Trait StreamExt

pub trait StreamExt: Stream {
    // Provided methods
    fn next(&mut self) -> Next<'_, Self> 
       where Self: Unpin { ... }
    fn into_future(self) -> StreamFuture<Self> 
       where Self: Sized + Unpin { ... }
    fn map<T, F>(self, f: F) -> Map<Self, F>
       where F: FnMut(Self::Item) -> T,
             Self: Sized { ... }
    fn enumerate(self) -> Enumerate<Self>
       where Self: Sized { ... }
    fn filter<Fut, F>(self, f: F) -> Filter<Self, Fut, F>
       where F: FnMut(&Self::Item) -> Fut,
             Fut: Future<Output = bool>,
             Self: Sized { ... }
    fn filter_map<Fut, T, F>(self, f: F) -> FilterMap<Self, Fut, F>
       where F: FnMut(Self::Item) -> Fut,
             Fut: Future<Output = Option<T>>,
             Self: Sized { ... }
    fn then<Fut, F>(self, f: F) -> Then<Self, Fut, F>
       where F: FnMut(Self::Item) -> Fut,
             Fut: Future,
             Self: Sized { ... }
    fn collect<C>(self) -> Collect<Self, C> 
       where C: Default + Extend<Self::Item>,
             Self: Sized { ... }
    fn unzip<A, B, FromA, FromB>(self) -> Unzip<Self, FromA, FromB> 
       where FromA: Default + Extend<A>,
             FromB: Default + Extend<B>,
             Self: Sized + Stream<Item = (A, B)> { ... }
    fn concat(self) -> Concat<Self> 
       where Self: Sized,
             Self::Item: Extend<<Self::Item as IntoIterator>::Item> + IntoIterator + Default { ... }
    fn count(self) -> Count<Self> 
       where Self: Sized { ... }
    fn cycle(self) -> Cycle<Self>
       where Self: Sized + Clone { ... }
    fn fold<T, Fut, F>(self, init: T, f: F) -> Fold<Self, Fut, T, F> 
       where F: FnMut(T, Self::Item) -> Fut,
             Fut: Future<Output = T>,
             Self: Sized { ... }
    fn any<Fut, F>(self, f: F) -> Any<Self, Fut, F> 
       where F: FnMut(Self::Item) -> Fut,
             Fut: Future<Output = bool>,
             Self: Sized { ... }
    fn all<Fut, F>(self, f: F) -> All<Self, Fut, F> 
       where F: FnMut(Self::Item) -> Fut,
             Fut: Future<Output = bool>,
             Self: Sized { ... }
    fn flatten(self) -> Flatten<Self>
       where Self::Item: Stream,
             Self: Sized { ... }
    fn flatten_unordered(
        self,
        limit: impl Into<Option<usize>>,
    ) -> FlattenUnorderedWithFlowController<Self, ()>
       where Self::Item: Stream + Unpin,
             Self: Sized { ... }
    fn flat_map<U, F>(self, f: F) -> FlatMap<Self, U, F>
       where F: FnMut(Self::Item) -> U,
             U: Stream,
             Self: Sized { ... }
    fn flat_map_unordered<U, F>(
        self,
        limit: impl Into<Option<usize>>,
        f: F,
    ) -> FlatMapUnordered<Self, U, F>
       where U: Stream + Unpin,
             F: FnMut(Self::Item) -> U,
             Self: Sized { ... }
    fn scan<S, B, Fut, F>(self, initial_state: S, f: F) -> Scan<Self, S, Fut, F>
       where F: FnMut(&mut S, Self::Item) -> Fut,
             Fut: Future<Output = Option<B>>,
             Self: Sized { ... }
    fn skip_while<Fut, F>(self, f: F) -> SkipWhile<Self, Fut, F>
       where F: FnMut(&Self::Item) -> Fut,
             Fut: Future<Output = bool>,
             Self: Sized { ... }
    fn take_while<Fut, F>(self, f: F) -> TakeWhile<Self, Fut, F>
       where F: FnMut(&Self::Item) -> Fut,
             Fut: Future<Output = bool>,
             Self: Sized { ... }
    fn take_until<Fut>(self, fut: Fut) -> TakeUntil<Self, Fut>
       where Fut: Future,
             Self: Sized { ... }
    fn for_each<Fut, F>(self, f: F) -> ForEach<Self, Fut, F> 
       where F: FnMut(Self::Item) -> Fut,
             Fut: Future<Output = ()>,
             Self: Sized { ... }
    fn for_each_concurrent<Fut, F>(
        self,
        limit: impl Into<Option<usize>>,
        f: F,
    ) -> ForEachConcurrent<Self, Fut, F> 
       where F: FnMut(Self::Item) -> Fut,
             Fut: Future<Output = ()>,
             Self: Sized { ... }
    fn take(self, n: usize) -> Take<Self>
       where Self: Sized { ... }
    fn skip(self, n: usize) -> Skip<Self>
       where Self: Sized { ... }
    fn fuse(self) -> Fuse<Self>
       where Self: Sized { ... }
    fn by_ref(&mut self) -> &mut Self { ... }
    fn catch_unwind(self) -> CatchUnwind<Self>
       where Self: Sized + UnwindSafe { ... }
    fn boxed<'a>(self) -> Pin<Box<dyn Stream<Item = Self::Item> + Send + 'a>>
       where Self: Sized + Send + 'a { ... }
    fn boxed_local<'a>(self) -> Pin<Box<dyn Stream<Item = Self::Item> + 'a>>
       where Self: Sized + 'a { ... }
    fn buffered(self, n: usize) -> Buffered<Self>
       where Self::Item: Future,
             Self: Sized { ... }
    fn buffer_unordered(self, n: usize) -> BufferUnordered<Self>
       where Self::Item: Future,
             Self: Sized { ... }
    fn zip<St>(self, other: St) -> Zip<Self, St>
       where St: Stream,
             Self: Sized { ... }
    fn chain<St>(self, other: St) -> Chain<Self, St>
       where St: Stream<Item = Self::Item>,
             Self: Sized { ... }
    fn peekable(self) -> Peekable<Self>
       where Self: Sized { ... }
    fn chunks(self, capacity: usize) -> Chunks<Self>
       where Self: Sized { ... }
    fn ready_chunks(self, capacity: usize) -> ReadyChunks<Self>
       where Self: Sized { ... }
    fn forward<S>(self, sink: S) -> Forward<Self, S> 
       where S: Sink<Self::Ok, Error = Self::Error>,
             Self: Sized + TryStream { ... }
    fn split<Item>(self) -> (SplitSink<Self, Item>, SplitStream<Self>)
       where Self: Sized + Sink<Item> { ... }
    fn inspect<F>(self, f: F) -> Inspect<Self, F>
       where F: FnMut(&Self::Item),
             Self: Sized { ... }
    fn left_stream<B>(self) -> Either<Self, B> 
       where B: Stream<Item = Self::Item>,
             Self: Sized { ... }
    fn right_stream<B>(self) -> Either<B, Self> 
       where B: Stream<Item = Self::Item>,
             Self: Sized { ... }
    fn poll_next_unpin(
        &mut self,
        cx: &mut Context<'_>,
    ) -> Poll<Option<Self::Item>>
       where Self: Unpin { ... }
    fn select_next_some(&mut self) -> SelectNextSome<'_, Self> 
       where Self: Unpin + FusedStream { ... }
}

