Trait Future

1.36.0 · source

pub trait Future {
    type Output;

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

Expand description

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§

1.36.0 · source

type Output

The type of value produced on completion.

Required Methods§

1.36.0 · source

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.

Trait FutureCopy item path

§The poll method

Required Associated Types§

type Output

Required Methods§

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

§Return value

§Runtime characteristics

§Panics

Trait Implementations§

impl<'a, T> UnsafeFutureObj<'a, T> for &'a mut (dyn Future<Output = T> + Unpin)

fn into_raw(self) -> *mut dyn Future<Output = T> + 'a

unsafe fn drop(_ptr: *mut dyn Future<Output = T> + 'a)

impl<'a, T> UnsafeFutureObj<'a, T> for Box<dyn Future<Output = T> + 'a>where T: 'a,

fn into_raw(self) -> *mut dyn Future<Output = T> + 'a

unsafe fn drop(ptr: *mut dyn Future<Output = T> + 'a)

impl<'a, T> UnsafeFutureObj<'a, T> for Box<dyn Future<Output = T> + Send + 'a>where T: 'a,

fn into_raw(self) -> *mut dyn Future<Output = T> + 'a

unsafe fn drop(ptr: *mut dyn Future<Output = T> + 'a)

impl<'a, T> UnsafeFutureObj<'a, T> for Pin<&'a mut dyn Future<Output = T>>

fn into_raw(self) -> *mut dyn Future<Output = T> + 'a

unsafe fn drop(_ptr: *mut dyn Future<Output = T> + 'a)

Implementors§

impl Future for TimeoutFuture

type Output = ()

impl Future for EventListener

type Output = ()

impl Future for YieldNow

type Output = ()

impl Future for Notified<'_>

type Output = ()

impl Future for LocalSet

type Output = ()

impl Future for Sleep

type Output = ()

impl<'a, R> Future for futures_lite::io::FillBuf<'a, R>where R: AsyncBufRead + Unpin + ?Sized,

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

impl<'a, R> Future for cosmic::iced_winit::graphics::futures::futures::io::FillBuf<'a, R>where R: AsyncBufRead + Unpin + ?Sized,

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

impl<'a, S> Future for NthFuture<'a, S>where S: Stream + Unpin + ?Sized,

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

impl<'a, S> Future for ordered_stream::adapters::Next<'a, S>where S: OrderedStream + ?Sized,

type Output = Option<<S as OrderedStream>::Data>

impl<'a, S> Future for NextBefore<'a, S>where S: OrderedStream + ?Sized,

type Output = PollResult<<S as OrderedStream>::Ordering, <S as OrderedStream>::Data>

impl<'a, S, B, F> Future for FindMapFuture<'a, S, F>where S: Stream + Unpin + ?Sized, F: FnMut(<S as Stream>::Item) -> Option<B>,

type Output = Option<B>

impl<'a, S, F, E> Future for TryForEachFuture<'a, S, F>where S: Stream + Unpin + ?Sized, F: FnMut(<S as Stream>::Item) -> Result<(), E>,

type Output = Result<(), E>

impl<'a, S, P> Future for FindFuture<'a, S, P>where S: Stream + Unpin + ?Sized, P: FnMut(&<S as Stream>::Item) -> bool,

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

impl<'a, S, P> Future for PositionFuture<'a, S, P>where S: Stream + Unpin + ?Sized, P: FnMut(<S as Stream>::Item) -> bool,

type Output = Option<usize>

impl<'a, St> Future for Peek<'a, St>where St: Stream,

type Output = Option<&'a <St as Stream>::Item>

impl<'a, St> Future for PeekMut<'a, St>where St: Stream,

type Output = Option<&'a mut <St as Stream>::Item>

impl<'a, T> Future for 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 MutexLockFuture<'a, T>where T: ?Sized,

type Output = MutexGuard<'a, T>

impl<'a, T, E, S, F, B> Future for TryFoldFuture<'a, S, F, B>where S: Stream<Item = Result<T, E>> + Unpin, F: FnMut(B, T) -> Result<B, E>,

