Trait core::future::Future [−][src]
pub trait Future {
type Output;
fn poll(self: PinMut<Self>, cx: &mut Context) -> Poll<Self::Output>;
}🔬 This is a nightly-only experimental API. (futures_api #50547)
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 polling 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
🔬 This is a nightly-only experimental API. (futures_api #50547)
futures in libcore are unstable
The result of the Future.
Required Methods
fn poll(self: PinMut<Self>, cx: &mut Context) -> 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. The future will also register the
interest of the current task in the value being produced. For example,
if the future represents the availability of data on a socket, then the
task is recorded so that when data arrives, it is woken up (via
cx.waker()). 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 if Pending is returned it only means that the current task
(represented by the argument cx) will receive a notification. Tasks
from previous calls to poll will not receive notifications.
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 for
futures, but only whenever the future itself is ready, as signaled via
the Waker inside task::Context. 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.
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.