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<h3 id="将控制权交还给运行时"><a class="header" href="#将控制权交还给运行时">将控制权交还给运行时</a></h3>
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<p><a href="https://github.com/rust-lang/book/blob/dec931300f1a00e6e26021907192b0634980950c/src/ch17-03-more-futures.md">ch17-03-more-futures.md</a></p>
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<p>回忆一下我们在<a href="ch17-01-futures-and-syntax.html#第一个异步程序">“第一个异步程序”</a><!-- ignore -->一节中提到的内容：在每个 await 点，如果被等待的 future 还没准备好，Rust 就会给运行时一个机会来暂停当前任务并切换到其他任务。反过来也成立：Rust <em>只会</em> 在 await 点暂停 async 代码块，并把控制权交还给运行时。await 点之间的所有内容都是同步执行的。</p>
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<p>这意味着，如果你在一个 async 代码块中做了大量工作，却没有任何 await 点，那么这个 future 就会阻止其他 future 取得进展。有时你会听到人们把这称为一个 future 让其他 future <em>starve</em>（饥饿）。在某些场景下，这也许不是什么大问题；但如果你在做昂贵的初始化、长时间运行的工作，或者你有一个会无限执行某项任务的 future，就需要认真考虑该在何时、何地把控制权交还给运行时。</p>
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<p>让我们通过模拟一个长时间运行的操作来展示这种“饥饿”问题，再看看该如何解决。示例 17-14 引入了一个 <code>slow</code> 函数。</p>
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<figure class="listing">
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<p><span class="file-name">文件名：src/main.rs</span></p>
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<pre class="playground"><code class="language-rust edition2024"><span class="boring">extern crate trpl; // required for mdbook test
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</span><span class="boring">
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</span><span class="boring">use std::{thread, time::Duration};
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</span><span class="boring">
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</span><span class="boring">fn main() {
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</span><span class="boring"> trpl::block_on(async {
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</span><span class="boring"> // We will call `slow` here later
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</span><span class="boring"> });
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</span><span class="boring">}
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</span><span class="boring">
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</span>fn slow(name: &str, ms: u64) {
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thread::sleep(Duration::from_millis(ms));
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println!("'{name}' ran for {ms}ms");
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}</code></pre>
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<figcaption>示例 17-14：使用 `thread::sleep` 来模拟缓慢的操作</figcaption>
