Trait futures_util::stream::Stream
source · [−]pub trait Stream {
type Item;
fn poll_next(
self: Pin<&mut Self>,
cx: &mut Context<'_>
) -> Poll<Option<Self::Item>>;
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.
Associated Types
Required methods
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::Pendingmeans 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 subsequentpoll_nextcalls. -
Poll::Ready(None)means that the stream has terminated, andpoll_nextshould 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
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.