Trait Future

pub trait Future {
    type Output;

    // Required method
    fn poll(self: Pin<&mut Self>, cx: &mut Context<'_>) -> Poll<Self::Output>;
}

A future represents an asynchronous computation obtained by use of async.

A future is a value that might 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.

The poll method

The 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.

Required Associated Types

type Output

The type of value produced on completion.

Required Methods

fn poll(self: Pin<&mut Self>, cx: &mut Context<'_>) -> Poll<Self::Output>

Attempts to resolve the future to a final value, registering the current task for wakeup if the value is not yet available.

Return value

This function returns:

• Poll::Pending if the future is not ready yet
• Poll::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.

Runtime characteristics

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 a while, the work should be offloaded to a thread pool (or something similar) to ensure that poll can return quickly.

Panics

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.

Implementors

impl Future for AcquireArc

type Output = SemaphoreGuardArc

impl Future for TimeoutFuture

type Output = ()

impl Future for event_listener::EventListener

type Output = ()

impl Future for YieldNow

type Output = ()

impl<'a> Future for BarrierWait<'a>

type Output = BarrierWaitResult

impl<'a> Future for Acquire<'a>

type Output = SemaphoreGuard<'a>

impl<'a, R> Future for FillBuf<'a, R>

where R: AsyncBufRead + Unpin + ?Sized,

type Output = Result<&'a [u8], Error>

impl<'a, T> Future for async_broadcast::Recv<'a, T>

where T: Clone,

type Output = Result<T, RecvError>

impl<'a, T> Future for async_broadcast::Send<'a, T>

where T: Clone,

type Output = Result<Option<T>, SendError<T>>

impl<'a, T> Future for async_channel::Recv<'a, T>

type Output = Result<T, RecvError>

impl<'a, T> Future for async_channel::Send<'a, T>

type Output = Result<(), SendError<T>>

impl<'a, T> Future for Lock<'a, T>

where T: ?Sized,

type Output = MutexGuard<'a, T>

impl<'a, T> Future for Read<'a, T>

where T: ?Sized,

type Output = RwLockReadGuard<'a, T>

impl<'a, T> Future for ReadArc<'a, T>

type Output = RwLockReadGuardArc<T>

impl<'a, T> Future for UpgradableRead<'a, T>

where T: ?Sized,

type Output = RwLockUpgradableReadGuard<'a, T>

impl<'a, T> Future for UpgradableReadArc<'a, T>

where T: ?Sized,

type Output = RwLockUpgradableReadGuardArc<T>

impl<'a, T> Future for Upgrade<'a, T>

where T: ?Sized,

type Output = RwLockWriteGuard<'a, T>

impl<'a, T> Future for Write<'a, T>

where T: ?Sized,

type Output = RwLockWriteGuard<'a, T>

impl<'a, T> Future for WriteArc<'a, T>

where T: ?Sized,

type Output = RwLockWriteGuardArc<T>

impl<'a, T, const STACK_SIZE: usize> Future for StackFuture<'a, T, STACK_SIZE>

type Output = T

impl<'s, 'l, F> Future for Readable<'s, 'l, F>

where F: AsFd,

type Output = ()

impl<'s, 'l, F> Future for Writable<'s, 'l, F>

where F: AsFd,

type Output = ()

impl<F1, F2> Future for Zip<F1, F2>

where F1: Future, F2: Future,

type Output = (<F1 as Future>::Output, <F2 as Future>::Output)

impl<F> Future for &mut F

where F: Future + Unpin + ?Sized,

type Output = <F as Future>::Output

impl<F> Future for FutureWrapper<F>

where F: EventListenerFuture + ?Sized,

type Output = <F as EventListenerFuture>::Output

impl<F> Future for CatchUnwind<F>

where F: Future + UnwindSafe,

Available on crate feature std only.