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</figure>
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<p>这段代码使用的是 <code>std::thread::sleep</code>，而不是 <code>trpl::sleep</code>，因此调用 <code>slow</code> 会让当前线程阻塞若干毫秒。我们可以把 <code>slow</code> 看作现实世界中那些既耗时又会阻塞的操作的替身。</p>
|
|
|
<p>在示例 17-15 中，我们用 <code>slow</code> 来模拟在一对 future 中执行这类 CPU 密集型工作。</p>
|
|
|
<figure class="listing">
|
|
|
<p><span class="file-name">文件名：src/main.rs</span></p>
|
|
|
<pre class="playground"><code class="language-rust edition2024"><span class="boring">extern crate trpl; // required for mdbook test
|
|
|
</span><span class="boring">
|
|
|
</span><span class="boring">use std::{thread, time::Duration};
|
|
|
</span><span class="boring">
|
|
|
</span><span class="boring">fn main() {
|
|
|
</span><span class="boring"> trpl::block_on(async {
|
|
|
</span> let a = async {
|
|
|
println!("'a' started.");
|
|
|
slow("a", 30);
|
|
|
slow("a", 10);
|
|
|
slow("a", 20);
|
|
|
trpl::sleep(Duration::from_millis(50)).await;
|
|
|
println!("'a' finished.");
|
|
|
};
|
|
|
|
|
|
let b = async {
|
|
|
println!("'b' started.");
|
|
|
slow("b", 75);
|
|
|
slow("b", 10);
|
|
|
slow("b", 15);
|
|
|
slow("b", 350);
|
|
|
trpl::sleep(Duration::from_millis(50)).await;
|
|
|
println!("'b' finished.");
|
|
|
};
|
|
|
|
|
|
trpl::select(a, b).await;
|
|
|
<span class="boring"> });
|
|
|
</span><span class="boring">}
|
|
|
</span><span class="boring">
|
|
|
</span><span class="boring">fn slow(name: &str, ms: u64) {
|
|
|
</span><span class="boring"> thread::sleep(Duration::from_millis(ms));
|
|
|
</span><span class="boring"> println!("'{name}' ran for {ms}ms");
|
|
|
</span><span class="boring">}</span></code></pre>
|
|
|
<figcaption>示例 17-15：调用 `slow` 函数来模拟缓慢操作</figcaption>
|
|
|
</figure>
|
|
|
<p>每个 future 都会在完成一大串缓慢操作之后，才把控制权交还给运行时。如果你运行这段代码，就会看到如下输出：</p>
|
|
|
<!-- manual-regeneration
|
|
|
cd listings/ch17-async-await/listing-17-15/
|
|
|
cargo run
|
|
|
copy just the output
|
|
|
-->
|
|
|
<pre><code class="language-text">'a' started.
|
|
|
'a' ran for 30ms
|
|
|
'a' ran for 10ms
|
|
|
'a' ran for 20ms
|
|
|
'b' started.
|
|
|
'b' ran for 75ms
|
|
|
'b' ran for 10ms
|
|
|
'b' ran for 15ms
|
|
|
'b' ran for 350ms
|
|
|
'a' finished.
|
|
|
</code></pre>
|
|
|
<p>和示例 17-5 中用 <code>trpl::select</code> 让两个 URL 获取任务竞争时一样，<code>select</code> 仍然会在 <code>a</code> 完成时立刻结束。不过，这两个 future 里的 <code>slow</code> 调用之间完全没有交错执行。<code>a</code> future 会一路把自己的工作做完，直到等待 <code>trpl::sleep</code> 调用；接着 <code>b</code> future 又一路做完自己的工作，直到它自己的 <code>trpl::sleep</code> 被等待；最后 <code>a</code> future 才完成。要想让两个 future 在这些缓慢任务之间都取得进展，我们就需要一些 await 点，好把控制权交还给运行时。也就是说，我们得有某种可以被 await 的东西！</p>
|
|
|
