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<h1 id="使用-join-和-select-同时运行多个-future"><a class="header" href="#使用-join-和-select-同时运行多个-future">使用 <code>join!</code> 和 <code>select!</code> 同时运行多个 Future</a></h1>
<p>招数单一，杀伤力惊人，说的就是 <code>.await</code> ，但是光用它，还真做不到一招鲜吃遍天。比如我们该如何同时运行多个任务，而不是使用 <code>.await</code> 慢悠悠地排队完成。</p>
<h2 id="join"><a class="header" href="#join">join!</a></h2>
<p><code>futures</code> 包中提供了很多实用的工具，其中一个就是 <code>join!</code> 宏， 它允许我们同时等待多个不同 <code>Future</code> 的完成，且可以并发地运行这些 <code>Future</code>。</p>
<p>先来看一个不是很给力的、使用<code>.await</code>的版本:</p>
<pre><pre class="playground"><code class="language-rust edition2021"><span class="boring">#![allow(unused)]
</span><span class="boring">fn main() {
</span>async fn enjoy_book_and_music() -&gt; (Book, Music) {
let book = enjoy_book().await;
let music = enjoy_music().await;
(book, music)
}
<span class="boring">}</span></code></pre></pre>
<p>这段代码可以顺利运行，但是有一个很大的问题，就是必须先看完书后，才能听音乐。咱们以前，谁又不是那个摇头晃脑爱读书(耳朵里偷偷塞着耳机，听的正 high)的好学生呢？</p>
<p>要支持同时看书和听歌，有些人可能会凭空生成下面代码：</p>
<pre><pre class="playground"><code class="language-rust edition2021"><span class="boring">#![allow(unused)]
</span><span class="boring">fn main() {
</span>// WRONG -- 别这么做
async fn enjoy_book_and_music() -&gt; (Book, Music) {
let book_future = enjoy_book();
let music_future = enjoy_music();
(book_future.await, music_future.await)
}
<span class="boring">}</span></code></pre></pre>
<p>看上去像模像样，嗯，在某些语言中也许可以，但是 Rust 不行。因为在某些语言中，<code>Future</code>一旦创建就开始运行，等到返回的时候，基本就可以同时结束并返回了。 但是 Rust 中的 <code>Future</code> 是惰性的，直到调用 <code>.await</code> 时，才会开始运行。而那两个 <code>await</code> 由于在代码中有先后顺序，因此它们是顺序运行的。</p>
<p>为了正确的并发运行两个 <code>Future</code> ， 我们来试试 <code>futures::join!</code> 宏:</p>
<pre><pre class="playground"><code class="language-rust edition2021"><span class="boring">#![allow(unused)]
</span><span class="boring">fn main() {
</span>use futures::join;
async fn enjoy_book_and_music() -&gt; (Book, Music) {
let book_fut = enjoy_book();
let music_fut = enjoy_music();
join!(book_fut, music_fut)
}
<span class="boring">}</span></code></pre></pre>
<p><code>Duang</code>，目标顺利达成。同时 <code>join!</code> 会返回一个元组，里面的值是对应的 <code>Future</code> 执行结束后输出的值。</p>
<blockquote>
<p>如果希望同时运行一个数组里的多个异步任务，可以使用 <code>futures::future::join_all</code> 方法</p>
</blockquote>
<h2 id="try_join"><a class="header" href="#try_join">try_join!</a></h2>
<p>由于 <code>join!</code> 必须等待它管理的所有 <code>Future</code> 完成后才能完成，如果你希望在某一个 <code>Future</code> 报错后就立即停止所有 <code>Future</code> 的执行，可以使用 <code>try_join!</code>，特别是当 <code>Future</code> 返回 <code>Result</code> 时:</p>
<pre><pre class="playground"><code class="language-rust edition2021"><span class="boring">#![allow(unused)]
</span><span class="boring">fn main() {
</span>use futures::try_join;
async fn get_book() -&gt; Result&lt;Book, String&gt; { /* ... */ Ok(Book) }
async fn get_music() -&gt; Result&lt;Music, String&gt; { /* ... */ Ok(Music) }
