blob: 4cc1380ec61919612bfe4e3ebf0c61038a5e3d29 [file] [log] [blame]
use crate::stream::{StreamExt, Fuse};
use core::marker::Unpin;
use core::pin::PinMut;
use futures_core::stream::Stream;
use futures_core::task::{self, Poll};
use pin_utils::{unsafe_pinned, unsafe_unpinned};
/// A `Stream` that implements a `peek` method.
///
/// The `peek` method can be used to retrieve a reference
/// to the next `Stream::Item` if available. A subsequent
/// call to `poll` will return the owned item.
#[derive(Debug)]
#[must_use = "streams do nothing unless polled"]
pub struct Peekable<St: Stream> {
stream: Fuse<St>,
peeked: Option<St::Item>,
}
impl<St: Stream + Unpin> Unpin for Peekable<St> {}
impl<St: Stream> Peekable<St> {
unsafe_pinned!(stream: Fuse<St>);
unsafe_unpinned!(peeked: Option<St::Item>);
pub(super) fn new(stream: St) -> Peekable<St> {
Peekable {
stream: stream.fuse(),
peeked: None
}
}
/// Peek retrieves a reference to the next item in the stream.
///
/// This method polls the underlying stream and return either a reference
/// to the next item if the stream is ready or passes through any errors.
pub fn peek<'a>(
self: &'a mut PinMut<Self>,
cx: &mut task::Context,
) -> Poll<Option<&'a St::Item>> {
if self.peeked().is_some() {
return Poll::Ready(self.peeked().as_ref())
}
match ready!(self.stream().poll_next(cx)) {
None => Poll::Ready(None),
Some(item) => {
*self.peeked() = Some(item);
Poll::Ready(self.peeked().as_ref())
}
}
}
}
impl<S: Stream> Stream for Peekable<S> {
type Item = S::Item;
fn poll_next(
mut self: PinMut<Self>,
cx: &mut task::Context
) -> Poll<Option<Self::Item>> {
if let Some(item) = self.peeked().take() {
return Poll::Ready(Some(item))
}
self.stream().poll_next(cx)
}
}
/* TODO
// Forwarding impl of Sink from the underlying stream
impl<S> Sink for Peekable<S>
where S: Sink + Stream
{
type SinkItem = S::SinkItem;
type SinkError = S::SinkError;
delegate_sink!(stream);
}
*/