An extension trait for Streams that provides a variety of convenient combinator functions.

Provided Methods

fn next(&mut self) -> Next<'_, Self>

where Self: Unpin,

Creates a future that resolves to the next item in the stream.

Note that because next doesn’t take ownership over the stream, the Stream type must be Unpin. If you want to use next with a !Unpin stream, you’ll first have to pin the stream. This can be done by boxing the stream using Box::pin or pinning it to the stack using the pin_mut! macro from the pin_utils crate.

Examples

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

let mut stream = stream::iter(1..=3);

assert_eq!(stream.next().await, Some(1));
assert_eq!(stream.next().await, Some(2));
assert_eq!(stream.next().await, Some(3));
assert_eq!(stream.next().await, None);

fn into_future(self) -> StreamFuture<Self>

where Self: Sized + Unpin,

Converts this stream into a future of (next_item, tail_of_stream). If the stream terminates, then the next item is None.

The returned future can be used to compose streams and futures together by placing everything into the “world of futures”.

Note that because into_future moves the stream, the Stream type must be Unpin. If you want to use into_future with a !Unpin stream, you’ll first have to pin the stream. This can be done by boxing the stream using Box::pin or pinning it to the stack using the pin_mut! macro from the pin_utils crate.

Examples

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

let stream = stream::iter(1..=3);

let (item, stream) = stream.into_future().await;
assert_eq!(Some(1), item);

let (item, stream) = stream.into_future().await;
assert_eq!(Some(2), item);

fn map<T, F>(self, f: F) -> Map<Self, F>

where F: FnMut(Self::Item) -> T, Self: Sized,

Maps this stream’s items to a different type, returning a new stream of the resulting type.

The provided closure is executed over all elements of this stream as they are made available. It is executed inline with calls to poll_next.

Note that this function consumes the stream passed into it and returns a wrapped version of it, similar to the existing map methods in the standard library.

See StreamExt::then if you want to use a closure that returns a future instead of a value.

Examples

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

let stream = stream::iter(1..=3);
let stream = stream.map(|x| x + 3);

assert_eq!(vec![4, 5, 6], stream.collect::<Vec<_>>().await);

fn enumerate(self) -> Enumerate<Self>

where Self: Sized,

Creates a stream which gives the current iteration count as well as the next value.

The stream returned yields pairs (i, val), where i is the current index of iteration and val is the value returned by the stream.

enumerate() keeps its count as a usize. If you want to count by a different sized integer, the zip function provides similar functionality.

