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 poll
ing 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§
Required Methods§
sourcefn poll(self: Pin<&mut Self>, cx: &mut Context<'_>) -> Poll<Self::Output>
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.
Return value
This function returns:
Poll::Pending
if the future is not ready yetPoll::Ready(val)
with the resultval
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 poll
ed 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.
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 Implementations§
Implementors§
source§impl Future for PushedResponseFuture
impl Future for PushedResponseFuture
source§impl Future for ResponseFuture
impl Future for ResponseFuture
§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, 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, 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 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, S, P> Future for PositionFuture<'a, S, P>where S: Stream + Unpin + ?Sized, P: FnMut(<S as Stream>::Item) -> bool,
§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<'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>,
source§impl<B> Future for ReadySendRequest<B>where
B: Buf + 'static,
impl<B> Future for ReadySendRequest<B>where B: Buf + 'static,
type Output = Result<SendRequest<B>, Error>
§impl<F> Future for CatchUnwind<F>where
F: Future + UnwindSafe,
Available on crate feature std
only.
impl<F> Future for CatchUnwind<F>where F: Future + UnwindSafe,
std
only.§impl<Fut1, Fut2> Future for TryJoin<Fut1, Fut2>where
Fut1: TryFuture,
Fut2: TryFuture<Error = <Fut1 as TryFuture>::Error>,
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 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 OrElse<Fut1, Fut2, F>where TryFlattenErr<MapErr<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 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> 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 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> 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<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<Fut1, Fut2, Fut3, Fut4, Fut5> Future for TryJoin5<Fut1, Fut2, Fut3, Fut4, Fut5>where
Fut1: TryFuture,
Fut2: TryFuture<Error = <Fut1 as TryFuture>::Error>,
Fut3: TryFuture<Error = <Fut1 as TryFuture>::Error>,
Fut4: TryFuture<Error = <Fut1 as TryFuture>::Error>,
Fut5: TryFuture<Error = <Fut1 as TryFuture>::Error>,
impl<Fut1, Fut2, Fut3, Fut4, Fut5> Future for TryJoin5<Fut1, Fut2, Fut3, Fut4, Fut5>where Fut1: TryFuture, Fut2: TryFuture<Error = <Fut1 as TryFuture>::Error>, Fut3: TryFuture<Error = <Fut1 as TryFuture>::Error>, Fut4: TryFuture<Error = <Fut1 as TryFuture>::Error>, Fut5: TryFuture<Error = <Fut1 as TryFuture>::Error>,
§impl<Fut> Future for CatchUnwind<Fut>where
Fut: Future + UnwindSafe,
impl<Fut> Future for CatchUnwind<Fut>where Fut: Future + UnwindSafe,
§impl<Fut> Future for NeverError<Fut>where
Map<Fut, OkFn<Infallible>>: Future,
impl<Fut> Future for NeverError<Fut>where Map<Fut, OkFn<Infallible>>: Future,
type Output = <Map<Fut, OkFn<Infallible>> as Future>::Output
§impl<Fut, F, G> Future for MapOkOrElse<Fut, F, G>where
Map<IntoFuture<Fut>, ChainFn<MapOkFn<F>, ChainFn<MapErrFn<G>, MergeResultFn>>>: Future,
