Streams

Stream trait 类似于Future，但可以在完成之前，yield 多个值，类似于标准库的 Iterator trait：

trait Stream {
    /// The type of the value yielded by the stream.
    type Item;

    /// Attempt to resolve the next item in the stream.
    /// Retuns `Poll::Pending` if not ready, `Poll::Ready(Some(x))` if a value
    /// is ready, and `Poll::Ready(None)` if the stream has completed.
    fn poll_next(self: Pin<&mut Self>, cx: &mut Context<'_>)
        -> Poll<Option<Self::Item>>;
}

一个Stream的常见例子是，这个Receiver用于futures箱子的 channel 类型。它会在每次Sender端发送一个值，都会 yield Some(val)，并且一旦Sender被 dropped 和接收到了所有 pending 消息，就会 yield None：

async fn send_recv() {
    const BUFFER_SIZE: usize = 10;
    let (mut tx, mut rx) = mpsc::channel::<i32>(BUFFER_SIZE);

    tx.send(1).await.unwrap();
    tx.send(2).await.unwrap();
    drop(tx);

    // `StreamExt::next` is similar to `Iterator::next`, but returns a
    // type that implements `Future<Output = Option<T>>`.
    assert_eq!(Some(1), rx.next().await);
    assert_eq!(Some(2), rx.next().await);
    assert_eq!(None, rx.next().await);
}