Overflow Behavior

The method does no guarding against overflows, so enumerating more than usize::MAX elements either produces the wrong result or panics. If debug assertions are enabled, a panic is guaranteed.

Panics

The returned stream might panic if the to-be-returned index would overflow a usize.

Examples

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

let stream = stream::iter(vec!['a', 'b', 'c']);

let mut stream = stream.enumerate();

assert_eq!(stream.next().await, Some((0, 'a')));
assert_eq!(stream.next().await, Some((1, 'b')));
assert_eq!(stream.next().await, Some((2, 'c')));
assert_eq!(stream.next().await, None);

fn filter<Fut, F>(self, f: F) -> Filter<Self, Fut, F>

where F: FnMut(&Self::Item) -> Fut, Fut: Future<Output = bool>, Self: Sized,

Filters the values produced by this stream according to the provided asynchronous predicate.

As values of this stream are made available, the provided predicate f will be run against them. If the predicate returns a Future which resolves to true, then the stream will yield the value, but if the predicate returns a Future which resolves to false, then the value will be discarded and the next value will be produced.

Note that this function consumes the stream passed into it and returns a wrapped version of it, similar to the existing filter methods in the standard library.

Examples

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

let stream = stream::iter(1..=10);
let events = stream.filter(|x| future::ready(x % 2 == 0));

assert_eq!(vec![2, 4, 6, 8, 10], events.collect::<Vec<_>>().await);

fn filter_map<Fut, T, F>(self, f: F) -> FilterMap<Self, Fut, F>

where F: FnMut(Self::Item) -> Fut, Fut: Future<Output = Option<T>>, Self: Sized,

Filters the values produced by this stream while simultaneously mapping them to a different type according to the provided asynchronous closure.

As values of this stream are made available, the provided function will be run on them. If the future returned by the predicate f resolves to Some(item) then the stream will yield the value item, but if it resolves to None then the next value will be produced.

Note that this function consumes the stream passed into it and returns a wrapped version of it, similar to the existing filter_map methods in the standard library.

Examples

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

let stream = stream::iter(1..=10);
let events = stream.filter_map(|x| async move {
    if x % 2 == 0 { Some(x + 1) } else { None }
});

assert_eq!(vec![3, 5, 7, 9, 11], events.collect::<Vec<_>>().await);

fn then<Fut, F>(self, f: F) -> Then<Self, Fut, F>

where F: FnMut(Self::Item) -> Fut, Fut: Future, Self: Sized,

Computes from this stream’s items new items of a different type using an asynchronous closure.

The provided closure f will be called with an Item once a value is ready, it returns a future which will then be run to completion to produce the next value on this stream.

Note that this function consumes the stream passed into it and returns a wrapped version of it.

See StreamExt::map if you want to use a closure that returns a value instead of a future.

Examples

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

let stream = stream::iter(1..=3);
let stream = stream.then(|x| async move { x + 3 });

assert_eq!(vec![4, 5, 6], stream.collect::<Vec<_>>().await);

fn collect<C>(self) -> Collect<Self, C>

where C: Default + Extend<Self::Item>, Self: Sized,

Transforms a stream into a collection, returning a future representing the result of that computation.

The returned future will be resolved when the stream terminates.

Examples

use futures::channel::mpsc;
use futures::stream::StreamExt;
use std::thread;

let (tx, rx) = mpsc::unbounded();

thread::spawn(move || {
    for i in 1..=5 {
        tx.unbounded_send(i).unwrap();
    }
});

let output = rx.collect::<Vec<i32>>().await;
assert_eq!(output, vec![1, 2, 3, 4, 5]);

fn unzip<A, B, FromA, FromB>(self) -> Unzip<Self, FromA, FromB>

where FromA: Default + Extend<A>, FromB: Default + Extend<B>, Self: Sized + Stream<Item = (A, B)>,

Converts a stream of pairs into a future, which resolves to pair of containers.

unzip() produces a future, which resolves to two collections: one from the left elements of the pairs, and one from the right elements.

The returned future will be resolved when the stream terminates.

Examples

use futures::channel::mpsc;
use futures::stream::StreamExt;
use std::thread;

let (tx, rx) = mpsc::unbounded();

thread::spawn(move || {
    tx.unbounded_send((1, 2)).unwrap();
    tx.unbounded_send((3, 4)).unwrap();
    tx.unbounded_send((5, 6)).unwrap();
});

let (o1, o2): (Vec<_>, Vec<_>) = rx.unzip().await;
assert_eq!(o1, vec![1, 3, 5]);
assert_eq!(o2, vec![2, 4, 6]);

fn concat(self) -> Concat<Self>

where Self: Sized, Self::Item: Extend<<Self::Item as IntoIterator>::Item> + IntoIterator + Default,

Concatenate all items of a stream into a single extendable destination, returning a future representing the end result.

This combinator will extend the first item with the contents of all the subsequent results of the stream. If the stream is empty, the default value will be returned.

