[−][src]Trait std::future::Future
🔬 This is a nightly-only experimental API. (futures_api #50547)
futures in libcore are unstable
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.
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 wake up is performed using
the waker argument of the poll() method, 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.
Associated Types
type Output
🔬 This is a nightly-only experimental API. (futures_api #50547)
futures in libcore are unstable
The type of value produced on completion.
Required methods
fn poll(self: Pin<&mut Self>, waker: &Waker) -> Poll<Self::Output>
🔬 This is a nightly-only experimental API. (futures_api #50547)
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::Pendingif the future is not ready yetPoll::Ready(val)with the resultvalof 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 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 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 most recent
Waker passed to poll 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 awhile, the work should be offloaded to a
thread pool (or something similar) to ensure that poll can return
quickly.
An implementation of poll may also never cause memory unsafety.
Panics
Once a future has completed (returned Ready from poll),
then any future calls to poll may panic, block forever, or otherwise
cause any kind of bad behavior except causing memory unsafety.
The Future trait itself provides no guarantees about the behavior
of poll after a future has completed.
Implementors
impl<'_, F> Future for &'_ mut F where
F: Unpin + Future + ?Sized, [src]
F: Unpin + Future + ?Sized,
type Output = <F as Future>::Output
🔬 This is a nightly-only experimental API. (futures_api #50547)
futures in libcore are unstable
fn poll(
self: Pin<&mut &'_ mut F>,
waker: &Waker
) -> Poll<<&'_ mut F as Future>::Output>[src]
self: Pin<&mut &'_ mut F>,
waker: &Waker
) -> Poll<<&'_ mut F as Future>::Output>
impl<F> Future for Box<F> where
F: Unpin + Future + ?Sized, [src]
F: Unpin + Future + ?Sized,
type Output = <F as Future>::Output
🔬 This is a nightly-only experimental API. (futures_api #50547)
futures in libcore are unstable
fn poll(
self: Pin<&mut Box<F>>,
waker: &Waker
) -> Poll<<Box<F> as Future>::Output>[src]
self: Pin<&mut Box<F>>,
waker: &Waker
) -> Poll<<Box<F> as Future>::Output>
impl<F: Future> Future for AssertUnwindSafe<F>[src]
type Output = F::Output
🔬 This is a nightly-only experimental API. (futures_api #50547)
futures in libcore are unstable
fn poll(self: Pin<&mut Self>, waker: &Waker) -> Poll<Self::Output>[src]
impl<P> Future for Pin<P> where
P: Unpin + DerefMut,
<P as Deref>::Target: Future, [src]
P: Unpin + DerefMut,
<P as Deref>::Target: Future,