,

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fn type_id(&self) -> TypeId

Gets the TypeId of self. Read more
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impl<T> Borrow<T> for T
where T: ?Sized,

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fn borrow(&self) -> &T

Immutably borrows from an owned value. Read more
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impl<T> BorrowMut<T> for T
where T: ?Sized,

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

Mutably borrows from an owned value. Read more
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impl<T> From<T> for T

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fn from(t: T) -> T

Returns the argument unchanged.

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impl<T, U> Into<U> for T
where U: From<T>,

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fn into(self) -> U

Calls U::from(self).

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

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impl<T> ToOwned for T
where T: Clone,

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type Owned = T

The resulting type after obtaining ownership.
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fn to_owned(&self) -> T

Creates owned data from borrowed data, usually by cloning. Read more
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fn clone_into(&self, target: &mut T)

Uses borrowed data to replace owned data, usually by cloning. Read more
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impl<T, U> TryFrom<U> for T
where U: Into<T>,

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type Error = Infallible

The type returned in the event of a conversion error.
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fn try_from(value: U) -> Result<T, <T as TryFrom<U>>::Error>

Performs the conversion.
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impl<T, U> TryInto<U> for T
where U: TryFrom<T>,

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type Error = <U as TryFrom<T>>::Error

The type returned in the event of a conversion error.
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fn try_into(self) -> Result<U, <U as TryFrom<T>>::Error>

Performs the conversion.