Trait tokio::stream::StreamExt

pub trait StreamExt: Stream {
    fn next(&mut self) -> Next<'_, Self>
    where
        Self: Unpin,
    { ... }
    fn try_next<T, E>(&mut self) -> TryNext<'_, Self>
    where
        Self: Stream<Item = Result<T, E>> + Unpin,
    { ... }
    fn map<T, F>(self, f: F) -> Map<Self, F>
    where
        F: FnMut(Self::Item) -> T,
        Self: Sized,
    { ... }
    fn merge<U>(self, other: U) -> Merge<Self, U>
    where
        U: Stream<Item = Self::Item>,
        Self: Sized,
    { ... }
    fn filter<F>(self, f: F) -> Filter<Self, F>
    where
        F: FnMut(&Self::Item) -> bool,
        Self: Sized,
    { ... }
    fn filter_map<T, F>(self, f: F) -> FilterMap<Self, F>
    where
        F: FnMut(Self::Item) -> Option<T>,
        Self: Sized,
    { ... }
    fn fuse(self) -> Fuse<Self>
    where
        Self: Sized,
    { ... }
    fn take(self, n: usize) -> Take<Self>
    where
        Self: Sized,
    { ... }
    fn take_while<F>(self, f: F) -> TakeWhile<Self, F>
    where
        F: FnMut(&Self::Item) -> bool,
        Self: Sized,
    { ... }
    fn skip(self, n: usize) -> Skip<Self>
    where
        Self: Sized,
    { ... }
    fn skip_while<F>(self, f: F) -> SkipWhile<Self, F>
    where
        F: FnMut(&Self::Item) -> bool,
        Self: Sized,
    { ... }
    fn all<F>(&mut self, f: F) -> AllFuture<'_, Self, F>
    where
        Self: Unpin,
        F: FnMut(Self::Item) -> bool,
    { ... }
    fn any<F>(&mut self, f: F) -> AnyFuture<'_, Self, F>
    where
        Self: Unpin,
        F: FnMut(Self::Item) -> bool,
    { ... }
    fn chain<U>(self, other: U) -> Chain<Self, U>
    where
        U: Stream<Item = Self::Item>,
        Self: Sized,
    { ... }
    fn fold<B, F>(self, init: B, f: F) -> FoldFuture<Self, B, F>
    where
        Self: Sized,
        F: FnMut(B, Self::Item) -> B,
    { ... }
    fn collect<T>(self) -> Collect<Self, T>
    where
        T: FromStream<Self::Item>,
        Self: Sized,
    { ... }
    fn timeout(self, duration: Duration) -> Timeout<Self>
    where
        Self: Sized,
    { ... }
    fn throttle(self, duration: Duration) -> Throttle<Self>
    where
        Self: Sized,
    { ... }
}

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

Be aware that the Stream trait in Tokio is a re-export of the trait found in the futures crate, however both Tokio and futures provide separate StreamExt utility traits, and some utilities are only available on one of these traits. Click here to see the other StreamExt trait in the futures crate.

If you need utilities from both StreamExt traits, you should prefer to import one of them, and use the other through the fully qualified call syntax. For example:

// import one of the traits:
use futures::stream::StreamExt;

let a = tokio::stream::iter(vec![1, 3, 5]);
let b = tokio::stream::iter(vec![2, 4, 6]);

// use the fully qualified call syntax for the other trait:
let merged = tokio::stream::StreamExt::merge(a, b);

// use normal call notation for futures::stream::StreamExt::collect
let output: Vec<_> = merged.collect().await;
assert_eq!(output, vec![1, 2, 3, 4, 5, 6]);

Provided methods

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

Consumes and returns the next value in the stream or None if the stream is finished.

Equivalent to:

async fn next(&mut self) -> Option<Self::Item>;

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 tokio::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 try_next<T, E>(&mut self) -> TryNext<'_, Self> where
    Self: Stream<Item = Result<T, E>> + Unpin, 

Consumes and returns the next item in the stream. If an error is encountered before the next item, the error is returned instead.

Equivalent to:

async fn try_next(&mut self) -> Result<Option<T>, E>;

This is similar to the next combinator, but returns a Result<Option<T>, E> rather than an Option<Result<T, E>>, making for easy use with the ? operator.