impl<Fut, F, G> Future for MapOkOrElse<Fut, F, G>where Map<IntoFuture<Fut>, ChainFn<MapOkFn<F>, ChainFn<MapErrFn<G>, MergeResultFn>>>: Future,
§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>,
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, C> Future for CollectFuture<S, C>where
S: Stream,
C: Default + Extend<<S as Stream>::Item>,
impl<S, C> Future for CollectFuture<S, C>where S: Stream, C: Default + Extend<<S as Stream>::Item>,
§impl<S, F, T> Future for FoldFuture<S, F, T>where
S: Stream,
F: FnMut(T, <S as Stream>::Item) -> T,
impl<S, F, T> Future for FoldFuture<S, F, T>where S: Stream, F: FnMut(T, <S as Stream>::Item) -> T,
§impl<S, P> Future for AllFuture<'_, S, P>where
S: Stream + Unpin + ?Sized,
P: FnMut(<S as Stream>::Item) -> bool,
impl<S, P> Future for AllFuture<'_, S, P>where S: Stream + Unpin + ?Sized, P: FnMut(<S as Stream>::Item) -> bool,
§impl<S, P> Future for AnyFuture<'_, S, P>where
S: Stream + Unpin + ?Sized,
P: FnMut(<S as Stream>::Item) -> bool,
impl<S, P> Future for AnyFuture<'_, S, P>where S: Stream + Unpin + ?Sized, P: FnMut(<S as Stream>::Item) -> bool,
§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>,
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>,
§impl<Si, St, Ok, Error> Future for SendAll<'_, Si, St>where
Si: Sink<Ok, Error = Error> + Unpin + ?Sized,
St: Stream<Item = Result<Ok, Error>> + Unpin + ?Sized,
impl<Si, St, Ok, Error> Future for SendAll<'_, Si, St>where Si: Sink<Ok, Error = Error> + Unpin + ?Sized, St: Stream<Item = Result<Ok, Error>> + Unpin + ?Sized,
§impl<St> Future for Concat<St>where
St: Stream,
<St as Stream>::Item: Extend<<<St as Stream>::Item as IntoIterator>::Item> + IntoIterator + Default,
impl<St> Future for Concat<St>where St: Stream, <St as Stream>::Item: Extend<<<St as Stream>::Item as IntoIterator>::Item> + IntoIterator + Default,
§impl<St> Future for TryConcat<St>where
St: TryStream,
<St as TryStream>::Ok: Extend<<<St as TryStream>::Ok as IntoIterator>::Item> + IntoIterator + Default,
impl<St> Future for TryConcat<St>where St: TryStream, <St as TryStream>::Ok: Extend<<<St as TryStream>::Ok as IntoIterator>::Item> + IntoIterator + Default,
§impl<St, A, B, FromA, FromB> Future for Unzip<St, FromA, FromB>where
St: Stream<Item = (A, B)>,
FromA: Default + Extend<A>,
FromB: Default + Extend<B>,
impl<St, A, B, FromA, FromB> Future for Unzip<St, FromA, FromB>where St: Stream<Item = (A, B)>, FromA: Default + Extend<A>, FromB: Default + Extend<B>,
type Output = (FromA, FromB)
§impl<St, C> Future for TryCollect<St, C>where
St: TryStream,
C: Default + Extend<<St as TryStream>::Ok>,
impl<St, C> Future for TryCollect<St, C>where St: TryStream, C: Default + Extend<<St as TryStream>::Ok>,
§impl<St, F> Future for NextIf<'_, St, F>where
St: Stream,
F: for<'a> FnOnce1<&'a <St as Stream>::Item, Output = bool>,
impl<St, F> Future for NextIf<'_, St, F>where St: Stream, F: for<'a> FnOnce1<&'a <St as Stream>::Item, Output = bool>,
§impl<St, Fut, F> Future for ForEach<St, Fut, F>where
St: Stream,
F: FnMut(<St as Stream>::Item) -> Fut,
Fut: Future<Output = ()>,
impl<St, Fut, F> Future for ForEach<St, Fut, F>where St: Stream, F: FnMut(<St as Stream>::Item) -> Fut, Fut: Future<Output = ()>,
§impl<St, Fut, F> Future for ForEachConcurrent<St, Fut, F>where
St: Stream,
F: FnMut(<St as Stream>::Item) -> Fut,
Fut: Future<Output = ()>,
impl<St, Fut, F> Future for ForEachConcurrent<St, Fut, F>where St: Stream, F: FnMut(<St as Stream>::Item) -> Fut, Fut: Future<Output = ()>,
