RUST 实现 Future trait
rust 的 Future 可以简单理解成一个状态机，async/await 是实现 Future trait 的语法糖
use std::{sync::{Arc, Mutex}, task::Waker, time::Duration};#[tokio::main]
async fn main() {println!("main start");SleepFut::new(Duration::from_secs(3)).await;println!("main end");
}struct SleepFut {duration: Duration,state: Arc<Mutex<SleepState>>,
}struct SleepState {waker: Option<Waker>,inner_state: SleepInnerState,
}enum SleepInnerState {Init,Sleeping,Done,
}impl SleepFut {fn new(duration: Duration) -> Self {SleepFut {duration,state: Arc::new(Mutex::new(SleepState {waker: None,inner_state: SleepInnerState::Init,})),}}
}impl Future for SleepFut {type Output = ();fn poll(self: std::pin::Pin<&mut Self>, cx: &mut std::task::Context<'_>) -> std::task::Poll<Self::Output> {println!("polling...");let mut guard = self.state.lock().unwrap();match guard.inner_state {SleepInnerState::Init => {println!("init");guard.inner_state = SleepInnerState::Sleeping;let duration = self.duration;let state = self.state.clone();std::thread::spawn(move || {println!("thread start");std::thread::sleep(duration);let mut guard = state.lock().unwrap();guard.inner_state = SleepInnerState::Done;if let Some(waker) = guard.waker.take() {waker.wake();}println!("thread end");});guard.waker = Some(cx.waker().clone());std::task::Poll::Pending}SleepInnerState::Sleeping => {// 只有当新的 waker 与现有 waker 不同时才更新// 这里使用 will_wake 进行优化match &guard.waker {Some(w) if w.will_wake(cx.waker()) => {// 已经是相同的 waker，不需要更新}_ => {// 更新 wakerguard.waker = Some(cx.waker().clone());}}std::task::Poll::Pending},SleepInnerState::Done => {println!("done");std::task::Poll::Ready(())},}}
}