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//! A synchronization primitive for task wakeup. use std::{cell::Cell, fmt, marker::PhantomData, rc, task::Waker}; /// A synchronization primitive for task wakeup. /// /// Sometimes the task interested in a given event will change over time. /// An `LocalWaker` can coordinate concurrent notifications with the consumer /// potentially "updating" the underlying task to wake up. This is useful in /// scenarios where a computation completes in another task and wants to /// notify the consumer, but the consumer is in the process of being migrated to /// a new logical task. /// /// Consumers should call `register` before checking the result of a computation /// and producers should call `wake` after producing the computation (this /// differs from the usual `thread::park` pattern). It is also permitted for /// `wake` to be called **before** `register`. This results in a no-op. /// /// A single `LocalWaker` may be reused for any number of calls to `register` or /// `wake`. #[derive(Default)] pub struct LocalWaker { waker: Cell<Option<Waker>>, _t: PhantomData<rc::Rc<()>>, } impl LocalWaker { /// Create an `LocalWaker`. pub fn new() -> Self { LocalWaker { waker: Cell::new(None), _t: PhantomData, } } #[inline] /// Registers the waker to be notified on calls to `wake`. /// /// Returns `true` if waker was registered before. pub fn register(&self, waker: &Waker) -> bool { self.waker.replace(Some(waker.clone())).is_some() } #[inline] /// Calls `wake` on the last `Waker` passed to `register`. /// /// If `register` has not been called yet, then this does nothing. pub fn wake(&self) { if let Some(waker) = self.take() { waker.wake(); } } /// Returns the last `Waker` passed to `register`, so that the user can wake it. /// /// If a waker has not been registered, this returns `None`. pub fn take(&self) -> Option<Waker> { self.waker.take() } } impl fmt::Debug for LocalWaker { fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result { write!(f, "LocalWaker") } }