§impl<St, Fut, F> Future for TryForEach<St, Fut, F>where
St: TryStream,
F: FnMut(<St as TryStream>::Ok) -> Fut,
Fut: TryFuture<Ok = (), Error = <St as TryStream>::Error>,
impl<St, Fut, F> Future for TryForEach<St, Fut, F>where St: TryStream, F: FnMut(<St as TryStream>::Ok) -> Fut, Fut: TryFuture<Ok = (), Error = <St as TryStream>::Error>,
§impl<St, Fut, F> Future for TryForEachConcurrent<St, Fut, F>where
St: TryStream,
F: FnMut(<St as TryStream>::Ok) -> Fut,
Fut: Future<Output = Result<(), <St as TryStream>::Error>>,
impl<St, Fut, F> Future for TryForEachConcurrent<St, Fut, F>where St: TryStream, F: FnMut(<St as TryStream>::Ok) -> Fut, Fut: Future<Output = Result<(), <St as TryStream>::Error>>,
§impl<St, Fut, T, F> Future for Fold<St, Fut, T, F>where
St: Stream,
F: FnMut(T, <St as Stream>::Item) -> Fut,
Fut: Future<Output = T>,
impl<St, Fut, T, F> Future for Fold<St, Fut, T, F>where St: Stream, F: FnMut(T, <St as Stream>::Item) -> Fut, Fut: Future<Output = T>,
§impl<St, Fut, T, F> Future for TryFold<St, Fut, T, F>where
St: TryStream,
F: FnMut(T, <St as TryStream>::Ok) -> Fut,
Fut: TryFuture<Ok = T, Error = <St as TryStream>::Error>,
impl<St, Fut, T, F> Future for TryFold<St, Fut, T, F>where St: TryStream, F: FnMut(T, <St as TryStream>::Ok) -> Fut, Fut: TryFuture<Ok = T, Error = <St as TryStream>::Error>,
§impl<St, Si> Future for Forward<St, Si>where
Forward<St, Si, <St as TryStream>::Ok>: Future,
St: TryStream,
impl<St, Si> Future for Forward<St, Si>where Forward<St, Si, <St as TryStream>::Ok>: Future, St: TryStream,
§impl<St, T> Future for NextIfEq<'_, St, T>where
St: Stream,
<St as Stream>::Item: PartialEq<T>,
T: ?Sized,
impl<St, T> Future for NextIfEq<'_, St, T>where St: Stream, <St as Stream>::Item: PartialEq<T>, T: ?Sized,
§impl<T1, T2, E, F1, F2> Future for TryZip<F1, F2>where
F1: Future<Output = Result<T1, E>>,
F2: Future<Output = Result<T2, E>>,
impl<T1, T2, E, F1, F2> Future for TryZip<F1, F2>where F1: Future<Output = Result<T1, E>>, F2: Future<Output = Result<T2, E>>,
source§impl<T> Future for WithDispatch<T>where
T: Future,
Available on crate feature std
only.
impl<T> Future for WithDispatch<T>where T: Future,
std
only.source§impl<T, B> Future for Connection<T, B>where
T: AsyncRead + AsyncWrite + Unpin,
B: Buf + 'static,
impl<T, B> Future for Connection<T, B>where T: AsyncRead + AsyncWrite + Unpin, B: Buf + 'static,
source§impl<T, B> Future for Handshake<T, B>where
B: Buf + 'static,
T: AsyncRead + AsyncWrite + Unpin,
impl<T, B> Future for Handshake<T, B>where B: Buf + 'static, T: AsyncRead + AsyncWrite + Unpin,
type Output = Result<Connection<T, B>, Error>
§impl<T, E, S> Future for TryNextFuture<'_, S>where
S: Stream<Item = Result<T, E>> + Unpin + ?Sized,
impl<T, E, S> Future for TryNextFuture<'_, S>where S: Stream<Item = Result<T, E>> + Unpin + ?Sized,
§impl<T, E, S, C> Future for TryCollectFuture<S, C>where
S: Stream<Item = Result<T, E>>,
C: Default + Extend<T>,
impl<T, E, S, C> Future for TryCollectFuture<S, C>where S: Stream<Item = Result<T, E>>, C: Default + Extend<T>,
§impl<T, F1, F2> Future for Race<F1, F2>where
F1: Future<Output = T>,
F2: Future<Output = T>,
Available on crate feature std
only.
impl<T, F1, F2> Future for Race<F1, F2>where F1: Future<Output = T>, F2: Future<Output = T>,
std
only.