async fn get_book_and_music() -&gt; Result&lt;(Book, Music), String&gt; {
let book_fut = get_book();
let music_fut = get_music();
try_join!(book_fut, music_fut)
}
<span class="boring">}</span></code></pre></pre>
<p>有一点需要注意，传给 <code>try_join!</code> 的所有 <code>Future</code> 都必须拥有相同的错误类型。如果错误类型不同，可以考虑使用来自 <code>futures::future::TryFutureExt</code> 模块的 <code>map_err</code> 和 <code>err_info</code> 方法将错误进行转换:</p>
<pre><pre class="playground"><code class="language-rust edition2021"><span class="boring">#![allow(unused)]
</span><span class="boring">fn main() {
</span>use futures::{
future::TryFutureExt,
try_join,
};
async fn get_book() -&gt; Result&lt;Book, ()&gt; { /* ... */ Ok(Book) }
async fn get_music() -&gt; Result&lt;Music, String&gt; { /* ... */ Ok(Music) }
async fn get_book_and_music() -&gt; Result&lt;(Book, Music), String&gt; {
let book_fut = get_book().map_err(|()| &quot;Unable to get book&quot;.to_string());
let music_fut = get_music();
try_join!(book_fut, music_fut)
}
<span class="boring">}</span></code></pre></pre>
<p><code>join!</code> 很好很强大，但是人无完人，J 无完 J, 它有一个很大的问题。</p>
<h2 id="select"><a class="header" href="#select">select!</a></h2>
<p><code>join!</code> 只有等所有 <code>Future</code> 结束后，才能集中处理结果，如果你想同时等待多个 <code>Future</code> ，且任何一个 <code>Future</code> 结束后，都可以立即被处理，可以考虑使用 <code>futures::select!</code>:</p>
<pre><pre class="playground"><code class="language-rust edition2021"><span class="boring">#![allow(unused)]
</span><span class="boring">fn main() {
</span>use futures::{
future::FutureExt, // for `.fuse()`
pin_mut,
select,
};
async fn task_one() { /* ... */ }
async fn task_two() { /* ... */ }
async fn race_tasks() {
let t1 = task_one().fuse();
let t2 = task_two().fuse();
pin_mut!(t1, t2);
select! {
() = t1 =&gt; println!(&quot;任务1率先完成&quot;),
() = t2 =&gt; println!(&quot;任务2率先完成&quot;),
}
}
<span class="boring">}</span></code></pre></pre>
<p>上面的代码会同时并发地运行 <code>t1</code> 和 <code>t2</code>， 无论两者哪个先完成，都会调用对应的 <code>println!</code> 打印相应的输出，然后函数结束且不会等待另一个任务的完成。</p>
<p>但是，在实际项目中，我们往往需要等待多个任务都完成后，再结束，像上面这种其中一个任务结束就立刻结束的场景着实不多。</p>
<h4 id="default-和-complete"><a class="header" href="#default-和-complete">default 和 complete</a></h4>
<p><code>select!</code>还支持 <code>default</code> 和 <code>complete</code> 分支:</p>
<ul>
<li><code>complete</code> 分支当所有的 <code>Future</code> 和 <code>Stream</code> 完成后才会被执行，它往往配合 <code>loop</code> 使用，<code>loop</code> 用于循环完成所有的 <code>Future</code></li>
<li><code>default</code> 分支，若没有任何 <code>Future</code> 或 <code>Stream</code> 处于 <code>Ready</code> 状态， 则该分支会被立即执行</li>
</ul>
<pre><pre class="playground"><code class="language-rust edition2021">use futures::future;
use futures::select;
pub fn main() {
let mut a_fut = future::ready(4);
let mut b_fut = future::ready(6);
let mut total = 0;
loop {
select! {
a = a_fut =&gt; total += a,
b = b_fut =&gt; total += b,
complete =&gt; break,
default =&gt; panic!(), // 该分支永远不会运行，因为 `Future` 会先运行，然后是 `complete`
