Struct futures_util::stream::Peekable

pub struct Peekable<St: Stream> {
    stream: Fuse<St>,
    peeked: Option<St::Item>,
}

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.

Fields

stream: Fuse<St>

peeked: Option<St::Item>

Implementations

impl<St: Stream> Peekable<St>

pub(super) fn new(stream: St) -> Self

pub fn get_ref(&self) -> &St

Acquires a reference to the underlying sink or stream that this combinator is pulling from.

pub fn get_mut(&mut self) -> &mut St

Acquires a mutable reference to the underlying sink or stream that this combinator is pulling from.

Note that care must be taken to avoid tampering with the state of the sink or stream which may otherwise confuse this combinator.

pub fn get_pin_mut(self: Pin<&mut Self>) -> Pin<&mut St>

Acquires a pinned mutable reference to the underlying sink or stream that this combinator is pulling from.

Note that care must be taken to avoid tampering with the state of the sink or stream which may otherwise confuse this combinator.

pub fn into_inner(self) -> St

Consumes this combinator, returning the underlying sink or stream.

Note that this may discard intermediate state of this combinator, so care should be taken to avoid losing resources when this is called.

pub fn peek(self: Pin<&mut Self>) -> Peek<'_, St>

Produces a future which retrieves a reference to the next item in the stream, or None if the underlying stream terminates.

pub fn poll_peek( self: Pin<&mut Self>, cx: &mut Context<'_>, ) -> Poll<Option<&St::Item>>

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_mut(self: Pin<&mut Self>) -> PeekMut<'_, St>

Produces a future which retrieves a mutable reference to the next item in the stream, or None if the underlying stream terminates.

Examples

use futures::stream::{self, StreamExt};
use futures::pin_mut;

let stream = stream::iter(vec![1, 2, 3]).peekable();
pin_mut!(stream);

assert_eq!(stream.as_mut().peek_mut().await, Some(&mut 1));
assert_eq!(stream.as_mut().next().await, Some(1));

// Peek into the stream and modify the value which will be returned next
if let Some(p) = stream.as_mut().peek_mut().await {
    if *p == 2 {
        *p = 5;
    }
}

assert_eq!(stream.collect::<Vec<_>>().await, vec![5, 3]);

pub fn poll_peek_mut( self: Pin<&mut Self>, cx: &mut Context<'_>, ) -> Poll<Option<&mut St::Item>>

Peek retrieves a mutable reference to the next item in the stream.

pub fn next_if<F>(self: Pin<&mut Self>, func: F) -> NextIf<'_, St, F>

where F: FnOnce(&St::Item) -> bool,

Creates a future which will consume and return the next value of this stream if a condition is true.

If func returns true for the next value of this stream, consume and return it. Otherwise, return None.

Examples

Consume a number if it’s equal to 0.

use futures::stream::{self, StreamExt};
use futures::pin_mut;

let stream = stream::iter(0..5).peekable();
pin_mut!(stream);
// The first item of the stream is 0; consume it.
assert_eq!(stream.as_mut().next_if(|&x| x == 0).await, Some(0));
// The next item returned is now 1, so `consume` will return `false`.
assert_eq!(stream.as_mut().next_if(|&x| x == 0).await, None);
// `next_if` saves the value of the next item if it was not equal to `expected`.
assert_eq!(stream.next().await, Some(1));

Consume any number less than 10.

use futures::stream::{self, StreamExt};
use futures::pin_mut;

let stream = stream::iter(1..20).peekable();
pin_mut!(stream);
// Consume all numbers less than 10
while stream.as_mut().next_if(|&x| x < 10).await.is_some() {}
// The next value returned will be 10
assert_eq!(stream.next().await, Some(10));

pub fn next_if_eq<'a, T>( self: Pin<&'a mut Self>, expected: &'a T, ) -> NextIfEq<'a, St, T>

where T: ?Sized, St::Item: PartialEq<T>,

Creates a future which will consume and return the next item if it is equal to expected.

Example

Consume a number if it’s equal to 0.

use futures::stream::{self, StreamExt};
use futures::pin_mut;

let stream = stream::iter(0..5).peekable();
pin_mut!(stream);
// The first item of the stream is 0; consume it.
assert_eq!(stream.as_mut().next_if_eq(&0).await, Some(0));
// The next item returned is now 1, so `consume` will return `false`.
assert_eq!(stream.as_mut().next_if_eq(&0).await, None);
// `next_if_eq` saves the value of the next item if it was not equal to `expected`.
assert_eq!(stream.next().await, Some(1));

Trait Implementations

impl<St: Debug + Stream> Debug for Peekable<St>

where St::Item: Debug,

fn fmt(&self, f: &mut Formatter<'_>) -> Result

Formats the value using the given formatter.

impl<St: Stream> FusedStream for Peekable<St>

fn is_terminated(&self) -> bool

Returns true if the stream should no longer be polled.

impl<S, Item> Sink<Item> for Peekable<S>

where S: Sink<Item> + Stream,

type Error = <S as Sink<Item>>::Error

The type of value produced by the sink when an error occurs.

fn poll_ready( self: Pin<&mut Self>, cx: &mut Context<'_>, ) -> Poll<Result<(), Self::Error>>

Attempts to prepare the Sink to receive a value.

fn start_send(self: Pin<&mut Self>, item: Item) -> Result<(), Self::Error>

Begin the process of sending a value to the sink. Each call to this function must be preceded by a successful call to poll_ready which returned Poll::Ready(Ok(())).

fn poll_flush( self: Pin<&mut Self>, cx: &mut Context<'_>, ) -> Poll<Result<(), Self::Error>>

Flush any remaining output from this sink.

fn poll_close( self: Pin<&mut Self>, cx: &mut Context<'_>, ) -> Poll<Result<(), Self::Error>>

Flush any remaining output and close this sink, if necessary.

impl<S: Stream> Stream for Peekable<S>

type Item = <S as Stream>::Item

Values yielded by the stream.

fn poll_next( self: Pin<&mut Self>, cx: &mut Context<'_>, ) -> Poll<Option<Self::Item>>

Attempt to pull out the next value of this stream, registering the current task for wakeup if the value is not yet available, and returning None if the stream is exhausted.

fn size_hint(&self) -> (usize, Option<usize>)

Returns the bounds on the remaining length of the stream.

impl<'__pin, St: Stream> Unpin for Peekable<St>

where PinnedFieldsOf<__Origin<'__pin, St>>: Unpin,