futures_util/stream/
peek.rsuse futures_core::{Async, Poll, Stream};
use futures_core::task;
use futures_sink::{Sink};
use stream::{StreamExt, Fuse};
#[derive(Debug)]
#[must_use = "streams do nothing unless polled"]
pub struct Peekable<S: Stream> {
stream: Fuse<S>,
peeked: Option<S::Item>,
}
pub fn new<S: Stream>(stream: S) -> Peekable<S> {
Peekable {
stream: stream.fuse(),
peeked: None
}
}
impl<S> Sink for Peekable<S>
where S: Sink + Stream
{
type SinkItem = S::SinkItem;
type SinkError = S::SinkError;
delegate_sink!(stream);
}
impl<S: Stream> Stream for Peekable<S> {
type Item = S::Item;
type Error = S::Error;
fn poll_next(&mut self, cx: &mut task::Context) -> Poll<Option<Self::Item>, Self::Error> {
if let Some(item) = self.peeked.take() {
return Ok(Async::Ready(Some(item)))
}
self.stream.poll_next(cx)
}
}
impl<S: Stream> Peekable<S> {
pub fn peek(&mut self, cx: &mut task::Context) -> Poll<Option<&S::Item>, S::Error> {
if self.peeked.is_some() {
return Ok(Async::Ready(self.peeked.as_ref()))
}
match try_ready!(self.poll_next(cx)) {
None => Ok(Async::Ready(None)),
Some(item) => {
self.peeked = Some(item);
Ok(Async::Ready(self.peeked.as_ref()))
}
}
}
}