task-model/real/tasks.md commit ffb00140a767d6e7a4a8875bf6965d10f830a271

任务

futures库没有直接定义Tasktrait, 取而代之地定义了更加通用的概念futures, 我们很快将深入了解它的细节. 现在, 让我们看看future的核心定义吧:

# #![allow(unused_variables)]
#fn main() {
/// An asynchronous computation that completes with a value or an error.
trait Future {
    type Item;
    type Error;

    /// Attempt to complete the future, yielding `Ok(Async::Pending)`
    /// if the future is blocked waiting for some other event to occur.
    fn poll(&mut self, cx: task::Context) -> Poll<Self::Item, Self::Error>;
}

type Poll<T, E> = Result<Async<T>, E>;
#}

Futures很像任务, 除了他们返回的是一个result(这允许它们进行组合). 换言之, 你可以 将任务理解为是任意实现了Future<Item = (), Error = !>的类型

然而, 它们还有其他区别: Futures不需要WakeHandle参数. 实践中, 这个参数几乎总是 从执行器处创建到将句柄(handle)排队时被传递下来并保持不变, 因此, 在futures库 里, 执行器在现成局部变量里面提供了方便的WakeHanle. 你可以用futures::task里 的curent_wake函数来获得它:

# #![allow(unused_variables)]
#fn main() {
/// When called within a task being executed, returns the wakeup handle for
/// that task. Panics if called outside of task execution.
fn current_wake() -> WakeHandle;
#}

理解Future和ToyTask的关联

能够知道Future和ToyTask如何精确地联系在一起是很有帮助的. 为此, 我们引入了 一个包装类型来将ToyTask转换成Future:

# #![allow(unused_variables)]
#fn main() {
use futures::task;
use futures::{Future, AsyncResult};

struct ToyTaskToFuture<T>(T);

impl<T: ToyTask> Future for ToyTaskToFuture<T> {
    type Item = ();
    type Error = !;

    fn get(&mut self) -> AsyncResult<(), !> {
        Ok(self.0.complete(task::current_wake()))
    }
}
#}