[−][src]Trait futures_util::stream::StreamExt

pub trait StreamExt: Stream {
    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: Default + Extend<Self::Item>>(self) -> Collect<Self, C>
    where
        Self: Sized,
    { ... }

    fn concat(self) -> Concat<Self>
    where
        Self: Sized,
        Self::Item: Extend<<Self::Item as IntoIterator>::Item> + IntoIterator + Default,
    { ... }

    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 flatten(self) -> Flatten<Self>
    where
        Self::Item: Stream,
        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 for_each<Fut, F>(self, f: F) -> ForEach<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 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 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.

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_value()] 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 evens = stream.filter(|x| future::ready(x % 2 == 0));

assert_eq!(vec![2, 4, 6, 8, 10], evens.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 evens = stream.filter_map(|x| async move {
    if x % 2 == 0 { Some(x + 1) } else { None }
});

assert_eq!(vec![3, 5, 7, 9, 11], evens.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.

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: Default + Extend<Self::Item>>(self) -> Collect<Self, C> where
    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 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 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 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 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 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 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 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 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][StreamExt::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][StreamExt::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);
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Implementors

impl<T: ?Sized> StreamExt for T where
    T: Stream, [src]

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