Examples

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

let mut stream = stream::iter(vec![Ok(1), Ok(2), Err("nope")]);

assert_eq!(stream.try_next().await, Ok(Some(1)));
assert_eq!(stream.try_next().await, Ok(Some(2)));
assert_eq!(stream.try_next().await, Err("nope"));

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 tokio::stream::{self, StreamExt};

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

assert_eq!(stream.next().await, Some(4));
assert_eq!(stream.next().await, Some(5));
assert_eq!(stream.next().await, Some(6));

fn merge<U>(self, other: U) -> Merge<Self, U> where
    U: Stream<Item = Self::Item>,
    Self: Sized, 

Combine two streams into one by interleaving the output of both as it is produced.

Values are produced from the merged stream in the order they arrive from the two source streams. If both source streams provide values simultaneously, the merge stream alternates between them. This provides some level of fairness. You should not chain calls to merge, as this will break the fairness of the merging.

The merged stream completes once both source streams complete. When one source stream completes before the other, the merge stream exclusively polls the remaining stream.

For merging multiple streams, consider using StreamMap instead.

Examples

use tokio::stream::StreamExt;
use tokio::sync::mpsc;
use tokio::time;

use std::time::Duration;

#[tokio::main]
async fn main() {
    let (tx1, rx1) = mpsc::channel(10);
    let (tx2, rx2) = mpsc::channel(10);

    let mut rx = rx1.merge(rx2);

    tokio::spawn(async move {
        // Send some values immediately
        tx1.send(1).await.unwrap();
        tx1.send(2).await.unwrap();

        // Let the other task send values
        time::sleep(Duration::from_millis(20)).await;

        tx1.send(4).await.unwrap();
    });

    tokio::spawn(async move {
        // Wait for the first task to send values
        time::sleep(Duration::from_millis(5)).await;

        tx2.send(3).await.unwrap();

        time::sleep(Duration::from_millis(25)).await;

        // Send the final value
        tx2.send(5).await.unwrap();
    });

   assert_eq!(1, rx.next().await.unwrap());
   assert_eq!(2, rx.next().await.unwrap());
   assert_eq!(3, rx.next().await.unwrap());
   assert_eq!(4, rx.next().await.unwrap());
   assert_eq!(5, rx.next().await.unwrap());

   // The merged stream is consumed
   assert!(rx.next().await.is_none());
}

fn filter<F>(self, f: F) -> Filter<Self, F> where
    F: FnMut(&Self::Item) -> bool,
    Self: Sized, 

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

As values of this stream are made available, the provided predicate f will be run against them. If the predicate resolves to true, then the stream will yield the value, but if the predicate 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 Iterator::filter method in the standard library.

Examples

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

let stream = stream::iter(1..=8);
let mut evens = stream.filter(|x| x % 2 == 0);

assert_eq!(Some(2), evens.next().await);
assert_eq!(Some(4), evens.next().await);
assert_eq!(Some(6), evens.next().await);
assert_eq!(Some(8), evens.next().await);
assert_eq!(None, evens.next().await);

fn filter_map<T, F>(self, f: F) -> FilterMap<Self, F> where
    F: FnMut(Self::Item) -> Option<T>,
    Self: Sized, 

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

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

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

Examples

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

let stream = stream::iter(1..=8);
let mut evens = stream.filter_map(|x| {
    if x % 2 == 0 { Some(x + 1) } else { None }
});

assert_eq!(Some(3), evens.next().await);
assert_eq!(Some(5), evens.next().await);
assert_eq!(Some(7), evens.next().await);
assert_eq!(Some(9), evens.next().await);
assert_eq!(None, evens.next().await);

fn fuse(self) -> Fuse<Self> where
    Self: Sized, 

Creates a stream which ends after the first None.

After a stream returns None, behavior is undefined. Future calls to poll_next may or may not return Some(T) again or they may panic. fuse() adapts a stream, ensuring that after None is given, it will return None forever.

Examples

use tokio::stream::{Stream, StreamExt};

use std::pin::Pin;
use std::task::{Context, Poll};

// a stream which alternates between Some and None
struct Alternate {
    state: i32,
}

impl Stream for Alternate {
    type Item = i32;

    fn poll_next(mut self: Pin<&mut Self>, _cx: &mut Context<'_>) -> Poll<Option<i32>> {
        let val = self.state;
        self.state = self.state + 1;

        // if it's even, Some(i32), else None
        if val % 2 == 0 {
            Poll::Ready(Some(val))
        } else {
            Poll::Ready(None)
        }
    }
}

#[tokio::main]
async fn main() {
    let mut stream = Alternate { state: 0 };

    // the stream goes back and forth
    assert_eq!(stream.next().await, Some(0));
    assert_eq!(stream.next().await, None);
    assert_eq!(stream.next().await, Some(2));
    assert_eq!(stream.next().await, None);

