tokio/tokio/tests/stream_chain.rs

use tokio::stream::{self, Stream, StreamExt};
use tokio::sync::mpsc;
use tokio_test::{assert_pending, assert_ready, task};

#[tokio::test]
async fn basic_usage() {
    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.size_hint(), (6, Some(6)));
    assert_eq!(stream.next().await, Some(1));

    assert_eq!(stream.size_hint(), (5, Some(5)));
    assert_eq!(stream.next().await, Some(2));

    assert_eq!(stream.size_hint(), (4, Some(4)));
    assert_eq!(stream.next().await, Some(3));

    assert_eq!(stream.size_hint(), (3, Some(3)));
    assert_eq!(stream.next().await, Some(4));

    assert_eq!(stream.size_hint(), (2, Some(2)));
    assert_eq!(stream.next().await, Some(5));

    assert_eq!(stream.size_hint(), (1, Some(1)));
    assert_eq!(stream.next().await, Some(6));

    assert_eq!(stream.size_hint(), (0, Some(0)));
    assert_eq!(stream.next().await, None);

    assert_eq!(stream.size_hint(), (0, Some(0)));
    assert_eq!(stream.next().await, None);
}

#[tokio::test]
async fn pending_first() {
    let (tx1, rx1) = mpsc::unbounded_channel();
    let (tx2, rx2) = mpsc::unbounded_channel();

    let mut stream = task::spawn(rx1.chain(rx2));
    assert_eq!(stream.size_hint(), (0, None));

    assert_pending!(stream.poll_next());

    tx2.send(2).unwrap();
    assert!(!stream.is_woken());

    assert_pending!(stream.poll_next());

    tx1.send(1).unwrap();
    assert!(stream.is_woken());
    assert_eq!(Some(1), assert_ready!(stream.poll_next()));

    assert_pending!(stream.poll_next());

    drop(tx1);

    assert_eq!(stream.size_hint(), (0, None));

    assert!(stream.is_woken());
    assert_eq!(Some(2), assert_ready!(stream.poll_next()));

    assert_eq!(stream.size_hint(), (0, None));

    drop(tx2);

    assert_eq!(stream.size_hint(), (0, None));
    assert_eq!(None, assert_ready!(stream.poll_next()));
}