Trait futures_util::stream::Stream

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pub trait Stream {
    type Item;

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

    // Provided method
    fn size_hint(&self) -> (usize, Option<usize>) { ... }
}

Expand description

A stream of values produced asynchronously.

If Future<Output = T> is an asynchronous version of T, then Stream<Item = T> is an asynchronous version of Iterator<Item = T>. A stream represents a sequence of value-producing events that occur asynchronously to the caller.

The trait is modeled after Future, but allows poll_next to be called even after a value has been produced, yielding None once the stream has been fully exhausted.

Required Associated Types§

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type Item

Values yielded by the stream.

Required Methods§

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

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

Return value

There are several possible return values, each indicating a distinct stream state:

Poll::Pending means that this stream’s next value is not ready yet. Implementations will ensure that the current task will be notified when the next value may be ready.
Poll::Ready(Some(val)) means that the stream has successfully produced a value, val, and may produce further values on subsequent poll_next calls.
Poll::Ready(None) means that the stream has terminated, and poll_next should not be invoked again.

Panics

Once a stream has finished (returned Ready(None) from poll_next), calling its poll_next method again may panic, block forever, or cause other kinds of problems; the Stream trait places no requirements on the effects of such a call. However, as the poll_next method is not marked unsafe, Rust’s usual rules apply: calls must never cause undefined behavior (memory corruption, incorrect use of unsafe functions, or the like), regardless of the stream’s state.

If this is difficult to guard against then the fuse adapter can be used to ensure that poll_next always returns Ready(None) in subsequent calls.

Provided Methods§

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

Returns the bounds on the remaining length of the stream.

Specifically, size_hint() returns a tuple where the first element is the lower bound, and the second element is the upper bound.

The second half of the tuple that is returned is an Option<usize>. A None here means that either there is no known upper bound, or the upper bound is larger than usize.

Implementation notes

It is not enforced that a stream implementation yields the declared number of elements. A buggy stream may yield less than the lower bound or more than the upper bound of elements.

size_hint() is primarily intended to be used for optimizations such as reserving space for the elements of the stream, but must not be trusted to e.g., omit bounds checks in unsafe code. An incorrect implementation of size_hint() should not lead to memory safety violations.

That said, the implementation should provide a correct estimation, because otherwise it would be a violation of the trait’s protocol.

The default implementation returns (0, None) which is correct for any stream.

Trait futures_util::stream::Stream

Required Associated Types§

type Item

Required Methods§

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

Provided Methods§

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

Implementations on Foreign Types§

impl<S> Stream for &mut Swhere S: Stream + Unpin + ?Sized,

type Item = <S as Stream>::Item

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

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

impl<S> Stream for Box<S, Global>where S: Stream + Unpin + ?Sized,

type Item = <S as Stream>::Item

fn poll_next( self: Pin<&mut Box<S, Global>>, cx: &mut Context<'_> ) -> Poll<Option<<Box<S, Global> as Stream>::Item>>

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

impl<P> Stream for Pin<P>where P: DerefMut + Unpin, <P as Deref>::Target: Stream,

type Item = <<P as Deref>::Target as Stream>::Item

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

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

impl<S> Stream for AssertUnwindSafe<S>where S: Stream,

type Item = <S as Stream>::Item

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

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

impl<T> Stream for UnboundedReceiver<T>

type Item = T

fn poll_next( self: Pin<&mut UnboundedReceiver<T>>, cx: &mut Context<'_> ) -> Poll<Option<T>>

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

impl<T> Stream for Receiver<T>

type Item = T

fn poll_next( self: Pin<&mut Receiver<T>>, cx: &mut Context<'_> ) -> Poll<Option<T>>

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

Implementors§

impl<A, B> Stream for Either<A, B>where A: Stream, B: Stream<Item = A::Item>,

type Item = <A as Stream>::Item

impl<A, F: FnMut() -> A> Stream for RepeatWith<F>

type Item = A

impl<B, St, S, Fut, F> Stream for Scan<St, S, Fut, F>where St: Stream, F: FnMut(&mut S, St::Item) -> Fut, Fut: Future<Output = Option<B>>,

type Item = B

impl<F> Stream for FlattenStream<F>where Flatten<F, <F as Future>::Output>: Stream, F: Future,

type Item = <Flatten<F, <F as Future>::Output> as Stream>::Item

impl<F> Stream for futures_util::future::IntoStream<F>where Once<F>: Stream,

type Item = <Once<F> as Stream>::Item

impl<Fut> Stream for TryFlattenStream<Fut>where TryFlatten<Fut, Fut::Ok>: Stream, Fut: TryFuture,

type Item = <TryFlatten<Fut, <Fut as TryFuture>::Ok> as Stream>::Item

impl<Fut, Si> Stream for FlattenSink<Fut, Si>where TryFlatten<Fut, Si>: Stream,

type Item = <TryFlatten<Fut, Si> as Stream>::Item

impl<Fut: Future> Stream for FuturesOrdered<Fut>

type Item = <Fut as Future>::Output

impl<Fut: Future> Stream for FuturesUnordered<Fut>

type Item = <Fut as Future>::Output

impl<Fut: Future> Stream for Once<Fut>

type Item = <Fut as Future>::Output

impl<I> Stream for Iter<I>where I: Iterator,

type Item = <I as Iterator>::Item

impl<R: AsyncBufRead> Stream for Lines<R>

type Item = Result<String, Error>

impl<S, Item> Stream for Buffer<S, Item>where S: Sink<Item> + Stream,

type Item = <S as Stream>::Item

impl<S, Item, E> Stream for SinkErrInto<S, Item, E>where S: Sink<Item> + Stream, S::Error: Into<E>,

type Item = <S as Stream>::Item

impl<S, Item, U, Fut, F> Stream for With<S, Item, U, Fut, F>where S: Stream + Sink<Item>, F: FnMut(U) -> Fut, Fut: Future,

type Item = <S as Stream>::Item

impl<S, Item, U, St, F> Stream for WithFlatMap<S, Item, U, St, F>where S: Stream + Sink<Item>, F: FnMut(U) -> St, St: Stream<Item = Result<Item, S::Error>>,

type Item = <S as Stream>::Item

impl<S: Stream> Stream for Fuse<S>

type Item = <S as Stream>::Item

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

type Item = <S as Stream>::Item

impl<S: Stream> Stream for SplitStream<S>

type Item = <S as Stream>::Item

impl<S: Stream, F> Stream for SinkMapErr<S, F>

type Item = <S as Stream>::Item

impl<St1, St2> Stream for Chain<St1, St2>where St1: Stream, St2: Stream<Item = St1::Item>,

type Item = <St1 as Stream>::Item

impl<St1, St2> Stream for Select<St1, St2>where St1: Stream, St2: Stream<Item = St1::Item>,

type Item = <St1 as Stream>::Item

impl<St1, St2> Stream for Zip<St1, St2>where St1: Stream, St2: Stream,

type Item = (<St1 as Stream>::Item, <St2 as Stream>::Item)

impl<St1, St2, Clos, State> Stream for SelectWithStrategy<St1, St2, Clos, State>where St1: Stream, St2: Stream<Item = St1::Item>, Clos: FnMut(&mut State) -> PollNext,