futures_util/stream/
poll_fn.rs

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//! Definition of the `PollFn` combinator

use super::assert_stream;
use core::fmt;
use core::pin::Pin;
use futures_core::stream::Stream;
use futures_core::task::{Context, Poll};

/// Stream for the [`poll_fn`] function.
#[must_use = "streams do nothing unless polled"]
pub struct PollFn<F> {
    f: F,
}

impl<F> Unpin for PollFn<F> {}

impl<F> fmt::Debug for PollFn<F> {
    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
        f.debug_struct("PollFn").finish()
    }
}

/// Creates a new stream wrapping a function returning `Poll<Option<T>>`.
///
/// Polling the returned stream calls the wrapped function.
///
/// # Examples
///
/// ```
/// use futures::stream::poll_fn;
/// use futures::task::Poll;
///
/// let mut counter = 1usize;
///
/// let read_stream = poll_fn(move |_| -> Poll<Option<String>> {
///     if counter == 0 { return Poll::Ready(None); }
///     counter -= 1;
///     Poll::Ready(Some("Hello, World!".to_owned()))
/// });
/// ```
pub fn poll_fn<T, F>(f: F) -> PollFn<F>
where
    F: FnMut(&mut Context<'_>) -> Poll<Option<T>>,
{
    assert_stream::<T, _>(PollFn { f })
}

impl<T, F> Stream for PollFn<F>
where
    F: FnMut(&mut Context<'_>) -> Poll<Option<T>>,
{
    type Item = T;

    fn poll_next(mut self: Pin<&mut Self>, cx: &mut Context<'_>) -> Poll<Option<T>> {
        (&mut self.f)(cx)
    }
}