Works with all collections that implement the Extend trait.

Examples

use futures::channel::mpsc;
use futures::stream::StreamExt;
use std::thread;

let (tx, rx) = mpsc::unbounded();

thread::spawn(move || {
    for i in (0..3).rev() {
        let n = i * 3;
        tx.unbounded_send(vec![n + 1, n + 2, n + 3]).unwrap();
    }
});

let result = rx.concat().await;

assert_eq!(result, vec![7, 8, 9, 4, 5, 6, 1, 2, 3]);

fn count(self) -> Count<Self>

where Self: Sized,

Drives the stream to completion, counting the number of items.

Overflow Behavior

The method does no guarding against overflows, so counting elements of a stream with more than usize::MAX elements either produces the wrong result or panics. If debug assertions are enabled, a panic is guaranteed.

Panics

This function might panic if the iterator has more than usize::MAX elements.

Examples

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

let stream = stream::iter(1..=10);
let count = stream.count().await;

assert_eq!(count, 10);

fn cycle(self) -> Cycle<Self>

where Self: Sized + Clone,

Repeats a stream endlessly.

The stream never terminates. Note that you likely want to avoid usage of collect or such on the returned stream as it will exhaust available memory as it tries to just fill up all RAM.

Examples

use futures::stream::{self, StreamExt};
let a = [1, 2, 3];
let mut s = stream::iter(a.iter()).cycle();

assert_eq!(s.next().await, Some(&1));
assert_eq!(s.next().await, Some(&2));
assert_eq!(s.next().await, Some(&3));
assert_eq!(s.next().await, Some(&1));
assert_eq!(s.next().await, Some(&2));
assert_eq!(s.next().await, Some(&3));
assert_eq!(s.next().await, Some(&1));

fn fold<T, Fut, F>(self, init: T, f: F) -> Fold<Self, Fut, T, F>

where F: FnMut(T, Self::Item) -> Fut, Fut: Future<Output = T>, Self: Sized,

Execute an accumulating asynchronous computation over a stream, collecting all the values into one final result.

This combinator will accumulate all values returned by this stream according to the closure provided. The initial state is also provided to this method and then is returned again by each execution of the closure. Once the entire stream has been exhausted the returned future will resolve to this value.

Examples

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

let number_stream = stream::iter(0..6);
let sum = number_stream.fold(0, |acc, x| async move { acc + x });
assert_eq!(sum.await, 15);

fn any<Fut, F>(self, f: F) -> Any<Self, Fut, F>

where F: FnMut(Self::Item) -> Fut, Fut: Future<Output = bool>, Self: Sized,

Execute predicate over asynchronous stream, and return true if any element in stream satisfied a predicate.

Examples

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

let number_stream = stream::iter(0..10);
let contain_three = number_stream.any(|i| async move { i == 3 });
assert_eq!(contain_three.await, true);

fn all<Fut, F>(self, f: F) -> All<Self, Fut, F>

where F: FnMut(Self::Item) -> Fut, Fut: Future<Output = bool>, Self: Sized,

Execute predicate over asynchronous stream, and return true if all element in stream satisfied a predicate.

Examples

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

let number_stream = stream::iter(0..10);
let less_then_twenty = number_stream.all(|i| async move { i < 20 });
assert_eq!(less_then_twenty.await, true);

fn flatten(self) -> Flatten<Self>

where Self::Item: Stream, Self: Sized,

Flattens a stream of streams into just one continuous stream.

Examples

use futures::channel::mpsc;
use futures::stream::StreamExt;
use std::thread;

let (tx1, rx1) = mpsc::unbounded();
let (tx2, rx2) = mpsc::unbounded();
let (tx3, rx3) = mpsc::unbounded();

thread::spawn(move || {
    tx1.unbounded_send(1).unwrap();
    tx1.unbounded_send(2).unwrap();
});
thread::spawn(move || {
    tx2.unbounded_send(3).unwrap();
    tx2.unbounded_send(4).unwrap();
});
thread::spawn(move || {
    tx3.unbounded_send(rx1).unwrap();
    tx3.unbounded_send(rx2).unwrap();
});

let output = rx3.flatten().collect::<Vec<i32>>().await;
assert_eq!(output, vec![1, 2, 3, 4]);

fn flatten_unordered( self, limit: impl Into<Option<usize>>, ) -> FlattenUnorderedWithFlowController<Self, ()>

where Self::Item: Stream + Unpin, Self: Sized,

Flattens a stream of streams into just one continuous stream. Polls inner streams produced by the base stream concurrently.

The only argument is an optional limit on the number of concurrently polled streams. If this limit is not None, no more than limit streams will be polled at the same time. The limit argument is of type Into<Option<usize>>, and so can be provided as either None, Some(10), or just 10. Note: a limit of zero is interpreted as no limit at all, and will have the same result as passing in None.