type Output = Result<B, E>

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<A> Future for ReadToEnd<'_, A>where A: AsyncRead + Unpin + ?Sized,

type Output = Result<usize, Error>

impl<A> Future for ReadToString<'_, A>where A: AsyncRead + Unpin + ?Sized,

type Output = Result<usize, Error>

impl<A, B> Future for cosmic::iced_winit::graphics::futures::futures::prelude::future::Either<A, B>where A: Future, B: Future<Output = <A as Future>::Output>,

type Output = <A as Future>::Output

impl<A, B> Future for Select<A, B>where A: Future + Unpin, B: Future + Unpin,

type Output = Either<(<A as Future>::Output, B), (<B as Future>::Output, A)>

impl<A, B> Future for TrySelect<A, B>where A: Unpin + TryFuture, B: Unpin + TryFuture,

type Output = Result<Either<(<A as TryFuture>::Ok, B), (<B as TryFuture>::Ok, A)>, Either<(<A as TryFuture>::Error, B), (<B as TryFuture>::Error, A)>>

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

Trait Future

§The `poll` method

impl<'a, T> UnsafeFutureObj<'a, T> for Box<dyn Future<Output = T> + 'a>
where T: 'a,

impl<'a, T> UnsafeFutureObj<'a, T> for Box<dyn Future<Output = T> + Send + 'a>
where T: 'a,

impl<'a, R> Future for futures_lite::io::FillBuf<'a, R>
where R: AsyncBufRead + Unpin + ?Sized,

impl<'a, R> Future for cosmic::iced_winit::graphics::futures::futures::io::FillBuf<'a, R>
where R: AsyncBufRead + Unpin + ?Sized,

impl<'a, S> Future for NthFuture<'a, S>
where S: Stream + Unpin + ?Sized,

impl<'a, S> Future for ordered_stream::adapters::Next<'a, S>
where S: OrderedStream + ?Sized,

impl<'a, S> Future for NextBefore<'a, S>
where S: OrderedStream + ?Sized,

impl<'a, S, B, F> Future for FindMapFuture<'a, S, F>
where S: Stream + Unpin + ?Sized, F: FnMut(<S as Stream>::Item) -> Option<B>,

impl<'a, S, F, E> Future for TryForEachFuture<'a, S, F>
where S: Stream + Unpin + ?Sized, F: FnMut(<S as Stream>::Item) -> Result<(), E>,

impl<'a, S, P> Future for FindFuture<'a, S, P>
where S: Stream + Unpin + ?Sized, P: FnMut(&<S as Stream>::Item) -> bool,

impl<'a, S, P> Future for PositionFuture<'a, S, P>
where S: Stream + Unpin + ?Sized, P: FnMut(<S as Stream>::Item) -> bool,

impl<'a, St> Future for Peek<'a, St>
where St: Stream,

impl<'a, St> Future for PeekMut<'a, St>
where St: Stream,

impl<'a, T> Future for Recv<'a, T>
where T: Clone,

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

impl<'a, T> Future for MutexLockFuture<'a, T>
where T: ?Sized,

impl<'a, T, E, S, F, B> Future for TryFoldFuture<'a, S, F, B>
where S: Stream<Item = Result<T, E>> + Unpin, F: FnMut(B, T) -> Result<B, E>,

impl<'s, 'l, F> Future for Readable<'s, 'l, F>
where F: AsFd,

impl<'s, 'l, F> Future for Writable<'s, 'l, F>
where F: AsFd,

impl<A> Future for ReadToEnd<'_, A>
where A: AsyncRead + Unpin + ?Sized,

impl<A> Future for ReadToString<'_, A>
where A: AsyncRead + Unpin + ?Sized,

impl<A, B> Future for cosmic::iced_winit::graphics::futures::futures::prelude::future::Either<A, B>
where A: Future, B: Future<Output = <A as Future>::Output>,

impl<A, B> Future for Select<A, B>
where A: Future + Unpin, B: Future + Unpin,

impl<A, B> Future for TrySelect<A, B>
where A: Unpin + TryFuture, B: Unpin + TryFuture,

