stream - 延迟 Tokio Stream

Question

给定一个 Stream，我想创建一个新的 Stream，其中的元素在它们之间有时间延迟。

我尝试使用 tokio_core::reactor::Timeout 和 Stream 的 and_then 组合器来编写代码，但是延迟不起作用:我立即获得所有元素，没有延迟。

这是一个独立的示例 ( playground ):

extern crate tokio_core;
extern crate futures;

use std::time::Duration;

use futures::{Future, Stream, stream, Sink};
use self::futures::sync::{mpsc};
use tokio_core::reactor;

const NUM_ITEMS: u32 = 8;

fn main() {
    let mut core = reactor::Core::new().unwrap();
    let handle = core.handle();
    let chandle = handle.clone();

    let (sink, stream) = mpsc::channel::<u32>(0);

    let send_stream = stream::iter_ok(0 .. NUM_ITEMS)
        .and_then(move |i: u32| {
                  let cchandle = chandle.clone();
                  println!("Creating a timeout object...");
                  reactor::Timeout::new(Duration::new(1,0), &cchandle)
                      .map_err(|_| ())
                      .and_then(|_| Ok(i))
        });

    let sink = sink.sink_map_err(|_| ());
    handle.spawn(sink.send_all(send_stream).and_then(|_| Ok(())));

    let mut incoming_items = Vec::new();
    {
        let keep_messages = stream.for_each(|item| {
            incoming_items.push(item);
            println!("item = {}", item);
            Ok(())
        });

        core.run(keep_messages).unwrap();
    }
    assert_eq!(incoming_items, (0 .. NUM_ITEMS).collect::<Vec<u32>>());
}

为了完整起见，这是我得到的输出:

Creating a timeout object...
Creating a timeout object...
item = 0
Creating a timeout object...
item = 1
Creating a timeout object...
item = 2
Creating a timeout object...
item = 3
Creating a timeout object...
item = 4
Creating a timeout object...
item = 5
Creating a timeout object...
item = 6
item = 7

我怀疑问题出在这些行中:

reactor::Timeout::new(Duration::new(1,0), &cchandle)
    .map_err(|_| ())
    .and_then(|_| Ok(i))

有可能我并没有真正等待返回的 Timeout 对象，尽管我不确定如何解决它。

Answer 1

正如我所怀疑的，问题出在对新创建的 Timeout 的操作(使用 and_then)。我们要么需要先解包调用 reactor::Timeout::new 的结果，如果手动完成可能会变得困惑，或者使用 into_future 来转换结果进入 Future，然后使用 Future 组合器处理它。

问题解决代码:

extern crate tokio_core;
extern crate futures;

use std::time::Duration;

use futures::{Future, Stream, stream, Sink, IntoFuture};
use self::futures::sync::{mpsc};
use tokio_core::reactor;

const NUM_ITEMS: u32 = 8;

fn main() {
    let mut core = reactor::Core::new().unwrap();
    let handle = core.handle();
    let chandle = handle.clone();

    let (sink, stream) = mpsc::channel::<u32>(0);

    let send_stream = stream::iter_ok(0 .. NUM_ITEMS)
        .and_then(move |i: u32| {
                  let cchandle = chandle.clone();
                  println!("Creating a timeout object...");
                  reactor::Timeout::new(Duration::new(1,0), &cchandle)
                      .into_future()
                      .and_then(move |timeout| timeout.and_then(move |_| Ok(i)))
                      .map_err(|_| ())
        });

    let sink = sink.sink_map_err(|_| ());
    handle.spawn(sink.send_all(send_stream).and_then(|_| Ok(())));

    let mut incoming_items = Vec::new();
    {
        let keep_messages = stream.for_each(|item| {
            incoming_items.push(item);
            println!("item = {}", item);
            Ok(())
        });

        core.run(keep_messages).unwrap();
    }
    assert_eq!(incoming_items, (0 .. NUM_ITEMS).collect::<Vec<u32>>());
}

请注意，使用了两个 and_then。第一个解包通过调用 reactor::Timeout::new 获得的 Result。第二个实际上等待 Timeout 触发。