    // however, once it is fused
    let mut stream = stream.fuse();

    assert_eq!(stream.next().await, Some(4));
    assert_eq!(stream.next().await, None);

    // it will always return `None` after the first time.
    assert_eq!(stream.next().await, None);
    assert_eq!(stream.next().await, None);
    assert_eq!(stream.next().await, None);
}

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 tokio::stream::{self, StreamExt};

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

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

fn take_while<F>(self, f: F) -> TakeWhile<Self, F> where
    F: FnMut(&Self::Item) -> bool,
    Self: Sized, 

Take elements from this stream while the provided 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 tokio::stream::{self, StreamExt};

let mut stream = stream::iter(1..=10).take_while(|x| *x <= 3);

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

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

Creates a new stream that will skip the n first items of the underlying stream.

Examples

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

let mut stream = stream::iter(1..=10).skip(7);

assert_eq!(Some(8), stream.next().await);
assert_eq!(Some(9), stream.next().await);
assert_eq!(Some(10), stream.next().await);
assert_eq!(None, stream.next().await);

fn skip_while<F>(self, f: F) -> SkipWhile<Self, F> where
    F: FnMut(&Self::Item) -> bool,
    Self: Sized, 

Skip elements from the underlying stream while the provided predicate resolves to true.

This function, like Iterator::skip_while, will ignore elemets from the stream until the predicate f resolves to false. Once one element returns false, the rest of the elements will be yielded.

Examples

use tokio::stream::{self, StreamExt};
let mut stream = stream::iter(vec![1,2,3,4,1]).skip_while(|x| *x < 3);

assert_eq!(Some(3), stream.next().await);
assert_eq!(Some(4), stream.next().await);
assert_eq!(Some(1), stream.next().await);
assert_eq!(None, stream.next().await);

fn all<F>(&mut self, f: F) -> AllFuture<'_, Self, F> where
    Self: Unpin,
    F: FnMut(Self::Item) -> bool, 

Tests if every element of the stream matches a predicate.

Equivalent to:

async fn all<F>(&mut self, f: F) -> bool;

all() takes a closure that returns true or false. It applies this closure to each element of the stream, and if they all return true, then so does all. If any of them return false, it returns false. An empty stream returns true.

all() is short-circuiting; in other words, it will stop processing as soon as it finds a false, given that no matter what else happens, the result will also be false.

An empty stream returns true.

Examples

Basic usage:

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

let a = [1, 2, 3];

assert!(stream::iter(&a).all(|&x| x > 0).await);

assert!(!stream::iter(&a).all(|&x| x > 2).await);

Stopping at the first false:

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

let a = [1, 2, 3];

let mut iter = stream::iter(&a);

assert!(!iter.all(|&x| x != 2).await);

// we can still use `iter`, as there are more elements.
assert_eq!(iter.next().await, Some(&3));

fn any<F>(&mut self, f: F) -> AnyFuture<'_, Self, F> where
    Self: Unpin,
    F: FnMut(Self::Item) -> bool, 

Tests if any element of the stream matches a predicate.

Equivalent to:

async fn any<F>(&mut self, f: F) -> bool;

any() takes a closure that returns true or false. It applies this closure to each element of the stream, and if any of them return true, then so does any(). If they all return false, it returns false.

any() is short-circuiting; in other words, it will stop processing as soon as it finds a true, given that no matter what else happens, the result will also be true.

An empty stream returns false.

Basic usage:

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

let a = [1, 2, 3];

assert!(stream::iter(&a).any(|&x| x > 0).await);

assert!(!stream::iter(&a).any(|&x| x > 5).await);

Stopping at the first true:

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

let a = [1, 2, 3];

let mut iter = stream::iter(&a);

assert!(iter.any(|&x| x != 2).await);

// we can still use `iter`, as there are more elements.
assert_eq!(iter.next().await, Some(&2));

fn chain<U>(self, other: U) -> Chain<Self, U> where
    U: Stream<Item = Self::Item>,
    Self: Sized, 

Combine two streams into one by first returning all values from the first stream then all values from the second stream.

As long as self still has values to emit, no values from other are emitted, even if some are ready.