Examples

use futures::channel::mpsc;
use futures::stream::StreamExt;
use std::thread;

let (tx1, rx1) = mpsc::unbounded();
let (tx2, rx2) = mpsc::unbounded();
let (tx3, rx3) = mpsc::unbounded();

thread::spawn(move || {
    tx1.unbounded_send(1).unwrap();
    tx1.unbounded_send(2).unwrap();
});
thread::spawn(move || {
    tx2.unbounded_send(3).unwrap();
    tx2.unbounded_send(4).unwrap();
});
thread::spawn(move || {
    tx3.unbounded_send(rx1).unwrap();
    tx3.unbounded_send(rx2).unwrap();
});

let mut output = rx3.flatten_unordered(None).collect::<Vec<i32>>().await;
output.sort();

assert_eq!(output, vec![1, 2, 3, 4]);

fn flat_map<U, F>(self, f: F) -> FlatMap<Self, U, F>

where F: FnMut(Self::Item) -> U, U: Stream, Self: Sized,

Maps a stream like StreamExt::map but flattens nested Streams.

StreamExt::map is very useful, but if it produces a Stream instead, you would have to chain combinators like .map(f).flatten() while this combinator provides ability to write .flat_map(f) instead of chaining.

The provided closure which produces inner streams is executed over all elements of stream as last inner stream is terminated and next stream item is available.

Note that this function consumes the stream passed into it and returns a wrapped version of it, similar to the existing flat_map methods in the standard library.

Examples

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

let stream = stream::iter(1..=3);
let stream = stream.flat_map(|x| stream::iter(vec![x + 3; x]));

assert_eq!(vec![4, 5, 5, 6, 6, 6], stream.collect::<Vec<_>>().await);

fn flat_map_unordered<U, F>( self, limit: impl Into<Option<usize>>, f: F, ) -> FlatMapUnordered<Self, U, F>

where U: Stream + Unpin, F: FnMut(Self::Item) -> U, Self: Sized,

Maps a stream like StreamExt::map but flattens nested Streams and polls them concurrently, yielding items in any order, as they made available.

StreamExt::map is very useful, but if it produces Streams instead, and you need to poll all of them concurrently, you would have to use something like for_each_concurrent and merge values by hand. This combinator provides ability to collect all values from concurrently polled streams into one stream.

The first argument is an optional limit on the number of concurrently polled streams. If this limit is not None, no more than limit streams will be polled at the same time. The limit argument is of type Into<Option<usize>>, and so can be provided as either None, Some(10), or just 10. Note: a limit of zero is interpreted as no limit at all, and will have the same result as passing in None.

The provided closure which produces inner streams is executed over all elements of stream as next stream item is available and limit of concurrently processed streams isn’t exceeded.

Note that this function consumes the stream passed into it and returns a wrapped version of it.

Examples

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

let stream = stream::iter(1..5);
let stream = stream.flat_map_unordered(1, |x| stream::iter(vec![x; x]));
let mut values = stream.collect::<Vec<_>>().await;
values.sort();

assert_eq!(vec![1usize, 2, 2, 3, 3, 3, 4, 4, 4, 4], values);

fn scan<S, B, Fut, F>(self, initial_state: S, f: F) -> Scan<Self, S, Fut, F>

where F: FnMut(&mut S, Self::Item) -> Fut, Fut: Future<Output = Option<B>>, Self: Sized,

Combinator similar to StreamExt::fold that holds internal state and produces a new stream.

Accepts initial state and closure which will be applied to each element of the stream until provided closure returns None. Once None is returned, stream will be terminated.

Examples

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

let stream = stream::iter(1..=10);

let stream = stream.scan(0, |state, x| {
    *state += x;
    future::ready(if *state < 10 { Some(x) } else { None })
});

assert_eq!(vec![1, 2, 3], stream.collect::<Vec<_>>().await);

fn skip_while<Fut, F>(self, f: F) -> SkipWhile<Self, Fut, F>

where F: FnMut(&Self::Item) -> Fut, Fut: Future<Output = bool>, Self: Sized,

Skip elements on this stream while the provided asynchronous predicate resolves to true.

This function, like Iterator::skip_while, will skip elements on the stream until the predicate f resolves to false. Once one element returns false, all future elements will be returned from the underlying stream.

Examples

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

let stream = stream::iter(1..=10);

let stream = stream.skip_while(|x| future::ready(*x <= 5));

assert_eq!(vec![6, 7, 8, 9, 10], stream.collect::<Vec<_>>().await);

fn take_while<Fut, F>(self, f: F) -> TakeWhile<Self, Fut, F>

where F: FnMut(&Self::Item) -> Fut, Fut: Future<Output = bool>, Self: Sized,

Take elements from this stream while the provided asynchronous predicate resolves to true.

This function, like Iterator::take_while, will take elements from the stream until the predicate f resolves to false. Once one element returns false, it will always return that the stream is done.

