extern crate futures; // 0.1.24

use futures::{Async, Poll, Stream, task::{self, Task}};

struct Evens {
    n: u32,
    pause: bool,
    task: Option<Task>,
}

impl Default for Evens {
    fn default() -> Evens {
        Evens {n: 0, pause: false, task: None}
    }
}

impl Stream for Evens {
    type Item = u32;
    type Error = ();

    fn poll(&mut self) -> Poll<Option<Self::Item>, Self::Error> {
        let pause = self.pause;
        self.pause = !pause;
        if pause {
            self.task = Some(task::current());
            Ok(Async::NotReady)
        } else {
            let n = self.n;
            self.n += 2;
            Ok(Async::Ready(Some(n)))
        }
    }
}

fn main() -> Result<(), ()> {
    let mut evens = Evens::default();
    loop {
        match evens.poll()? {
            Async::NotReady => if let Some(task) = evens.task.take() {
                task.notify();
            },
            Async::Ready(val) => println!("{:?}", val),
        }
    }
}