<p>实际上，我们已经能在示例 17-15 中看到这种“交接”是如何发生的：如果去掉 <code>a</code> future 末尾的 <code>trpl::sleep</code>，那么它会直接完成，而 <code>b</code> future <em>根本不会运行</em>。让我们先从 <code>trpl::sleep</code> 入手，试着让这些操作能够轮流取得进展，如示例 17-16 所示。</p>
|
|
|
<figure class="listing">
|
|
|
<p><span class="file-name">文件名：src/main.rs</span></p>
|
|
|
<pre class="playground"><code class="language-rust edition2024"><span class="boring">extern crate trpl; // required for mdbook test
|
|
|
</span><span class="boring">
|
|
|
</span><span class="boring">use std::{thread, time::Duration};
|
|
|
</span><span class="boring">
|
|
|
</span><span class="boring">fn main() {
|
|
|
</span><span class="boring"> trpl::block_on(async {
|
|
|
</span> let one_ms = Duration::from_millis(1);
|
|
|
|
|
|
let a = async {
|
|
|
println!("'a' started.");
|
|
|
slow("a", 30);
|
|
|
trpl::sleep(one_ms).await;
|
|
|
slow("a", 10);
|
|
|
trpl::sleep(one_ms).await;
|
|
|
slow("a", 20);
|
|
|
trpl::sleep(one_ms).await;
|
|
|
println!("'a' finished.");
|
|
|
};
|
|
|
|
|
|
let b = async {
|
|
|
println!("'b' started.");
|
|
|
slow("b", 75);
|
|
|
trpl::sleep(one_ms).await;
|
|
|
slow("b", 10);
|
|
|
trpl::sleep(one_ms).await;
|
|
|
slow("b", 15);
|
|
|
trpl::sleep(one_ms).await;
|
|
|
slow("b", 350);
|
|
|
trpl::sleep(one_ms).await;
|
|
|
println!("'b' finished.");
|
|
|
};
|
|
|
<span class="boring">
|
|
|
</span><span class="boring"> trpl::select(a, b).await;
|
|
|
</span><span class="boring"> });
|
|
|
</span><span class="boring">}
|
|
|
</span><span class="boring">
|
|
|
</span><span class="boring">fn slow(name: &str, ms: u64) {
|
|
|
</span><span class="boring"> thread::sleep(Duration::from_millis(ms));
|
|
|
</span><span class="boring"> println!("'{name}' ran for {ms}ms");
|
|
|
</span><span class="boring">}</span></code></pre>
|
|
|
<figcaption>示例 17-16：使用 `trpl::sleep` 让操作轮流推进</figcaption>
|
|
|
</figure>
|
|
|
<p>我们在每次调用 <code>slow</code> 之间都插入了 <code>trpl::sleep</code> 调用和 await 点。现在，两个 future 的工作就交错在一起了：</p>
|
|
|
<!-- manual-regeneration
|
|
|
cd listings/ch17-async-await/listing-17-16
|
|
|
cargo run
|
|
|
copy just the output
|
|
|
-->
|
|
|
<pre><code class="language-text">'a' started.
|
|
|
'a' ran for 30ms
|
|
|
'b' started.
|
|
|
'b' ran for 75ms
|
|
|
'a' ran for 10ms
|
|
|
'b' ran for 10ms
|
|
|
'a' ran for 20ms
|
|
|
'b' ran for 15ms
|
|
|
'a' finished.
|
|
|
</code></pre>
|
|
|
<p><code>a</code> future 仍然会在第一次把控制权交给 <code>b</code> 之前先运行一阵，因为它是在第一次调用 <code>trpl::sleep</code> 之前先执行了 <code>slow</code>；但在那之后，每当其中一个 future 命中 await 点，它们就会来回切换。在这个例子中，我们是在每次 <code>slow</code> 之后这么做的，不过实际上也可以按任何对你最合理的方式来拆分工作。</p>
|
|
|
<p>但我们其实并不是真的想在这里<em>休眠</em>（<em>sleep</em>）；我们只是希望程序尽可能快地前进。我们真正需要的只是把控制权交还给运行时。可以直接通过 <code>trpl::yield_now</code> 来做到这一点。在示例 17-17 中，我们把前面的 <code>trpl::sleep</code> 全部替换成 <code>trpl::yield_now</code>。</p>
|
|
|
<figure class="listing">
|
|
|
