stream - 一旦其中一个底层流耗尽，就使流组合耗尽

Question

如果我想将多个相同类型的流合并为一个，我会使用 Stream::select :

let combined = first_stream.select(second_stream)

但是，一旦其中一个流耗尽，另一个流仍然可以为组合流产生结果。一旦任一基础流耗尽，我可以使用什么来耗尽组合流？

Answer 1

编写您自己的流组合器:

use futures::{Async, Poll, Stream}; // 0.1.25

struct WhileBoth<S1, S2>(S1, S2)
where
    S1: Stream,
    S2: Stream<Item = S1::Item, Error = S1::Error>;

impl<S1, S2> Stream for WhileBoth<S1, S2>
where
    S1: Stream,
    S2: Stream<Item = S1::Item, Error = S1::Error>,
{
    type Item = S1::Item;
    type Error = S1::Error;

    fn poll(&mut self) -> Poll<Option<Self::Item>, Self::Error> {
        match self.0.poll() {
            // Return errors or ready values (including the `None`
            // that indicates the stream is empty) immediately.
            r @ Err(_) | r @ Ok(Async::Ready(_)) => r,
            // If the first stream is not ready, try the second one.
            Ok(Async::NotReady) => self.1.poll(),
        }
    }
}

另见:

• How can I add new methods to Iterator?