| //! Definition of the MaybeDone combinator |
| |
| use super::assert_future; |
| use core::mem; |
| use core::pin::Pin; |
| use futures_core::future::{FusedFuture, Future}; |
| use futures_core::ready; |
| use futures_core::task::{Context, Poll}; |
| |
| /// A future that may have completed. |
| /// |
| /// This is created by the [`maybe_done()`] function. |
| #[derive(Debug)] |
| pub enum MaybeDone<Fut: Future> { |
| /// A not-yet-completed future |
| Future(/* #[pin] */ Fut), |
| /// The output of the completed future |
| Done(Fut::Output), |
| /// The empty variant after the result of a [`MaybeDone`] has been |
| /// taken using the [`take_output`](MaybeDone::take_output) method. |
| Gone, |
| } |
| |
| impl<Fut: Future + Unpin> Unpin for MaybeDone<Fut> {} |
| |
| /// Wraps a future into a `MaybeDone` |
| /// |
| /// # Examples |
| /// |
| /// ``` |
| /// # futures::executor::block_on(async { |
| /// use futures::future; |
| /// use futures::pin_mut; |
| /// |
| /// let future = future::maybe_done(async { 5 }); |
| /// pin_mut!(future); |
| /// assert_eq!(future.as_mut().take_output(), None); |
| /// let () = future.as_mut().await; |
| /// assert_eq!(future.as_mut().take_output(), Some(5)); |
| /// assert_eq!(future.as_mut().take_output(), None); |
| /// # }); |
| /// ``` |
| pub fn maybe_done<Fut: Future>(future: Fut) -> MaybeDone<Fut> { |
| assert_future::<(), _>(MaybeDone::Future(future)) |
| } |
| |
| impl<Fut: Future> MaybeDone<Fut> { |
| /// Returns an [`Option`] containing a mutable reference to the output of the future. |
| /// The output of this method will be [`Some`] if and only if the inner |
| /// future has been completed and [`take_output`](MaybeDone::take_output) |
| /// has not yet been called. |
| #[inline] |
| pub fn output_mut(self: Pin<&mut Self>) -> Option<&mut Fut::Output> { |
| unsafe { |
| match self.get_unchecked_mut() { |
| MaybeDone::Done(res) => Some(res), |
| _ => None, |
| } |
| } |
| } |
| |
| /// Attempt to take the output of a `MaybeDone` without driving it |
| /// towards completion. |
| #[inline] |
| pub fn take_output(self: Pin<&mut Self>) -> Option<Fut::Output> { |
| match &*self { |
| Self::Done(_) => {} |
| Self::Future(_) | Self::Gone => return None, |
| } |
| unsafe { |
| match mem::replace(self.get_unchecked_mut(), Self::Gone) { |
| MaybeDone::Done(output) => Some(output), |
| _ => unreachable!(), |
| } |
| } |
| } |
| } |
| |
| impl<Fut: Future> FusedFuture for MaybeDone<Fut> { |
| fn is_terminated(&self) -> bool { |
| match self { |
| Self::Future(_) => false, |
| Self::Done(_) | Self::Gone => true, |
| } |
| } |
| } |
| |
| impl<Fut: Future> Future for MaybeDone<Fut> { |
| type Output = (); |
| |
| fn poll(mut self: Pin<&mut Self>, cx: &mut Context<'_>) -> Poll<Self::Output> { |
| unsafe { |
| match self.as_mut().get_unchecked_mut() { |
| MaybeDone::Future(f) => { |
| let res = ready!(Pin::new_unchecked(f).poll(cx)); |
| self.set(Self::Done(res)); |
| } |
| MaybeDone::Done(_) => {} |
| MaybeDone::Gone => panic!("MaybeDone polled after value taken"), |
| } |
| } |
| Poll::Ready(()) |
| } |
| } |