已应用：生成一个执行器

Rust 的Future是懒惰的：除非是向着'完成'这一个目标积极前进，否则他们不会做任何事情。向 Future 完成前进的一种方法是，在async函数里面，对它.await，但这只会将问题升了个级：谁来管理，从顶层 async函数返回的 Futures ？答案是：我们需要一个Future执行者（executor）。

Future executor 获取一组顶层Future，并每当Future可以前进时，通过调用poll，让它们驶向完成。通常一旦开始，executor 会poll一个 Future 。当Future表示，因wake()的调用准备好前进，会将它们先放回到一个队列，才再次poll，重复直到Future已经完成。

在本节中，我们将编写自己的简单 executor，该 executor 能够让大量顶层 Future 同时驶向完成。

在此示例中，我们依赖futures箱子，ArcWake trait 会用到，它提供了一种轻松的方法来构建Waker。

[package]
name = "xyz"
version = "0.1.0"
authors = ["XYZ Author"]
edition = "2018"

[dependencies]
futures-preview = "=0.3.0-alpha.17"

接下来，我们需要在顶部，添加以下内容src/main.rs：

use {
    futures::{
        future::{FutureExt, BoxFuture},
        task::{ArcWake, waker_ref},
    },
    std::{
        future::Future,
        sync::{Arc, Mutex},
        sync::mpsc::{sync_channel, SyncSender, Receiver},
        task::{Context, Poll},
        time::Duration,
    },
    // The timer we wrote in the previous section:
    timer_future::TimerFuture,
};

我们的 executor 的工作是，将通过发送任务，在通道上运行。executor 将事件从通道中拉出，并运行它们。当一个任务准备做更多的工作（被唤醒）时，它可以安排自己重新回到通道上，以计划再次进行轮询。

在这种设计中，executor 本身仅需要任务通道的接收端。用户将获得发送端，以便他们可以生成新的 Future 。任务本身就是 Future，是可以重新计划自己的。因此，我们会将它们与一个 sender 每每存储在一起，这样，任务就可以用来让自己重新排队。

/// Task executor that receives tasks off of a channel and runs them.
struct Executor {
    ready_queue: Receiver<Arc<Task>>,
}

/// `Spawner` spawns new futures onto the task channel.
#[derive(Clone)]
struct Spawner {
    task_sender: SyncSender<Arc<Task>>,
}

/// A future that can reschedule itself to be polled by an `Executor`.
struct Task {
    /// In-progress future that should be pushed to completion.
    ///
    /// The `Mutex` is not necessary for correctness, since we only have
    /// one thread executing tasks at once. However, Rust isn't smart
    /// enough to know that `future` is only mutated from one thread,
    /// so we need use the `Mutex` to prove thread-safety. A production
    /// executor would not need this, and could use `UnsafeCell` instead.
    future: Mutex<Option<BoxFuture<'static, ()>>>,

    /// Handle to place the task itself back onto the task queue.
    task_sender: SyncSender<Arc<Task>>,
}

fn new_executor_and_spawner() -> (Executor, Spawner) {
    // Maximum number of tasks to allow queueing in the channel at once.
    // This is just to make `sync_channel` happy, and wouldn't be present in
    // a real executor.
    const MAX_QUEUED_TASKS: usize = 10_000;
    let (task_sender, ready_queue) = sync_channel(MAX_QUEUED_TASKS);
    (Executor { ready_queue }, Spawner { task_sender })
}

我们还向 spawner 添加一种方法，以使其易于生成新的 Future 。此方法将拿到一个 Future 类型，将其装箱，并放入 FutureObj 中，然后创建一个新类型Arc<Task>，它的内部可以在 executor 上，排队。

impl Spawner {
    fn spawn(&self, future: impl Future<Output = ()> + 'static + Send) {
        let future = future.boxed();
        let task = Arc::new(Task {
            future: Mutex::new(Some(future)),
            task_sender: self.task_sender.clone(),
        });
        self.task_sender.send(task).expect("too many tasks queued");
    }
}

要轮询 Future ，我们需要创建一个Waker。正如在唤醒章节，Waker负责安排，一旦wake调用了，就再次轮询的任务。记住，Wakers 是会告诉 executor，确切的那些任务已经准备就绪，只轮询准备好前进的 Future。创建一个新的Waker最简单的方法是，通过实现ArcWake trait ，然后使用waker_ref要么.into_waker()函数，将Arc<impl ArcWake>转变成一个Waker。让我们，为我们的任务实现ArcWake，这样就可以转变为Waker，和被唤醒啦：

impl ArcWake for Task {
    fn wake_by_ref(arc_self: &Arc<Self>) {
        // Implement `wake` by sending this task back onto the task channel
        // so that it will be polled again by the executor.
        let cloned = arc_self.clone();
        arc_self.task_sender.send(cloned).expect("too many tasks queued");
    }
}

当一个新建的Waker，从Arc<Task>而来，那么我们在它上面调用wake()，将导致Arc的一个 copy 发送到任务通道。然后，我们的 executor 需要选择任务，并进行轮询。让我们实现一下：

impl Executor {
    fn run(&self) {
        while let Ok(task) = self.ready_queue.recv() {
            // Take the future, and if it has not yet completed (is still Some),
            // poll it in an attempt to complete it.
            let mut future_slot = task.future.lock().unwrap();
            if let Some(mut future) = future_slot.take() {
                // Create a `LocalWaker` from the task itself
                let waker = waker_ref(&task);
                let context = &mut Context::from_waker(&*waker);
                // `BoxFuture<T>` is a type alias for
                // `Pin<Box<dyn Future<Output = T> + Send + 'static>>`.
                // We can get a `Pin<&mut dyn Future + Send + 'static>`
                // from it by calling the `Pin::as_mut` method.
                if let Poll::Pending = future.as_mut().poll(context) {
                    // We're not done processing the future, so put it
                    // back in its task to be run again in the future.
                    *future_slot = Some(future);
                }
            }
        }
    }
}

恭喜你！我们现在有一个能工作的 Future executor。我们甚至可以使用它，来运行async/.await代码和自定义 Future ，例如，我们之前写过的TimerFuture：

fn main() {
    let (executor, spawner) = new_executor_and_spawner();

    // Spawn a task to print before and after waiting on a timer.
    spawner.spawn(async {
        println!("howdy!");
        // Wait for our timer future to complete after two seconds.
        TimerFuture::new(Duration::new(2, 0)).await;
        println!("done!");
    });

    // Drop the spawner so that our executor knows it is finished and won't
    // receive more incoming tasks to run.
    drop(spawner);

    // Run the executor until the task queue is empty.
    // This will print "howdy!", pause, and then print "done!".
    executor.run();
}