示例

extern crate want;


// Some message that is expensive to produce.
struct Expensive;

// Some futures-aware MPSC channel...
let (mut tx, mut rx) = mpsc_channel();

// And our `want` channel!
let (mut gv, mut tk) = want::new();


// Our receiving task...
spawn(async move {
    // Maybe something comes up that prevents us from ever
    // using the expensive message.
    //
    // Without `want`, the "send" task may have started to
    // produce the expensive message even though we wouldn't
    // be able to use it.
    if !we_still_want_message() {
        return;
    }

    // But we can use it! So tell the `want` channel.
    tk.want();

    match rx.recv().await {
        Some(_msg) => println!("got a message"),
        None => println!("DONE"),
    }
});

// Our sending task
spawn(async move {
    // It's expensive to create a new message, so we wait until the
    // receiving end truly *wants* the message.
    if let Err(_closed) = gv.want().await {
        // Looks like they will never want it...
        return;
    }

    // They want it, let's go!
    tx.send(Expensive);
});