};
}
assert_eq!(total, 10);
}</code></pre></pre>
<p>以上代码 <code>default</code> 分支由于最后一个运行，而在它之前 <code>complete</code> 分支已经通过 <code>break</code> 跳出了循环，因此 <code>default</code> 永远不会被执行。</p>
<p>如果你希望 <code>default</code> 也有机会露下脸，可以将 <code>complete</code> 的 <code>break</code> 修改为其它的，例如 <code>println!(&quot;completed!&quot;)</code>，然后再观察下运行结果。</p>
<p>再回到 <code>select</code> 的第一个例子中，里面有一段代码长这样：</p>
<pre><pre class="playground"><code class="language-rust edition2021"><span class="boring">#![allow(unused)]
</span><span class="boring">fn main() {
</span>let t1 = task_one().fuse();
let t2 = task_two().fuse();
pin_mut!(t1, t2);
<span class="boring">}</span></code></pre></pre>
<p>当时没有展开讲，相信大家也有疑惑，下面我们来一起看看。</p>
<h4 id="跟-unpin-和-fusedfuture-进行交互"><a class="header" href="#跟-unpin-和-fusedfuture-进行交互">跟 <code>Unpin</code> 和 <code>FusedFuture</code> 进行交互</a></h4>
<p>首先，<code>.fuse()</code> 方法可以让 <code>Future</code> 实现 <code>FusedFuture</code> 特征， 而 <code>pin_mut!</code> 宏会为 <code>Future</code> 实现 <code>Unpin</code> 特征，这两个特征恰恰是使用 <code>select</code> 所必须的:</p>
<ul>
<li><code>Unpin</code>，由于 <code>select</code> 不会通过拿走所有权的方式使用 <code>Future</code>，而是通过可变引用的方式去使用，这样当 <code>select</code> 结束后，该 <code>Future</code> 若没有被完成，它的所有权还可以继续被其它代码使用。</li>
<li><code>FusedFuture</code> 的原因跟上面类似，当 <code>Future</code> 一旦完成后，那 <code>select</code> 就不能再对其进行轮询使用。<code>Fuse</code> 意味着熔断，相当于 <code>Future</code> 一旦完成，再次调用 <code>poll</code> 会直接返回 <code>Poll::Pending</code>。</li>
</ul>
<p>只有实现了 <code>FusedFuture</code>，<code>select</code> 才能配合 <code>loop</code> 一起使用。假如没有实现，就算一个 <code>Future</code> 已经完成了，它依然会被 <code>select</code> 不停的轮询执行。</p>
<p><code>Stream</code> 稍有不同，它们使用的特征是 <code>FusedStream</code>。 通过 <code>.fuse()</code>(也可以手动实现)实现了该特征的 <code>Stream</code>，对其调用 <code>.next()</code> 或 <code>.try_next()</code> 方法可以获取实现了 <code>FusedFuture</code> 特征的<code>Future</code>:</p>
<pre><pre class="playground"><code class="language-rust edition2021"><span class="boring">#![allow(unused)]
</span><span class="boring">fn main() {
</span>use futures::{
stream::{Stream, StreamExt, FusedStream},
select,
};
async fn add_two_streams(
mut s1: impl Stream&lt;Item = u8&gt; + FusedStream + Unpin,
mut s2: impl Stream&lt;Item = u8&gt; + FusedStream + Unpin,
) -&gt; u8 {
let mut total = 0;
loop {
let item = select! {
x = s1.next() =&gt; x,
x = s2.next() =&gt; x,
complete =&gt; break,
};
if let Some(next_num) = item {
total += next_num;
}
}
total
}
<span class="boring">}</span></code></pre></pre>
<h2 id="在-select-循环中并发"><a class="header" href="#在-select-循环中并发">在 select 循环中并发</a></h2>
<p>一个很实用但又鲜为人知的函数是 <code>Fuse::terminated()</code> ，可以使用它构建一个空的 <code>Future</code> ，空自然没啥用，但是如果它能在后面再被填充呢？</p>
<p>考虑以下场景：当你要在 <code>select</code> 循环中运行一个任务，但是该任务却是在 <code>select</code> 循环内部创建时，上面的函数就非常好用了。</p>
<pre><pre class="playground"><code class="language-rust edition2021"><span class="boring">#![allow(unused)]