<p><span class="file-name">文件名：src/main.rs</span></p>
|
|
|
<pre class="playground"><code class="language-rust edition2024"><span class="boring">extern crate trpl; // required for mdbook test
|
|
|
</span><span class="boring">
|
|
|
</span><span class="boring">use std::{thread, time::Duration};
|
|
|
</span><span class="boring">
|
|
|
</span><span class="boring">fn main() {
|
|
|
</span><span class="boring"> trpl::block_on(async {
|
|
|
</span> let a = async {
|
|
|
println!("'a' started.");
|
|
|
slow("a", 30);
|
|
|
trpl::yield_now().await;
|
|
|
slow("a", 10);
|
|
|
trpl::yield_now().await;
|
|
|
slow("a", 20);
|
|
|
trpl::yield_now().await;
|
|
|
println!("'a' finished.");
|
|
|
};
|
|
|
|
|
|
let b = async {
|
|
|
println!("'b' started.");
|
|
|
slow("b", 75);
|
|
|
trpl::yield_now().await;
|
|
|
slow("b", 10);
|
|
|
trpl::yield_now().await;
|
|
|
slow("b", 15);
|
|
|
trpl::yield_now().await;
|
|
|
slow("b", 350);
|
|
|
trpl::yield_now().await;
|
|
|
println!("'b' finished.");
|
|
|
};
|
|
|
<span class="boring">
|
|
|
</span><span class="boring"> trpl::select(a, b).await;
|
|
|
</span><span class="boring"> });
|
|
|
</span><span class="boring">}
|
|
|
</span><span class="boring">
|
|
|
</span><span class="boring">fn slow(name: &str, ms: u64) {
|
|
|
</span><span class="boring"> thread::sleep(Duration::from_millis(ms));
|
|
|
</span><span class="boring"> println!("'{name}' ran for {ms}ms");
|
|
|
</span><span class="boring">}</span></code></pre>
|
|
|
<figcaption>示例 17-17：使用 `yield_now` 让操作轮流推进</figcaption>
|
|
|
</figure>
|
|
|
<p>这段代码不仅更清楚地表达了真实意图，而且通常也会比使用 <code>sleep</code> 快得多，因为像 <code>sleep</code> 使用的那类计时器，往往都会受到最小粒度限制。比如我们这里使用的 <code>sleep</code>，即使你传入的是 1 纳秒的 <code>Duration</code>，它也至少会睡眠 1 毫秒。再说一次，现代计算机是非常<em>快</em>的：1 毫秒里已经能完成大量工作了！</p>
|
|
|
<p>这说明：即使面对计算密集型任务，async 依然可能是有用的，具体取决于你的程序还在做什么，因为它提供了一种很实用的手段，用来组织程序不同部分之间的关系（当然代价是 async 状态机本身也有一定开销）。这是一种 <em>协作式多任务</em>（<em>cooperative multitasking</em>）：每个 future 都可以通过 await 点来决定何时交出控制权，因此每个 future 也都负有避免长时间阻塞的责任。在某些基于 Rust 的嵌入式操作系统中，这甚至是<em>唯一</em>的多任务形式！</p>
|
|
|
<p>当然，在真实代码里，你通常不会在每一行之间都交替插入函数调用和 await 点。像这样主动交出控制权虽然相对便宜，但并不是没有代价。在很多情况下，试图把一个计算密集型任务切得太碎，反而可能显著拖慢它的执行速度，所以有时为了<em>整体</em>性能，让某个操作短暂阻塞一下反而更好。还是那句老话：一定要靠测量来确认代码真正的性能瓶颈。不过，如果你发现原本预期会并发执行的工作，实际上却大量串行发生，那么就要记住这里的底层机制。</p>
|
|
|
<h3 id="构建我们自己的异步抽象"><a class="header" href="#构建我们自己的异步抽象">构建我们自己的异步抽象</a></h3>
|
|
|
<p>我们还可以把多个 future 组合在一起，创造出新的模式。比如，完全可以用手头已有的 async 构件来写一个 <code>timeout</code> 函数。等我们做完，它本身就会成为另一个可以继续拿来构建更多 async 抽象的基础模块。</p>
|
|
|
<p>示例 17-18 展示了我们希望这个 <code>timeout</code> 在面对一个缓慢 future 时应该如何工作。</p>
|
|
|
<figure class="listing">
|
|
|
<p><span class="file-name">文件名：src/main.rs</span></p>
|
|
|
<pre><code class="language-rust ignore does_not_compile"><span class="boring">extern crate trpl; // required for mdbook test