Examples

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

let stream = stream::iter(1..=10);

let stream = stream.take_while(|x| future::ready(*x <= 5));

assert_eq!(vec![1, 2, 3, 4, 5], stream.collect::<Vec<_>>().await);

fn take_until<Fut>(self, fut: Fut) -> TakeUntil<Self, Fut>

where Fut: Future, Self: Sized,

Take elements from this stream until the provided future resolves.

This function will take elements from the stream until the provided stopping future fut resolves. Once the fut future becomes ready, this stream combinator will always return that the stream is done.

The stopping future may return any type. Once the stream is stopped the result of the stopping future may be accessed with TakeUntil::take_result(). The stream may also be resumed with TakeUntil::take_future(). See the documentation of TakeUntil for more information.

Examples

use futures::future;
use futures::stream::{self, StreamExt};
use futures::task::Poll;

let stream = stream::iter(1..=10);

let mut i = 0;
let stop_fut = future::poll_fn(|_cx| {
    i += 1;
    if i <= 5 {
        Poll::Pending
    } else {
        Poll::Ready(())
    }
});

let stream = stream.take_until(stop_fut);

assert_eq!(vec![1, 2, 3, 4, 5], stream.collect::<Vec<_>>().await);

fn for_each<Fut, F>(self, f: F) -> ForEach<Self, Fut, F>

where F: FnMut(Self::Item) -> Fut, Fut: Future<Output = ()>, Self: Sized,

Runs this stream to completion, executing the provided asynchronous closure for each element on the stream.

The closure provided will be called for each item this stream produces, yielding a future. That future will then be executed to completion before moving on to the next item.

The returned value is a Future where the Output type is (); it is executed entirely for its side effects.

To process each item in the stream and produce another stream instead of a single future, use then instead.

Examples

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

let mut x = 0;

{
    let fut = stream::repeat(1).take(3).for_each(|item| {
        x += item;
        future::ready(())
    });
    fut.await;
}

assert_eq!(x, 3);

fn for_each_concurrent<Fut, F>( self, limit: impl Into<Option<usize>>, f: F, ) -> ForEachConcurrent<Self, Fut, F>

where F: FnMut(Self::Item) -> Fut, Fut: Future<Output = ()>, Self: Sized,

Runs this stream to completion, executing the provided asynchronous closure for each element on the stream concurrently as elements become available.

This is similar to StreamExt::for_each, but the futures produced by the closure are run concurrently (but not in parallel– this combinator does not introduce any threads).

The closure provided will be called for each item this stream produces, yielding a future. That future will then be executed to completion concurrently with the other futures produced by the closure.

The first argument is an optional limit on the number of concurrent futures. If this limit is not None, no more than limit futures will be run concurrently. The limit argument is of type Into<Option<usize>>, and so can be provided as either None, Some(10), or just 10. Note: a limit of zero is interpreted as no limit at all, and will have the same result as passing in None.

This method is only available when the std or alloc feature of this library is activated, and it is activated by default.

Examples

use futures::channel::oneshot;
use futures::stream::{self, StreamExt};

let (tx1, rx1) = oneshot::channel();
let (tx2, rx2) = oneshot::channel();
let (tx3, rx3) = oneshot::channel();

let fut = stream::iter(vec![rx1, rx2, rx3]).for_each_concurrent(
    /* limit */ 2,
    |rx| async move {
        rx.await.unwrap();
    }
);
tx1.send(()).unwrap();
tx2.send(()).unwrap();
tx3.send(()).unwrap();
fut.await;

fn take(self, n: usize) -> Take<Self>

where Self: Sized,

Creates a new stream of at most n items of the underlying stream.

Once n items have been yielded from this stream then it will always return that the stream is done.

Examples

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

let stream = stream::iter(1..=10).take(3);

assert_eq!(vec![1, 2, 3], stream.collect::<Vec<_>>().await);

fn skip(self, n: usize) -> Skip<Self>

where Self: Sized,

Creates a new stream which skips n items of the underlying stream.

Once n items have been skipped from this stream then it will always return the remaining items on this stream.

Examples

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

let stream = stream::iter(1..=10).skip(5);

assert_eq!(vec![6, 7, 8, 9, 10], stream.collect::<Vec<_>>().await);

fn fuse(self) -> Fuse<Self>

where Self: Sized,

Fuse a stream such that poll_next will never again be called once it has finished. This method can be used to turn any Stream into a FusedStream.

Normally, once a stream has returned None from poll_next any further calls could exhibit bad behavior such as block forever, panic, never return, etc. If it is known that poll_next may be called after stream has already finished, then this method can be used to ensure that it has defined semantics.

The poll_next method of a fused stream is guaranteed to return None after the underlying stream has finished.

Examples

use futures::executor::block_on_stream;
use futures::stream::{self, StreamExt};
use futures::task::Poll;

let mut x = 0;
let stream = stream::poll_fn(|_| {
    x += 1;
    match x {
        0..=2 => Poll::Ready(Some(x)),
        3 => Poll::Ready(None),
        _ => panic!("should not happen")
    }
}).fuse();

let mut iter = block_on_stream(stream);
assert_eq!(Some(1), iter.next());
assert_eq!(Some(2), iter.next());
assert_eq!(None, iter.next());
assert_eq!(None, iter.next());
// ...

fn by_ref(&mut self) -> &mut Self

Borrows a stream, rather than consuming it.