</span><span class="boring">fn main() {
</span>use futures::{
future::{Fuse, FusedFuture, FutureExt},
stream::{FusedStream, Stream, StreamExt},
pin_mut,
select,
};
async fn get_new_num() -&gt; u8 { /* ... */ 5 }
async fn run_on_new_num(_: u8) { /* ... */ }
async fn run_loop(
mut interval_timer: impl Stream&lt;Item = ()&gt; + FusedStream + Unpin,
starting_num: u8,
) {
let run_on_new_num_fut = run_on_new_num(starting_num).fuse();
let get_new_num_fut = Fuse::terminated();
pin_mut!(run_on_new_num_fut, get_new_num_fut);
loop {
select! {
() = interval_timer.select_next_some() =&gt; {
// 定时器已结束，若`get_new_num_fut`没有在运行，就创建一个新的
if get_new_num_fut.is_terminated() {
get_new_num_fut.set(get_new_num().fuse());
}
},
new_num = get_new_num_fut =&gt; {
// 收到新的数字 -- 创建一个新的`run_on_new_num_fut`并丢弃掉旧的
run_on_new_num_fut.set(run_on_new_num(new_num).fuse());
},
// 运行 `run_on_new_num_fut`
() = run_on_new_num_fut =&gt; {},
// 若所有任务都完成，直接 `panic`， 原因是 `interval_timer` 应该连续不断的产生值，而不是结束
//后，执行到 `complete` 分支
complete =&gt; panic!(&quot;`interval_timer` completed unexpectedly&quot;),
}
}
}
<span class="boring">}</span></code></pre></pre>
<p>当某个 <code>Future</code> 有多个拷贝都需要同时运行时，可以使用 <code>FuturesUnordered</code> 类型。下面的例子跟上个例子大体相似，但是它会将 <code>run_on_new_num_fut</code> 的每一个拷贝都运行到完成，而不是像之前那样一旦创建新的就终止旧的。</p>
<pre><pre class="playground"><code class="language-rust edition2021"><span class="boring">#![allow(unused)]
</span><span class="boring">fn main() {
</span>use futures::{
future::{Fuse, FusedFuture, FutureExt},
stream::{FusedStream, FuturesUnordered, Stream, StreamExt},
pin_mut,
select,
};
async fn get_new_num() -&gt; u8 { /* ... */ 5 }
async fn run_on_new_num(_: u8) -&gt; u8 { /* ... */ 5 }
// 使用从 `get_new_num` 获取的最新数字 来运行 `run_on_new_num`
//
// 每当计时器结束后，`get_new_num` 就会运行一次，它会立即取消当前正在运行的`run_on_new_num` ,
// 并且使用新返回的值来替换
async fn run_loop(
mut interval_timer: impl Stream&lt;Item = ()&gt; + FusedStream + Unpin,
starting_num: u8,
) {
let mut run_on_new_num_futs = FuturesUnordered::new();
run_on_new_num_futs.push(run_on_new_num(starting_num));
let get_new_num_fut = Fuse::terminated();
pin_mut!(get_new_num_fut);
loop {
select! {
() = interval_timer.select_next_some() =&gt; {
// 定时器已结束，若 `get_new_num_fut` 没有在运行，就创建一个新的
if get_new_num_fut.is_terminated() {
get_new_num_fut.set(get_new_num().fuse());
}
},
new_num = get_new_num_fut =&gt; {
// 收到新的数字 -- 创建一个新的 `run_on_new_num_fut` (并没有像之前的例子那样丢弃掉旧值)
run_on_new_num_futs.push(run_on_new_num(new_num));
},
// 运行 `run_on_new_num_futs`, 并检查是否有已经完成的
res = run_on_new_num_futs.select_next_some() =&gt; {
println!(&quot;run_on_new_num_fut returned {:?}&quot;, res);
},
// 若所有任务都完成，直接 `panic`， 原因是 `interval_timer` 应该连续不断的产生值，而不是结束
//后，执行到 `complete` 分支
complete =&gt; panic!(&quot;`interval_timer` completed unexpectedly&quot;),
}
}
}
<span class="boring">}</span></code></pre></pre>
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