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

impl<F> Future for &mut F
where F: Future + Unpin + ?Sized,

impl<F> Future for futures_lite::future::CatchUnwind<F>
where F: Future + UnwindSafe,

impl<F> Future for Unconstrained<F>
where F: Future,

impl<F> Future for Flatten<F>
where Flatten<F, <F as Future>::Output>: Future, F: Future,

impl<F> Future for JoinAll<F>
where F: Future,

impl<F> Future for OptionFuture<F>
where F: Future,

impl<F> Future for TryJoinAll<F>
where F: TryFuture,

impl<F> Future for AssertUnwindSafe<F>
where F: Future,

impl<F, A> Future for Box<F, A>
where F: Future + Unpin + ?Sized, A: Allocator,

impl<F, R> Future for Lazy<F>
where F: FnOnce(&mut Context<'_>) -> R,

impl<Fut1, Fut2> Future for Join<Fut1, Fut2>
where Fut1: Future, Fut2: Future,

impl<Fut1, Fut2> Future for TryFlatten<Fut1, Fut2>
where TryFlatten<Fut1, Fut2>: Future,

impl<Fut1, Fut2> Future for TryJoin<Fut1, Fut2>
where Fut1: TryFuture, Fut2: TryFuture<Error = <Fut1 as TryFuture>::Error>,

impl<Fut1, Fut2, F> Future for AndThen<Fut1, Fut2, F>
where TryFlatten<MapOk<Fut1, F>, Fut2>: Future,

impl<Fut1, Fut2, F> Future for OrElse<Fut1, Fut2, F>
where TryFlattenErr<MapErr<Fut1, F>, Fut2>: Future,

impl<Fut1, Fut2, F> Future for Then<Fut1, Fut2, F>
where Flatten<Map<Fut1, F>, Fut2>: Future,

impl<Fut1, Fut2, Fut3> Future for Join3<Fut1, Fut2, Fut3>
where Fut1: Future, Fut2: Future, Fut3: Future,

impl<Fut1, Fut2, Fut3> Future for TryJoin3<Fut1, Fut2, Fut3>
where Fut1: TryFuture, Fut2: TryFuture<Error = <Fut1 as TryFuture>::Error>, Fut3: TryFuture<Error = <Fut1 as TryFuture>::Error>,

impl<Fut1, Fut2, Fut3, Fut4> Future for Join4<Fut1, Fut2, Fut3, Fut4>
where Fut1: Future, Fut2: Future, Fut3: Future, Fut4: Future,

impl<Fut1, Fut2, Fut3, Fut4> Future for TryJoin4<Fut1, Fut2, Fut3, Fut4>
where Fut1: TryFuture, Fut2: TryFuture<Error = <Fut1 as TryFuture>::Error>, Fut3: TryFuture<Error = <Fut1 as TryFuture>::Error>, Fut4: TryFuture<Error = <Fut1 as TryFuture>::Error>,

impl<Fut1, Fut2, Fut3, Fut4, Fut5> Future for Join5<Fut1, Fut2, Fut3, Fut4, Fut5>
where Fut1: Future, Fut2: Future, Fut3: Future, Fut4: Future, Fut5: Future,

impl<Fut> Future for MaybeDone<Fut>
where Fut: Future,

impl<Fut> Future for TryMaybeDone<Fut>
where Fut: TryFuture,

impl<Fut> Future for cosmic::iced_winit::graphics::futures::futures::prelude::future::CatchUnwind<Fut>
where Fut: Future + UnwindSafe,

impl<Fut> Future for Fuse<Fut>
where Fut: Future,

impl<Fut> Future for IntoFuture<Fut>
where Fut: TryFuture,

impl<Fut> Future for NeverError<Fut>
where Map<Fut, OkFn<Infallible>>: Future,

impl<Fut> Future for Remote<Fut>
where Fut: Future,

impl<Fut> Future for SelectAll<Fut>
where Fut: Future + Unpin,

impl<Fut> Future for SelectOk<Fut>
where Fut: TryFuture + Unpin,