pub struct Notified<'a> { /* private fields */ }
Expand description
Future returned from Notify::notified()
.
This future is fused, so once it has completed, any future calls to poll
will immediately return Poll::Ready
.
Implementations§
Source§impl Notified<'_>
impl Notified<'_>
Sourcepub fn enable(self: Pin<&mut Self>) -> bool
pub fn enable(self: Pin<&mut Self>) -> bool
Adds this future to the list of futures that are ready to receive
wakeups from calls to notify_one
.
Polling the future also adds it to the list, so this method should only
be used if you want to add the future to the list before the first call
to poll
. (In fact, this method is equivalent to calling poll
except
that no Waker
is registered.)
This has no effect on notifications sent using notify_waiters
, which
are received as long as they happen after the creation of the Notified
regardless of whether enable
or poll
has been called.
This method returns true if the Notified
is ready. This happens in the
following situations:
- The
notify_waiters
method was called between the creation of theNotified
and the call to this method. - This is the first call to
enable
orpoll
on this future, and theNotify
was holding a permit from a previous call tonotify_one
. The call consumes the permit in that case. - The future has previously been enabled or polled, and it has since
then been marked ready by either consuming a permit from the
Notify
, or by a call tonotify_one
ornotify_waiters
that removed it from the list of futures ready to receive wakeups.
If this method returns true, any future calls to poll on the same future
will immediately return Poll::Ready
.
§Examples
Unbound multi-producer multi-consumer (mpmc) channel.
The call to enable
is important because otherwise if you have two
calls to recv
and two calls to send
in parallel, the following could
happen:
- Both calls to
try_recv
returnNone
. - Both new elements are added to the vector.
- The
notify_one
method is called twice, adding only a single permit to theNotify
. - Both calls to
recv
reach theNotified
future. One of them consumes the permit, and the other sleeps forever.
By adding the Notified
futures to the list by calling enable
before
try_recv
, the notify_one
calls in step three would remove the
futures from the list and mark them notified instead of adding a permit
to the Notify
. This ensures that both futures are woken.
use tokio::sync::Notify;
use std::collections::VecDeque;
use std::sync::Mutex;
struct Channel<T> {
messages: Mutex<VecDeque<T>>,
notify_on_sent: Notify,
}
impl<T> Channel<T> {
pub fn send(&self, msg: T) {
let mut locked_queue = self.messages.lock().unwrap();
locked_queue.push_back(msg);
drop(locked_queue);
// Send a notification to one of the calls currently
// waiting in a call to `recv`.
self.notify_on_sent.notify_one();
}
pub fn try_recv(&self) -> Option<T> {
let mut locked_queue = self.messages.lock().unwrap();
locked_queue.pop_front()
}
pub async fn recv(&self) -> T {
let future = self.notify_on_sent.notified();
tokio::pin!(future);
loop {
// Make sure that no wakeup is lost if we get
// `None` from `try_recv`.
future.as_mut().enable();
if let Some(msg) = self.try_recv() {
return msg;
}
// Wait for a call to `notify_one`.
//
// This uses `.as_mut()` to avoid consuming the future,
// which lets us call `Pin::set` below.
future.as_mut().await;
// Reset the future in case another call to
// `try_recv` got the message before us.
future.set(self.notify_on_sent.notified());
}
}
}