|
|
|
</span><span class="boring">
|
|
|
</span><span class="boring">use std::time::Duration;
|
|
|
</span><span class="boring">
|
|
|
</span><span class="boring">fn main() {
|
|
|
</span><span class="boring"> trpl::block_on(async {
|
|
|
</span> let slow = async {
|
|
|
trpl::sleep(Duration::from_secs(5)).await;
|
|
|
"Finally finished"
|
|
|
};
|
|
|
|
|
|
match timeout(slow, Duration::from_secs(2)).await {
|
|
|
Ok(message) => println!("Succeeded with '{message}'"),
|
|
|
Err(duration) => {
|
|
|
println!("Failed after {} seconds", duration.as_secs())
|
|
|
}
|
|
|
}
|
|
|
<span class="boring"> });
|
|
|
</span><span class="boring">}</span></code></pre>
|
|
|
<figcaption>示例 17-18：使用我们设想的 `timeout` 为一个缓慢操作设置时间限制</figcaption>
|
|
|
</figure>
|
|
|
<p>让我们来实现它。首先先想想 <code>timeout</code> 的 API：</p>
|
|
|
<ul>
|
|
|
<li>它本身需要是一个 async 函数，这样我们才能等待它。</li>
|
|
|
<li>它的第一个参数应该是一个要执行的 future。我们可以把它设计成泛型，从而支持任意 future。</li>
|
|
|
<li>它的第二个参数应该是最大等待时间。如果用 <code>Duration</code>，就能很方便地直接传给 <code>trpl::sleep</code>。</li>
|
|
|
<li>它应该返回一个 <code>Result</code>。如果传入的 future 成功完成，结果应当是 <code>Ok</code>，内部带上该 future 产生的值；如果先发生超时，结果就应当是 <code>Err</code>，内部带上等待的时长。</li>
|
|
|
</ul>
|
|
|
<p>示例 17-19 展示了这个声明。</p>
|
|
|
<!-- This is not tested because it intentionally does not compile. -->
|
|
|
<figure class="listing">
|
|
|
<p><span class="file-name">文件名：src/main.rs</span></p>
|
|
|
<pre><code class="language-rust ignore does_not_compile"><span class="boring">extern crate trpl; // required for mdbook test
|
|
|
</span><span class="boring">
|
|
|
</span><span class="boring">use std::time::Duration;
|
|
|
</span><span class="boring">
|
|
|
</span><span class="boring">fn main() {
|
|
|
</span><span class="boring"> trpl::block_on(async {
|
|
|
</span><span class="boring"> let slow = async {
|
|
|
</span><span class="boring"> trpl::sleep(Duration::from_secs(5)).await;
|
|
|
</span><span class="boring"> "Finally finished"
|
|
|
</span><span class="boring"> };
|
|
|
</span><span class="boring">
|
|
|
</span><span class="boring"> match timeout(slow, Duration::from_secs(2)).await {
|
|
|
</span><span class="boring"> Ok(message) => println!("Succeeded with '{message}'"),
|
|
|
</span><span class="boring"> Err(duration) => {
|
|
|
</span><span class="boring"> println!("Failed after {} seconds", duration.as_secs())
|
|
|
</span><span class="boring"> }
|
|
|
</span><span class="boring"> }
|
|
|
</span><span class="boring"> });
|
|
|
</span><span class="boring">}
|
|
|
</span><span class="boring">
|
|
|
</span>async fn timeout<F: Future>(
|
|
|
future_to_try: F,
|
|
|
max_time: Duration,
|
|
|
) -> Result<F::Output, Duration> {
|
|
|
// Here is where our implementation will go!
