pub trait Future {
type Output;
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.
Associated Types
Required methods
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 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 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.