1 //! Definition of the `PollFn` adapter combinator
2
3 use super::assert_future;
4 use core::fmt;
5 use core::pin::Pin;
6 use futures_core::future::Future;
7 use futures_core::task::{Context, Poll};
8
9 /// Future for the [`poll_fn`] function.
10 #[must_use = "futures do nothing unless you `.await` or poll them"]
11 pub struct PollFn<F> {
12 f: F,
13 }
14
15 impl<F> Unpin for PollFn<F> {}
16
17 /// Creates a new future wrapping around a function returning [`Poll`].
18 ///
19 /// Polling the returned future delegates to the wrapped function.
20 ///
21 /// # Examples
22 ///
23 /// ```
24 /// # futures::executor::block_on(async {
25 /// use futures::future::poll_fn;
26 /// use futures::task::{Context, Poll};
27 ///
28 /// fn read_line(_cx: &mut Context<'_>) -> Poll<String> {
29 /// Poll::Ready("Hello, World!".into())
30 /// }
31 ///
32 /// let read_future = poll_fn(read_line);
33 /// assert_eq!(read_future.await, "Hello, World!".to_owned());
34 /// # });
35 /// ```
poll_fn<T, F>(f: F) -> PollFn<F> where F: FnMut(&mut Context<'_>) -> Poll<T>36 pub fn poll_fn<T, F>(f: F) -> PollFn<F>
37 where
38 F: FnMut(&mut Context<'_>) -> Poll<T>
39 {
40 assert_future::<T, _>(PollFn { f })
41 }
42
43 impl<F> fmt::Debug for PollFn<F> {
fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result44 fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
45 f.debug_struct("PollFn").finish()
46 }
47 }
48
49 impl<T, F> Future for PollFn<F>
50 where F: FnMut(&mut Context<'_>) -> Poll<T>,
51 {
52 type Output = T;
53
poll(mut self: Pin<&mut Self>, cx: &mut Context<'_>) -> Poll<T>54 fn poll(mut self: Pin<&mut Self>, cx: &mut Context<'_>) -> Poll<T> {
55 (&mut self.f)(cx)
56 }
57 }
58