|
|
|
}</code></pre>
|
|
|
<figcaption>示例 17-19：定义 `timeout` 的签名</figcaption>
|
|
|
</figure>
|
|
|
<p>这样类型层面的目标就满足了。接下来想想我们需要的<em>行为</em>：我们希望让传入的 future 和这个时长“竞争”。可以用 <code>trpl::sleep</code> 根据这个时长构造一个计时器 future，再用 <code>trpl::select</code> 让计时器和调用者传入的 future 一起运行。</p>
|
|
|
<p>在示例 17-20 中，我们通过匹配 <code>trpl::select</code> 的等待结果来实现 <code>timeout</code>。</p>
|
|
|
<figure class="listing">
|
|
|
<p><span class="file-name">文件名：src/main.rs</span></p>
|
|
|
<pre class="playground"><code class="language-rust edition2024"><span class="boring">extern crate trpl; // required for mdbook test
|
|
|
</span><span class="boring">
|
|
|
</span><span class="boring">use std::time::Duration;
|
|
|
</span><span class="boring">
|
|
|
</span>use trpl::Either;
|
|
|
|
|
|
// --snip--
|
|
|
|
|
|
<span class="boring">fn main() {
|
|
|
</span><span class="boring"> trpl::block_on(async {
|
|
|
</span><span class="boring"> let slow = async {
|
|
|
</span><span class="boring"> trpl::sleep(Duration::from_secs(5)).await;
|
|
|
</span><span class="boring"> "Finally finished"
|
|
|
</span><span class="boring"> };
|
|
|
</span><span class="boring">
|
|
|
</span><span class="boring"> match timeout(slow, Duration::from_secs(2)).await {
|
|
|
</span><span class="boring"> Ok(message) => println!("Succeeded with '{message}'"),
|
|
|
</span><span class="boring"> Err(duration) => {
|
|
|
</span><span class="boring"> println!("Failed after {} seconds", duration.as_secs())
|
|
|
</span><span class="boring"> }
|
|
|
</span><span class="boring"> }
|
|
|
</span><span class="boring"> });
|
|
|
</span><span class="boring">}
|
|
|
</span><span class="boring">
|
|
|
</span>async fn timeout<F: Future>(
|
|
|
future_to_try: F,
|
|
|
max_time: Duration,
|
|
|
) -> Result<F::Output, Duration> {
|
|
|
match trpl::select(future_to_try, trpl::sleep(max_time)).await {
|
|
|
Either::Left(output) => Ok(output),
|
|
|
Either::Right(_) => Err(max_time),
|
|
|
}
|
|
|
}</code></pre>
|
|
|
<figcaption>示例 17-20：使用 `select` 和 `sleep` 定义 `timeout`</figcaption>
|
|
|
</figure>
|
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|
<p><code>trpl::select</code> 的实现并不是公平的：它总是按参数传入的顺序进行轮询（其他一些 <code>select</code> 实现会随机选择先轮询哪个参数）。因此，我们把 <code>future_to_try</code> 作为第一个参数传给 <code>select</code>，好让它即使在 <code>max_time</code> 很短的情况下，也仍然有机会先完成。如果 <code>future_to_try</code> 先完成，<code>select</code> 会返回 <code>Left</code>，其中包含 <code>future_to_try</code> 的输出；如果 <code>timer</code> 先完成，<code>select</code> 就会返回 <code>Right</code>，其中包含计时器的输出 <code>()</code>。</p>
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<p>如果 <code>future_to_try</code> 成功完成，并且我们得到了 <code>Left(output)</code>，那么就返回 <code>Ok(output)</code>。如果相反是睡眠计时器先结束，我们得到 <code>Right(())</code>，那就用 <code>_</code> 忽略这个 <code>()</code>，并返回 <code>Err(max_time)</code>。</p>
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<p>这样一来，我们就用另外两个 async 小工具拼出了一个可工作的 <code>timeout</code>。如果运行代码，它会在超时后打印出失败信息：</p>
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<pre><code class="language-text">Failed after 2 seconds
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</code></pre>
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<p>因为 future 可以和其他 future 组合，你就能利用更小的 async 构件构建出非常强大的工具。比如，完全可以用同样的方法把 timeout 和 retry 组合起来，再进一步把它们用在网络请求一类的操作上（例如示例 17-5 里的那些）。</p>
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<p>在实践中，你通常会主要直接使用 <code>async</code> 和 <code>await</code>，其次才是像 <code>select</code> 这样的函数，以及像 <code>join!</code> 这样的宏，来控制最外层的 future 应该如何执行。</p>
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<p>到这里，我们已经看过好几种同时处理多个 future 的方式了。接下来，我们将看看如何借助 <em>stream</em>，按照时间顺序处理一串 future。</p>
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