source: example/maybe-await.rs

use futures::executor::block_on;
use futures::future::Future;
use std::pin::Pin;
use std::sync::{Arc,Mutex};
use std::task::{Context, Poll, Waker};


struct FutureState {
        set: bool,
        waker: Option<Waker>,
}

#[derive(Clone)]
struct MyFuture {
        state: Arc<Mutex<FutureState>>
}

impl MyFuture {
        fn new() -> MyFuture {
                MyFuture{
                        state: Arc::new(Mutex::new(FutureState{
                                set: false,
                                waker: None
                        }))
                }
        }

        fn fulfill(&self) {
                let mut state = self.state.lock().unwrap();
                state.set = true;
                if let Some(waker) = state.waker.take() {
                        waker.wake()
                }
        }
}

impl Future for MyFuture {
        type Output = ();
        fn poll(self: Pin<&mut Self>, cx: &mut Context<'_>) -> Poll<Self::Output> {
                println!("Polling Future");
                let mut state = self.state.lock().unwrap();
                if state.set {
                        println!("Ready");
                        Poll::Ready(())
                } else {
                        println!("Pending");
                        state.waker = Some(cx.waker().clone());
                        Poll::Pending
                }
        }
}

async fn hello_world(f1: MyFuture, f2: MyFuture) {
        println!("Enter");
        f1.await;
        println!("Between");
        f2.await;
        println!("Done");
}

fn main() {
        block_on(async {
                let f1 = MyFuture::new();
                let f2 = MyFuture::new();
                let f3 = MyFuture::new();
                let future = hello_world(f1.clone(), f2.clone());

                println!("Before first fulfill");
                f1.fulfill();
                println!("Before second fulfill");
                f2.fulfill();
                println!("Before first await");
                future.await;
                println!("Before second await");
                // future.await;
        });
}