[−][src]Trait futures_core::future::Future
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futures in libcore are unstable
A future represents an asychronous 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.
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 wake up is performed using
cx.waker()
, a handle for waking up the current task.
When using a future, you generally won't call poll
directly, but instead
await!
the value.
Associated Types
type Output
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futures in libcore are unstable
The result of the Future
.
Required Methods
fn poll(self: Pin<&mut Self>, lw: &LocalWaker) -> Poll<Self::Output>
🔬 This is a nightly-only experimental API. (futures_api
)
futures in libcore are unstable
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 LocalWaker
to be woken once the future can
make progress. For example, a future waiting for a socket to become
readable would call .clone()
on the LocalWaker
and store it.
When a signal arrives elsewhere indicating that the socket is readable,
[LocalWaker::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 most recent
LocalWaker
passed to poll
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 must
never 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.
LocalWaker
, Waker
and thread-safety
The poll
function takes a LocalWaker
, an object which knows how to
awaken the current task. LocalWaker
is not Send
nor Sync
, so in
order to make thread-safe futures the LocalWaker::into_waker
method
should be used to convert the LocalWaker
into a thread-safe version.
LocalWaker::wake
implementations have the ability to be more
efficient, however, so when thread safety is not necessary,
LocalWaker
should be preferred.
Panics
Once a future has completed (returned Ready
from poll
),
then any future calls to poll
may panic, block forever, or otherwise
cause bad behavior. The Future
trait itself provides no guarantees
about the behavior of poll
after a future has completed.
Implementations on Foreign Types
impl<'a, F> Future for AssertUnwindSafe<F> where
F: Future,
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impl<'a, F> Future for AssertUnwindSafe<F> where
F: Future,
type Output = <F as Future>::Output
🔬 This is a nightly-only experimental API. (futures_api
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futures in libcore are unstable
fn poll(
self: Pin<&mut AssertUnwindSafe<F>>,
lw: &LocalWaker
) -> Poll<<AssertUnwindSafe<F> as Future>::Output>
[src]
fn poll(
self: Pin<&mut AssertUnwindSafe<F>>,
lw: &LocalWaker
) -> Poll<<AssertUnwindSafe<F> as Future>::Output>
🔬 This is a nightly-only experimental API. (futures_api
)
futures in libcore are unstable
impl<'a, F> Future for &'a mut F where
F: Unpin + Future + ?Sized,
[src]
impl<'a, F> Future for &'a mut F where
F: Unpin + Future + ?Sized,
type Output = <F as Future>::Output
🔬 This is a nightly-only experimental API. (futures_api
)
futures in libcore are unstable
fn poll(
self: Pin<&mut &'a mut F>,
lw: &LocalWaker
) -> Poll<<&'a mut F as Future>::Output>
[src]
fn poll(
self: Pin<&mut &'a mut F>,
lw: &LocalWaker
) -> Poll<<&'a mut F as Future>::Output>
🔬 This is a nightly-only experimental API. (futures_api
)
futures in libcore are unstable
impl<P> Future for Pin<P> where
P: DerefMut,
<P as Deref>::Target: Future,
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impl<P> Future for Pin<P> where
P: DerefMut,
<P as Deref>::Target: Future,
type Output = <<P as Deref>::Target as Future>::Output
🔬 This is a nightly-only experimental API. (futures_api
)
futures in libcore are unstable
fn poll(
self: Pin<&mut Pin<P>>,
lw: &LocalWaker
) -> Poll<<Pin<P> as Future>::Output>
[src]
fn poll(
self: Pin<&mut Pin<P>>,
lw: &LocalWaker
) -> Poll<<Pin<P> as Future>::Output>
🔬 This is a nightly-only experimental API. (futures_api
)
futures in libcore are unstable
impl<F> Future for Box<F> where
F: Unpin + Future + ?Sized,
[src]
impl<F> Future for Box<F> where
F: Unpin + Future + ?Sized,
type Output = <F as Future>::Output
🔬 This is a nightly-only experimental API. (futures_api
)
futures in libcore are unstable
fn poll(
self: Pin<&mut Box<F>>,
lw: &LocalWaker
) -> Poll<<Box<F> as Future>::Output>
[src]
fn poll(
self: Pin<&mut Box<F>>,
lw: &LocalWaker
) -> Poll<<Box<F> as Future>::Output>
🔬 This is a nightly-only experimental API. (futures_api
)
futures in libcore are unstable
Implementors
impl<'a, T> Future for FutureObj<'a, T>
[src]
impl<'a, T> Future for FutureObj<'a, T>
type Output = T
🔬 This is a nightly-only experimental API. (futures_api
)
futures in libcore are unstable
fn poll(self: Pin<&mut Self>, lw: &LocalWaker) -> Poll<T>
[src]
fn poll(self: Pin<&mut Self>, lw: &LocalWaker) -> Poll<T>
🔬 This is a nightly-only experimental API. (futures_api
)
futures in libcore are unstable
impl<'a, T> Future for LocalFutureObj<'a, T>
[src]
impl<'a, T> Future for LocalFutureObj<'a, T>