type Output = Result<<F as Future>::Output, Box<dyn Any + Send>>

impl<F> Future for AssertUnwindSafe<F>

where F: Future,

type Output = <F as Future>::Output

impl<F, A> Future for bevy::ecs::label::Box<F, A>

where F: Future + Unpin + ?Sized, A: Allocator,

type Output = <F as Future>::Output

impl<F, A> Future for Box<F, A>

where F: Future + Unpin + ?Sized, A: Allocator + 'static,

type Output = <F as Future>::Output

impl<Fut> Future for Fuse<Fut>

where Fut: Future,

type Output = <Fut as Future>::Output

impl<L, R> Future for Either<L, R>

where L: Future, R: Future<Output = <L as Future>::Output>,

Either<L, R> is a future if both L and R are futures.

type Output = <L as Future>::Output

impl<P> Future for Pin<P>

where P: DerefMut, <P as Deref>::Target: Future,

type Output = <<P as Deref>::Target as Future>::Output

impl<R> Future for ReadExactFuture<'_, R>

where R: AsyncRead + Unpin + ?Sized,

type Output = Result<(), Error>

impl<R> Future for ReadFuture<'_, R>

where R: AsyncRead + Unpin + ?Sized,

type Output = Result<usize, Error>

impl<R> Future for ReadLineFuture<'_, R>

where R: AsyncBufRead + Unpin + ?Sized,

type Output = Result<usize, Error>

impl<R> Future for ReadToEndFuture<'_, R>

where R: AsyncRead + Unpin + ?Sized,

type Output = Result<usize, Error>

impl<R> Future for ReadToStringFuture<'_, R>

where R: AsyncRead + Unpin + ?Sized,

type Output = Result<usize, Error>

impl<R> Future for ReadUntilFuture<'_, R>

where R: AsyncBufRead + Unpin + ?Sized,

type Output = Result<usize, Error>

impl<R> Future for ReadVectoredFuture<'_, R>

where R: AsyncRead + Unpin + ?Sized,

type Output = Result<usize, Error>

impl<S> Future for SeekForwardFuture<'_, S>

where S: AsyncSeekForward + Unpin + ?Sized,

type Output = Result<u64, Error>

impl<S> Future for SeekFuture<'_, S>

where S: AsyncSeek + Unpin + ?Sized,

type Output = Result<u64, Error>

impl<S> Future for CountFuture<S>

where S: Stream + ?Sized,

type Output = usize

impl<S> Future for LastFuture<S>

where S: Stream,

type Output = Option<<S as Stream>::Item>

impl<S> Future for NextFuture<'_, S>

where S: Stream + Unpin + ?Sized,

type Output = Option<<S as Stream>::Item>

impl<S> Future for NthFuture<'_, S>

where S: Stream + Unpin + ?Sized,

type Output = Option<<S as Stream>::Item>

impl<S, A, B, FromA, FromB> Future for UnzipFuture<S, FromA, FromB>

where S: Stream<Item = (A, B)>, FromA: Default + Extend<A>, FromB: Default + Extend<B>,

type Output = (FromA, FromB)

impl<S, B, F> Future for FindMapFuture<'_, S, F>

where S: Stream + Unpin + ?Sized, F: FnMut(<S as Stream>::Item) -> Option<B>,

type Output = Option<B>

impl<S, C> Future for CollectFuture<S, C>

where S: Stream, C: Default + Extend<<S as Stream>::Item>,

type Output = C

impl<S, F> Future for ForEachFuture<S, F>

where S: Stream, F: FnMut(<S as Stream>::Item),

type Output = ()

impl<S, F, E> Future for TryForEachFuture<'_, S, F>

where S: Stream + Unpin + ?Sized, F: FnMut(<S as Stream>::Item) -> Result<(), E>,

