#[must_use = "futures do nothing unless you `.await` or poll them"]
#[lang = "future_trait"]pub trait Future {
type Output;
#[lang = "poll"] fn poll(self: Pin<&mut Self>, cx: &mut Context<'_>) -> Poll<Self::Output>;
}
A future represents an asynchronous computation.
A future is a value that may not have finished computing yet. This kind of "asynchronous value" makes it possible for a thread to continue doing useful work while it waits for the value to become available.
poll methodThe core method of future, poll, attempts to resolve the future into a final value. This method does not block if the value is not ready. Instead, the current task is scheduled to be woken up when it's possible to make further progress by polling again. The context passed to the poll method can provide a Waker, which is a handle for waking up the current task.
When using a future, you generally won't call poll directly, but instead .await the value.
type OutputThe type of value produced on completion.
#[lang = "poll"]fn poll(self: Pin<&mut Self>, cx: &mut Context<'_>) -> Poll<Self::Output>Attempt to resolve the future to a final value, registering the current task for wakeup if the value is not yet available.
This function returns:
Poll::Pending if the future is not ready yetPoll::Ready(val) with the result val of this future if it finished successfully.Once a future has finished, clients should not poll it again.
When a future is not ready yet, poll returns Poll::Pending and stores a clone of the Waker copied from the current Context. This Waker is then woken once the future can make progress. For example, a future waiting for a socket to become readable would call .clone() on the Waker and store it. When a signal arrives elsewhere indicating that the socket is readable, Waker::wake is called and the socket future's task is awoken. Once a task has been woken up, it should attempt to poll the future again, which may or may not produce a final value.
Note that on multiple calls to poll, only the Waker from the Context passed to the most recent call should be scheduled to receive a wakeup.
Futures alone are inert; they must be actively polled to make progress, meaning that each time the current task is woken up, it should actively re-poll pending futures that it still has an interest in.
The poll function is not called repeatedly in a tight loop -- instead, it should only be called when the future indicates that it is ready to make progress (by calling wake()). If you're familiar with the poll(2) or select(2) syscalls on Unix it's worth noting that futures typically do not suffer the same problems of "all wakeups must poll all events"; they are more like epoll(4).
An implementation of poll should strive to return quickly, and should not block. Returning quickly prevents unnecessarily clogging up threads or event loops. If it is known ahead of time that a call to poll may end up taking awhile, the work should be offloaded to a thread pool (or something similar) to ensure that poll can return quickly.
Once a future has completed (returned Ready from poll), calling its poll method again may panic, block forever, or cause other kinds of problems; the Future trait places no requirements on the effects of such a call. However, as the poll method is not marked unsafe, Rust's usual rules apply: calls must never cause undefined behavior (memory corruption, incorrect use of unsafe functions, or the like), regardless of the future's state.
impl<T, F> Future for PollFn<F> where
F: FnMut(&mut Context<'_>) -> Poll<T>, [src]
type Output = Tfn poll(self: Pin<&mut PollFn<F>>, cx: &mut Context<'_>) -> Poll<T>[src]
impl<'_, F> Future for &'_ mut F where
F: Unpin + Future + ?Sized, [src]
type Output = <F as Future>::Outputfn poll(
self: Pin<&mut &'_ mut F>,
cx: &mut Context<'_>
) -> Poll<<&'_ mut F as Future>::Output>[src]
impl<F> Future for Box<F> where
F: Unpin + Future + ?Sized, [src]
type Output = <F as Future>::Outputfn poll(
self: Pin<&mut Box<F>>,
cx: &mut Context<'_>
) -> Poll<<Box<F> as Future>::Output>[src]
impl<F: Future> Future for AssertUnwindSafe<F>[src]
type Output = F::Outputfn poll(self: Pin<&mut Self>, cx: &mut Context<'_>) -> Poll<Self::Output>[src]
impl<P> Future for Pin<P> where
P: Unpin + DerefMut,
<P as Deref>::Target: Future, [src]
type Output = <<P as Deref>::Target as Future>::Outputfn poll(
self: Pin<&mut Pin<P>>,
cx: &mut Context<'_>
) -> Poll<<Pin<P> as Future>::Output>[src]
impl<T> Future for Pending<T>[src]
impl<T> Future for Ready<T>[src]
© 2010 The Rust Project Developers
Licensed under the Apache License, Version 2.0 or the MIT license, at your option.
https://doc.rust-lang.org/std/future/trait.Future.html