This is useful to allow applying stream adaptors while still retaining ownership of the original stream.

Examples

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

let mut stream = stream::iter(1..5);

let sum = stream.by_ref()
                .take(2)
                .fold(0, |a, b| async move { a + b })
                .await;
assert_eq!(sum, 3);

// You can use the stream again
let sum = stream.take(2)
                .fold(0, |a, b| async move { a + b })
                .await;
assert_eq!(sum, 7);

fn catch_unwind(self) -> CatchUnwind<Self>

where Self: Sized + UnwindSafe,

Catches unwinding panics while polling the stream.

Caught panic (if any) will be the last element of the resulting stream.

In general, panics within a stream can propagate all the way out to the task level. This combinator makes it possible to halt unwinding within the stream itself. It’s most commonly used within task executors. This method should not be used for error handling.

Note that this method requires the UnwindSafe bound from the standard library. This isn’t always applied automatically, and the standard library provides an AssertUnwindSafe wrapper type to apply it after-the fact. To assist using this method, the Stream trait is also implemented for AssertUnwindSafe<St> where St implements Stream.

This method is only available when the std feature of this library is activated, and it is activated by default.

Examples

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

let stream = stream::iter(vec![Some(10), None, Some(11)]);
// Panic on second element
let stream_panicking = stream.map(|o| o.unwrap());
// Collect all the results
let stream = stream_panicking.catch_unwind();

let results: Vec<Result<i32, _>> = stream.collect().await;
match results[0] {
    Ok(10) => {}
    _ => panic!("unexpected result!"),
}
assert!(results[1].is_err());
assert_eq!(results.len(), 2);

fn boxed<'a>(self) -> Pin<Box<dyn Stream<Item = Self::Item> + Send + 'a>>

where Self: Sized + Send + 'a,

Wrap the stream in a Box, pinning it.

This method is only available when the std or alloc feature of this library is activated, and it is activated by default.

fn boxed_local<'a>(self) -> Pin<Box<dyn Stream<Item = Self::Item> + 'a>>

where Self: Sized + 'a,

Wrap the stream in a Box, pinning it.

Similar to boxed, but without the Send requirement.

This method is only available when the std or alloc feature of this library is activated, and it is activated by default.

fn buffered(self, n: usize) -> Buffered<Self>

where Self::Item: Future, Self: Sized,

An adaptor for creating a buffered list of pending futures.

If this stream’s item can be converted into a future, then this adaptor will buffer up to at most n futures and then return the outputs in the same order as the underlying stream. No more than n futures will be buffered at any point in time, and less than n may also be buffered depending on the state of each future.

The returned stream will be a stream of each future’s output.

This method is only available when the std or alloc feature of this library is activated, and it is activated by default.

fn buffer_unordered(self, n: usize) -> BufferUnordered<Self>

where Self::Item: Future, Self: Sized,

An adaptor for creating a buffered list of pending futures (unordered).

If this stream’s item can be converted into a future, then this adaptor will buffer up to n futures and then return the outputs in the order in which they complete. No more than n futures will be buffered at any point in time, and less than n may also be buffered depending on the state of each future.

The returned stream will be a stream of each future’s output.

This method is only available when the std or alloc feature of this library is activated, and it is activated by default.

Examples

use futures::channel::oneshot;
use futures::stream::{self, StreamExt};

let (send_one, recv_one) = oneshot::channel();
let (send_two, recv_two) = oneshot::channel();

let stream_of_futures = stream::iter(vec![recv_one, recv_two]);
let mut buffered = stream_of_futures.buffer_unordered(10);

send_two.send(2i32)?;
assert_eq!(buffered.next().await, Some(Ok(2i32)));

send_one.send(1i32)?;
assert_eq!(buffered.next().await, Some(Ok(1i32)));

assert_eq!(buffered.next().await, None);

fn zip<St>(self, other: St) -> Zip<Self, St>

where St: Stream, Self: Sized,

An adapter for zipping two streams together.

The zipped stream waits for both streams to produce an item, and then returns that pair. If either stream ends then the zipped stream will also end.

Examples

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

let stream1 = stream::iter(1..=3);
let stream2 = stream::iter(5..=10);

let vec = stream1.zip(stream2)
                 .collect::<Vec<_>>()
                 .await;
assert_eq!(vec![(1, 5), (2, 6), (3, 7)], vec);

fn chain<St>(self, other: St) -> Chain<Self, St>

where St: Stream<Item = Self::Item>, Self: Sized,

Adapter for chaining two streams.

The resulting stream emits elements from the first stream, and when first stream reaches the end, emits the elements from the second stream.

