pub trait StreamExt: Stream {
Show 17 methods
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,
{ ... }
}Expand description
An extension trait for Streams that provides a variety of convenient
combinator functions.
Provided methods
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);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"));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));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 (mut tx1, rx1) = mpsc::channel(10);
let (mut 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::delay_for(Duration::from_millis(20)).await;
tx1.send(4).await.unwrap();
});
tokio::spawn(async move {
// Wait for the first task to send values
time::delay_for(Duration::from_millis(5)).await;
tx2.send(3).await.unwrap();
time::delay_for(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());
}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);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);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);
}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);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);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);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);Tests if every element of the stream matches a predicate.
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));Tests if any element of the stream matches a predicate.
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));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);
}A combinator that applies a function to every element in a stream producing a single, final value.
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,
fn collect<T>(self) -> Collect<Self, T> where
T: FromStream<Self::Item>,
Self: Sized,
Drain stream pushing all emitted values into a collection.
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);
}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));