futures_util/stream/
poll_fn.rs

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1//! Definition of the `PollFn` combinator
2
3use super::assert_stream;
4use core::fmt;
5use core::pin::Pin;
6use futures_core::stream::Stream;
7use futures_core::task::{Context, Poll};
8
9/// Stream for the [`poll_fn`] function.
10#[must_use = "streams do nothing unless polled"]
11pub struct PollFn<F> {
12    f: F,
13}
14
15impl<F> Unpin for PollFn<F> {}
16
17impl<F> fmt::Debug for PollFn<F> {
18    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/// ```
41pub fn poll_fn<T, F>(f: F) -> PollFn<F>
42where
43    F: FnMut(&mut Context<'_>) -> Poll<Option<T>>,
44{
45    assert_stream::<T, _>(PollFn { f })
46}
47
48impl<T, F> Stream for PollFn<F>
49where
50    F: FnMut(&mut Context<'_>) -> Poll<Option<T>>,
51{
52    type Item = T;
53
54    fn poll_next(mut self: Pin<&mut Self>, cx: &mut Context<'_>) -> Poll<Option<T>> {
55        (&mut self.f)(cx)
56    }
57}
futures_util/stream/poll_fn.rs

futures_util/stream/
poll_fn.rs