type Output = Result<(), E>

impl<S, F, T> Future for FoldFuture<S, F, T>

where S: Stream, F: FnMut(T, <S as Stream>::Item) -> T,

type Output = T

impl<S, P> Future for AllFuture<'_, S, P>

where S: Stream + Unpin + ?Sized, P: FnMut(<S as Stream>::Item) -> bool,

type Output = bool

impl<S, P> Future for AnyFuture<'_, S, P>

where S: Stream + Unpin + ?Sized, P: FnMut(<S as Stream>::Item) -> bool,

type Output = bool

impl<S, P> Future for FindFuture<'_, S, P>

where S: Stream + Unpin + ?Sized, P: FnMut(&<S as Stream>::Item) -> bool,

type Output = Option<<S as Stream>::Item>

impl<S, P> Future for PositionFuture<'_, S, P>

where S: Stream + Unpin + ?Sized, P: FnMut(<S as Stream>::Item) -> bool,

type Output = Option<usize>

impl<S, P, B> Future for PartitionFuture<S, P, B>

where S: Stream, P: FnMut(&<S as Stream>::Item) -> bool, B: Default + Extend<<S as Stream>::Item>,

type Output = (B, B)

impl<T1, T2, E, F1, F2> Future for TryZip<F1, T1, F2, T2>

where F1: Future<Output = Result<T1, E>>, F2: Future<Output = Result<T2, E>>,

type Output = Result<(T1, T2), E>

impl<T> Future for LockArc<T>

where T: ?Sized,

type Output = MutexGuardArc<T>

impl<T> Future for UpgradeArc<T>

where T: ?Sized,

type Output = RwLockWriteGuardArc<T>

impl<T> Future for event_listener::EventListener<T>

type Output = T

impl<T> Future for Instrumented<T>

where T: Future,

type Output = <T as Future>::Output

impl<T> Future for WithDispatch<T>

where T: Future,

Available on crate feature std only.

type Output = <T as Future>::Output

impl<T> Future for bevy::tasks::Task<T>

type Output = T

impl<T> Future for Pending<T>

type Output = T

impl<T> Future for Ready<T>

type Output = T

impl<T> Future for AsyncDropInPlace<T>

where T: ?Sized,

type Output = ()

impl<T> Future for Exclusive<T>

where T: Future + ?Sized,

type Output = <T as Future>::Output

impl<T, E, S> Future for TryNextFuture<'_, S>

where S: Stream<Item = Result<T, E>> + Unpin + ?Sized,

type Output = Result<Option<T>, E>

impl<T, E, S, C> Future for TryCollectFuture<S, C>

where S: Stream<Item = Result<T, E>>, C: Default + Extend<T>,

type Output = Result<C, E>

impl<T, E, S, F, B> Future for TryFoldFuture<'_, S, F, B>

where S: Stream<Item = Result<T, E>> + Unpin, F: FnMut(B, T) -> Result<B, E>,

type Output = Result<B, E>

impl<T, F1, F2> Future for Or<F1, F2>

where F1: Future<Output = T>, F2: Future<Output = T>,

type Output = T

impl<T, F1, F2> Future for Race<F1, F2>

where F1: Future<Output = T>, F2: Future<Output = T>,

Available on crate feature race only.

type Output = T

impl<T, F> Future for bevy::tasks::futures_lite::future::PollFn<F>

where F: FnMut(&mut Context<'_>) -> Poll<T>,

type Output = T

impl<T, F> Future for PollOnce<F>

where F: Future<Output = T>,

type Output = Option<T>

impl<T, F> Future for core::future::poll_fn::PollFn<F>

where F: FnMut(&mut Context<'_>) -> Poll<T>,

type Output = T

impl<T, M> Future for FallibleTask<T, M>

type Output = Option<T>

impl<T, M> Future for async_task::task::Task<T, M>

type Output = T

impl<W> Future for CloseFuture<'_, W>

where W: AsyncWrite + Unpin + ?Sized,

type Output = Result<(), Error>

impl<W> Future for FlushFuture<'_, W>

where W: AsyncWrite + Unpin + ?Sized,

type Output = Result<(), Error>

impl<W> Future for WriteAllFuture<'_, W>

where W: AsyncWrite + Unpin + ?Sized,

type Output = Result<(), Error>

impl<W> Future for WriteFuture<'_, W>

where W: AsyncWrite + Unpin + ?Sized,

type Output = Result<usize, Error>

impl<W> Future for WriteVectoredFuture<'_, W>

where W: AsyncWrite + Unpin + ?Sized,

type Output = Result<usize, Error>