1 //! Definition of the `PollFn` combinator
2
3 use super::assert_stream;
4 use core::fmt;
5 use core::pin::Pin;
6 use futures_core::stream::Stream;
7 use futures_core::task::{Context, Poll};
8
9 /// Stream for the [`poll_fn`] function.
10 #[must_use = "streams do nothing unless polled"]
11 pub struct PollFn<F> {
12 f: F,
13 }
14
15 impl<F> Unpin for PollFn<F> {}
16
17 impl<F> fmt::Debug for PollFn<F> {
fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result18 fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
19 f.debug_struct("PollFn").finish()
20 }
21 }
22
23 /// Creates a new stream wrapping a function returning `Poll<Option<T>>`.
24 ///
25 /// Polling the returned stream calls the wrapped function.
26 ///
27 /// # Examples
28 ///
29 /// ```
30 /// use futures::stream::poll_fn;
31 /// use futures::task::Poll;
32 ///
33 /// let mut counter = 1usize;
34 ///
35 /// let read_stream = poll_fn(move |_| -> Poll<Option<String>> {
36 /// if counter == 0 { return Poll::Ready(None); }
37 /// counter -= 1;
38 /// Poll::Ready(Some("Hello, World!".to_owned()))
39 /// });
40 /// ```
poll_fn<T, F>(f: F) -> PollFn<F> where F: FnMut(&mut Context<'_>) -> Poll<Option<T>>,41 pub fn poll_fn<T, F>(f: F) -> PollFn<F>
42 where
43 F: FnMut(&mut Context<'_>) -> Poll<Option<T>>,
44 {
45 assert_stream::<T, _>(PollFn { f })
46 }
47
48 impl<T, F> Stream for PollFn<F>
49 where
50 F: FnMut(&mut Context<'_>) -> Poll<Option<T>>,
51 {
52 type Item = T;
53
poll_next(mut self: Pin<&mut Self>, cx: &mut Context<'_>) -> Poll<Option<T>>54 fn poll_next(mut self: Pin<&mut Self>, cx: &mut Context<'_>) -> Poll<Option<T>> {
55 (&mut self.f)(cx)
56 }
57 }
58