Examples

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

#[tokio::main]
async fn main() {
    let one = stream::iter(vec![1, 2, 3]);
    let two = stream::iter(vec![4, 5, 6]);

    let mut stream = one.chain(two);

    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, Some(4));
    assert_eq!(stream.next().await, Some(5));
    assert_eq!(stream.next().await, Some(6));
    assert_eq!(stream.next().await, None);
}

fn fold<B, F>(self, init: B, f: F) -> FoldFuture<Self, B, F> where
    Self: Sized,
    F: FnMut(B, Self::Item) -> B, 

A combinator that applies a function to every element in a stream producing a single, final value.

Equivalent to:

async fn fold<B, F>(self, init: B, f: F) -> B;

Examples

Basic usage:

use tokio::stream::{self, *};

let s = stream::iter(vec![1u8, 2, 3]);
let sum = s.fold(0, |acc, x| acc + x).await;

assert_eq!(sum, 6);

fn collect<T>(self) -> Collect<Self, T> where
    T: FromStream<Self::Item>,
    Self: Sized, 

Drain stream pushing all emitted values into a collection.

Equivalent to:

async fn collect<T>(self) -> T;

collect streams all values, awaiting as needed. Values are pushed into a collection. A number of different target collection types are supported, including Vec, String, and Bytes.

Result

collect() can also be used with streams of type Result<T, E> where T: FromStream<_>. In this case, collect() will stream as long as values yielded from the stream are Ok(_). If Err(_) is encountered, streaming is terminated and collect() returns the Err.

Notes

FromStream is currently a sealed trait. Stabilization is pending enhancements to the Rust language.

Examples

Basic usage:

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

#[tokio::main]
async fn main() {
    let doubled: Vec<i32> =
        stream::iter(vec![1, 2, 3])
            .map(|x| x * 2)
            .collect()
            .await;

    assert_eq!(vec![2, 4, 6], doubled);
}

Collecting a stream of Result values

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

#[tokio::main]
async fn main() {
    // A stream containing only `Ok` values will be collected
    let values: Result<Vec<i32>, &str> =
        stream::iter(vec![Ok(1), Ok(2), Ok(3)])
            .collect()
            .await;

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

    // A stream containing `Err` values will return the first error.
    let results = vec![Ok(1), Err("no"), Ok(2), Ok(3), Err("nein")];

    let values: Result<Vec<i32>, &str> =
        stream::iter(results)
            .collect()
            .await;

    assert_eq!(Err("no"), values);
}

fn timeout(self, duration: Duration) -> Timeout<Self> where
    Self: Sized, 

Applies a per-item timeout to the passed stream.

timeout() takes a Duration that represents the maximum amount of time each element of the stream has to complete before timing out.

If the wrapped stream yields a value before the deadline is reached, the value is returned. Otherwise, an error is returned. The caller may decide to continue consuming the stream and will eventually get the next source stream value once it becomes available.

Notes

This function consumes the stream passed into it and returns a wrapped version of it.

Polling the returned stream will continue to poll the inner stream even if one or more items time out.

Examples

Suppose we have a stream int_stream that yields 3 numbers (1, 2, 3):

use tokio::stream::{self, StreamExt};
use std::time::Duration;

let mut int_stream = int_stream.timeout(Duration::from_secs(1));

// When no items time out, we get the 3 elements in succession:
assert_eq!(int_stream.try_next().await, Ok(Some(1)));
assert_eq!(int_stream.try_next().await, Ok(Some(2)));
assert_eq!(int_stream.try_next().await, Ok(Some(3)));
assert_eq!(int_stream.try_next().await, Ok(None));

// If the second item times out, we get an error and continue polling the stream:
assert_eq!(int_stream.try_next().await, Ok(Some(1)));
assert!(int_stream.try_next().await.is_err());
assert_eq!(int_stream.try_next().await, Ok(Some(2)));
assert_eq!(int_stream.try_next().await, Ok(Some(3)));
assert_eq!(int_stream.try_next().await, Ok(None));

// If we want to stop consuming the source stream the first time an
// element times out, we can use the `take_while` operator:
let mut int_stream = int_stream.take_while(Result::is_ok);

assert_eq!(int_stream.try_next().await, Ok(Some(1)));
assert_eq!(int_stream.try_next().await, Ok(None));

fn throttle(self, duration: Duration) -> Throttle<Self> where
    Self: Sized, 

Slows down a stream by enforcing a delay between items.

Example

Create a throttled stream.

use std::time::Duration;
use tokio::stream::StreamExt;

let mut item_stream = futures::stream::repeat("one").throttle(Duration::from_secs(2));

loop {
    // The string will be produced at most every 2 seconds
    println!("{:?}", item_stream.next().await);
}

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Implementors

impl<St: ?Sized> StreamExt for St where
    St: Stream, 

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