2. The Future Trait / 2. Future Trait 🟡

What you’ll learn / 你将学到：

• The Future trait: Output, poll(), Context, Waker / Future trait 的组成：Output、poll()、Context 与 Waker
• How a waker tells the executor “poll me again” / Waker 如何告知执行器“请再次轮询我”
• The contract: never call wake() = your program silently hangs / 契约：如果不调用 wake()，程序就会静默挂起
• Implementing a real future by hand (Delay) / 手动落实一个真实的 future（Delay）

Anatomy of a Future / Future 的解剖

Everything in async Rust ultimately implements this trait:

Async Rust 中的一切最终都实现了这个 trait：

#![allow(unused)]
fn main() {
pub trait Future {
    type Output;

    fn poll(self: Pin<&mut Self>, cx: &mut Context<'_>) -> Poll<Self::Output>;
}

pub enum Poll<T> {
    Ready(T),   // The future has completed with value T
    Pending,    // The future is not ready yet — call me back later
}
}

That’s it. A Future is anything that can be polled — asked “are you done yet?” — and responds with either “yes, here’s the result” or “not yet, I’ll wake you up when I’m ready.”

就这么简单。Future 是任何可以被 poll（轮询）的对象 —— 即被询问“你做完了吗？” —— 并回答“做完了，这是结果”或“还没呢，等我准备好了会叫醒你”。

Output, poll(), Context, Waker / Output、poll()、Context 与 Waker

sequenceDiagram
    participant E as Executor
    participant F as Future
    participant R as Resource (I/O)

    E->>F: poll(cx)
    F->>R: Check: is data ready?
    R-->>F: Not yet
    F->>R: Register waker from cx
    F-->>E: Poll::Pending

    Note over R: ... time passes, data arrives ...

    R->>E: waker.wake() — "I'm ready!"
    E->>F: poll(cx) — try again
    F->>R: Check: is data ready?
    R-->>F: Yes! Here's the data
    F-->>E: Poll::Ready(data)

Let’s break down each piece:

让我们分解每个部分：

#![allow(unused)]
fn main() {
use std::future::Future;
use std::pin::Pin;
use std::task::{Context, Poll};

// A future that returns 42 immediately
struct Ready42;

impl Future for Ready42 {
    type Output = i32; // What the future eventually produces

    fn poll(self: Pin<&mut Self>, _cx: &mut Context<'_>) -> Poll<i32> {
        Poll::Ready(42) // Always ready — no waiting
    }
}
}

The components / 组成部分:

• Output — the type of value produced when the future completes / Future 完成时产生的值的类型
• poll() — called by the executor to check progress; returns Ready(value) or Pending / 由执行器调用以检查进度；返回 Ready(value) 或 Pending
• Pin<&mut Self> — ensures the future won’t be moved in memory (we’ll cover why in Ch. 4) / 确保 future 不会在内存中被移动（我们将在第 4 章解释原因）
• Context — carries the Waker so the future can signal the executor when it’s ready to make progress / 携带 Waker，以便 future 在准备好继续时通知执行器

The Waker Contract / Waker 契约

The Waker is the callback mechanism. When a future returns Pending, it must arrange for waker.wake() to be called later — otherwise the executor will never poll it again and the program hangs.

Waker 是一种回调机制。当 future 返回 Pending 时，它 必须 安排在之后调用 waker.wake() —— 否则执行器永远不会再次轮询它，程序就会挂起。

#![allow(unused)]
fn main() {
use std::task::{Context, Poll, Waker};
use std::pin::Pin;
use std::future::Future;
use std::sync::{Arc, Mutex};
use std::thread;
use std::time::Duration;

/// A future that completes after a delay (toy implementation)
struct Delay {
    completed: Arc<Mutex<bool>>,
    waker_stored: Arc<Mutex<Option<Waker>>>,
    duration: Duration,
    started: bool,
}

impl Delay {
    fn new(duration: Duration) -> Self {
        Delay {
            completed: Arc::new(Mutex::new(false)),
            waker_stored: Arc::new(Mutex::new(None)),
            duration,
            started: false,
        }
    }
}

impl Future for Delay {
    type Output = ();

    fn poll(mut self: Pin<&mut Self>, cx: &mut Context<'_>) -> Poll<()> {
        // Check if already completed
        if *self.completed.lock().unwrap() {
            return Poll::Ready(());
        }

        // Store the waker so the background thread can wake us
        *self.waker_stored.lock().unwrap() = Some(cx.waker().clone());

        // Start the background timer on first poll
        if !self.started {
            self.started = true;
            let completed = Arc::clone(&self.completed);
            let waker = Arc::clone(&self.waker_stored);
            let duration = self.duration;

            thread::spawn(move || {
                thread::sleep(duration);
                *completed.lock().unwrap() = true;

                // CRITICAL: wake the executor so it polls us again
                if let Some(w) = waker.lock().unwrap().take() {
                    w.wake(); // "Hey executor, I'm ready — poll me again!"
                }
            });
        }

        Poll::Pending // Not done yet
    }
}
}

Key insight: In C#, the TaskScheduler handles waking automatically. In Rust, you (or the I/O library you use) are responsible for calling waker.wake(). Forget it, and your program silently hangs.

关键洞察：在 C# 中，TaskScheduler 会自动处理唤醒。而在 Rust 中，你（或者你使用的 I/O 库）负责调用 waker.wake()。如果忘了这一步，你的程序就会静默挂起。

Exercise: Implement a CountdownFuture / 练习：实现一个倒计时 Future

#![allow(unused)]
fn main() {
use std::future::Future;
use std::pin::Pin;
use std::task::{Context, Poll};

struct CountdownFuture {
    count: u32,
}

impl CountdownFuture {
    fn new(start: u32) -> Self {
        CountdownFuture { count: start }
    }
}

impl Future for CountdownFuture {
    type Output = &'static str;

    fn poll(mut self: Pin<&mut Self>, cx: &mut Context<'_>) -> Poll<Self::Output> {
        if self.count == 0 {
            println!("Liftoff!");
            Poll::Ready("Liftoff!")
        } else {
            println!("{}...", self.count);
            self.count -= 1;
            cx.waker().wake_by_ref(); // Schedule re-poll immediately
            Poll::Pending
        }
    }
}
}

Key Takeaways — The Future Trait / 关键要点：Future Trait

• Future::poll() returns Poll::Ready(value) or Poll::Pending / Future::poll() 返回 Poll::Ready(value) 或 Poll::Pending
• A future must register a Waker before returning Pending — the executor uses it to know when to re-poll / Future 在返回 Pending 之前必须注册一个 Waker —— 执行器通过它知道何时重新轮询
• Pin<&mut Self> guarantees the future won’t be moved in memory (needed for self-referential state machines — see Ch 4) / Pin<&mut Self> 保证 future 不会在内存中移动（自引用状态机需要此特性 —— 见第 4 章）
• Everything in async Rust — async fn, .await, combinators — is built on this one trait / Async Rust 中的一切 —— async fn、.await、组合器 —— 都构建在这一 trait 之上

See also / 延伸阅读： Ch 3 — How Poll Works / 第 3 章：poll 的工作机制 for the executor loop, Ch 6 — Building Futures by Hand / 第 6 章：手写 Future for more complex implementations