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

let stream1 = stream::iter(vec![Ok(10), Err(false)]);
let stream2 = stream::iter(vec![Err(true), Ok(20)]);

let stream = stream1.chain(stream2);

let result: Vec<_> = stream.collect().await;
assert_eq!(result, vec![
    Ok(10),
    Err(false),
    Err(true),
    Ok(20),
]);

fn peekable(self) -> Peekable<Self>

where Self: Sized,

Creates a new stream which exposes a peek method.

Calling peek returns a reference to the next item in the stream.

fn chunks(self, capacity: usize) -> Chunks<Self>

where Self: Sized,

An adaptor for chunking up items of the stream inside a vector.

This combinator will attempt to pull items from this stream and buffer them into a local vector. At most capacity items will get buffered before they’re yielded from the returned stream.

Note that the vectors returned from this iterator may not always have capacity elements. If the underlying stream ended and only a partial vector was created, it’ll be returned. Additionally if an error happens from the underlying stream then the currently buffered items will be yielded.

This method is only available when the std or alloc feature of this library is activated, and it is activated by default.

Panics

This method will panic if capacity is zero.

fn ready_chunks(self, capacity: usize) -> ReadyChunks<Self>

where Self: Sized,

An adaptor for chunking up ready items of the stream inside a vector.

This combinator will attempt to pull ready items from this stream and buffer them into a local vector. At most capacity items will get buffered before they’re yielded from the returned stream. If underlying stream returns Poll::Pending, and collected chunk is not empty, it will be immediately returned.

If the underlying stream ended and only a partial vector was created, it will be returned.

This method is only available when the std or alloc feature of this library is activated, and it is activated by default.

Panics

This method will panic if capacity is zero.

fn forward<S>(self, sink: S) -> Forward<Self, S>

where S: Sink<Self::Ok, Error = Self::Error>, Self: Sized + TryStream,

A future that completes after the given stream has been fully processed into the sink and the sink has been flushed and closed.

This future will drive the stream to keep producing items until it is exhausted, sending each item to the sink. It will complete once the stream is exhausted, the sink has received and flushed all items, and the sink is closed. Note that neither the original stream nor provided sink will be output by this future. Pass the sink by Pin<&mut S> (for example, via forward(&mut sink) inside an async fn/block) in order to preserve access to the Sink. If the stream produces an error, that error will be returned by this future without flushing/closing the sink.

fn split<Item>(self) -> (SplitSink<Self, Item>, SplitStream<Self>)

where Self: Sized + Sink<Item>,

Splits this Stream + Sink object into separate Sink and Stream objects.

This can be useful when you want to split ownership between tasks, or allow direct interaction between the two objects (e.g. via Sink::send_all).

This method is only available when the std or alloc feature of this library is activated, and it is activated by default.

fn inspect<F>(self, f: F) -> Inspect<Self, F>

where F: FnMut(&Self::Item), Self: Sized,

Do something with each item of this stream, afterwards passing it on.

This is similar to the Iterator::inspect method in the standard library where it allows easily inspecting each value as it passes through the stream, for example to debug what’s going on.

fn left_stream<B>(self) -> Either<Self, B>

where B: Stream<Item = Self::Item>, Self: Sized,

Wrap this stream in an Either stream, making it the left-hand variant of that Either.

This can be used in combination with the right_stream method to write if statements that evaluate to different streams in different branches.

fn right_stream<B>(self) -> Either<B, Self>

where B: Stream<Item = Self::Item>, Self: Sized,

Wrap this stream in an Either stream, making it the right-hand variant of that Either.

This can be used in combination with the left_stream method to write if statements that evaluate to different streams in different branches.

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

where Self: Unpin,

A convenience method for calling Stream::poll_next on Unpin stream types.

fn select_next_some(&mut self) -> SelectNextSome<'_, Self>

where Self: Unpin + FusedStream,

Returns a Future that resolves when the next item in this stream is ready.

This is similar to the next method, but it won’t resolve to None if used on an empty Stream. Instead, the returned future type will return true from FusedFuture::is_terminated when the Stream is empty, allowing select_next_some to be easily used with the select! macro.

If the future is polled after this Stream is empty it will panic. Using the future with a FusedFuture-aware primitive like the select! macro will prevent this.

Examples

use futures::{future, select};
use futures::stream::{StreamExt, FuturesUnordered};

let mut fut = future::ready(1);
let mut async_tasks = FuturesUnordered::new();
let mut total = 0;
loop {
    select! {
        num = fut => {
            // First, the `ready` future completes.
            total += num;
            // Then we spawn a new task onto `async_tasks`,
            async_tasks.push(async { 5 });
        },
        // On the next iteration of the loop, the task we spawned
        // completes.
        num = async_tasks.select_next_some() => {
            total += num;
        }
        // Finally, both the `ready` future and `async_tasks` have
        // finished, so we enter the `complete` branch.
        complete => break,
    }
}
assert_eq!(total, 6);

Object Safety

This trait is not object safe.

Implementors

impl<T> StreamExt for T

where T: Stream + ?Sized,