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<h1 id="stream"><a class="header" href="#stream">Stream</a></h1>
<p>大家有没有想过, Rust 中的迭代器在迭代时能否异步进行?若不可以,是不是有相应的解决方案?</p>
<p>以上的问题其实很重要,因为在实际场景中,迭代一个集合,然后异步的去执行是很常见的需求,好在 Tokio 为我们提供了 <code>stream</code>,我们可以在异步函数中对其进行迭代,甚至和迭代器 <code>Iterator</code> 一样,<code>stream</code> 还能使用适配器,例如 <code>map</code> ! Tokio 在 <a href="https://docs.rs/tokio-stream/0.1.8/tokio_stream/trait.StreamExt.html"><code>StreamExt</code></a> 特征上定义了常用的适配器。</p>
<p>要使用 <code>stream</code> ,目前还需要手动引入对应的包:</p>
<pre><pre class="playground"><code class="language-rust edition2021"><span class="boring">#![allow(unused)]
</span><span class="boring">fn main() {
</span>tokio-stream = &quot;0.1&quot;
<span class="boring">}</span></code></pre></pre>
<blockquote>
<p>stream 没有放在 <code>tokio</code> 包的原因在于标准库中的 <code>Stream</code> 特征还没有稳定,一旦稳定后,<code>stream</code> 将移动到 <code>tokio</code> 中来</p>
</blockquote>
<h2 id="迭代"><a class="header" href="#迭代">迭代</a></h2>
<p>目前, Rust 语言还不支持异步的 <code>for</code> 循环,因此我们需要 <code>while let</code> 循环和 <a href="https://docs.rs/tokio-stream/0.1.8/tokio_stream/trait.StreamExt.html#method.next"><code>StreamExt::next()</code></a> 一起使用来实现迭代的目的:</p>
<pre><pre class="playground"><code class="language-rust edition2021">use tokio_stream::StreamExt;
#[tokio::main]
async fn main() {
let mut stream = tokio_stream::iter(&amp;[1, 2, 3]);
while let Some(v) = stream.next().await {
println!(&quot;GOT = {:?}&quot;, v);
}
}</code></pre></pre>
<p>和迭代器 <code>Iterator</code> 类似,<code>next()</code> 方法返回一个 <code>Option&lt;T&gt;</code>,其中 <code>T</code> 是从 <code>stream</code> 中获取的值的类型。若收到 <code>None</code> 则意味着 <code>stream</code> 迭代已经结束。</p>
<h4 id="mini-redis-广播"><a class="header" href="#mini-redis-广播">mini-redis 广播</a></h4>
<p>下面我们来实现一个复杂一些的 mini-redis 客户端,完整代码见<a href="https://github.com/tokio-rs/website/blob/master/tutorial-code/streams/src/main.rs">这里</a></p>
<p>在开始之前,首先启动一下完整的 mini-redis 服务器端:</p>
<pre><code class="language-console">$ mini-redis-server
</code></pre>
<pre><pre class="playground"><code class="language-rust edition2021">use tokio_stream::StreamExt;
use mini_redis::client;
async fn publish() -&gt; mini_redis::Result&lt;()&gt; {
let mut client = client::connect(&quot;127.0.0.1:6379&quot;).await?;
// 发布一些数据
client.publish(&quot;numbers&quot;, &quot;1&quot;.into()).await?;
client.publish(&quot;numbers&quot;, &quot;two&quot;.into()).await?;
client.publish(&quot;numbers&quot;, &quot;3&quot;.into()).await?;
client.publish(&quot;numbers&quot;, &quot;four&quot;.into()).await?;
client.publish(&quot;numbers&quot;, &quot;five&quot;.into()).await?;
client.publish(&quot;numbers&quot;, &quot;6&quot;.into()).await?;
Ok(())
}
async fn subscribe() -&gt; mini_redis::Result&lt;()&gt; {
let client = client::connect(&quot;127.0.0.1:6379&quot;).await?;
let subscriber = client.subscribe(vec![&quot;numbers&quot;.to_string()]).await?;
let messages = subscriber.into_stream();
tokio::pin!(messages);
while let Some(msg) = messages.next().await {
println!(&quot;got = {:?}&quot;, msg);
}
Ok(())
}
#[tokio::main]
async fn main() -&gt; mini_redis::Result&lt;()&gt; {
tokio::spawn(async {
publish().await
});
subscribe().await?;
println!(&quot;DONE&quot;);
Ok(())
}</code></pre></pre>
<p>上面生成了一个异步任务专门用于发布消息到 min-redis 服务器端的 <code>numbers</code> 消息通道中。然后,在 <code>main</code> 中,我们订阅了 <code>numbers</code> 消息通道,并且打印从中接收到的消息。</p>
<p>还有几点值得注意的:</p>
<ul>
<li><a href="https://docs.rs/mini-redis/0.4.1/mini_redis/client/struct.Subscriber.html#method.into_stream"><code>into_stream</code></a> 会将 <code>Subscriber</code> 变成一个 <code>stream</code></li>
<li><code>stream</code> 上调用 <code>next</code> 方法要求该 <code>stream</code> 被固定住(<a href="https://doc.rust-lang.org/std/pin/index.html"><code>pinned</code></a>),因此需要调用 <code>tokio::pin!</code></li>
</ul>
<blockquote>
<p>关于 Pin 的详细解读,可以阅读<a href="https://course.rs/async/pin-unpin.html">这篇文章</a></p>
</blockquote>
<p>大家可以去掉 <code>pin!</code> 的调用,然后观察下报错,若以后你遇到这种错误,可以尝试使用下 <code>pin!</code></p>
<p>此时,可以运行下我们的客户端代码看看效果(别忘了先启动前面提到的 mini-redis 服务端):</p>
<pre><code class="language-console">got = Ok(Message { channel: &quot;numbers&quot;, content: b&quot;1&quot; })
got = Ok(Message { channel: &quot;numbers&quot;, content: b&quot;two&quot; })
got = Ok(Message { channel: &quot;numbers&quot;, content: b&quot;3&quot; })
got = Ok(Message { channel: &quot;numbers&quot;, content: b&quot;four&quot; })
got = Ok(Message { channel: &quot;numbers&quot;, content: b&quot;five&quot; })
got = Ok(Message { channel: &quot;numbers&quot;, content: b&quot;6&quot; })
</code></pre>
<p>在了解了 <code>stream</code> 的基本用法后,我们再来看看如何使用适配器来扩展它。</p>
<h2 id="适配器"><a class="header" href="#适配器">适配器</a></h2>
<p>在前面章节中,我们了解了迭代器有<a href="https://course.rs/advance/functional-programing/iterator.html#%E6%B6%88%E8%B4%B9%E8%80%85%E4%B8%8E%E9%80%82%E9%85%8D%E5%99%A8">两种适配器</a></p>
<ul>
<li>迭代器适配器,会将一个迭代器转变成另一个迭代器,例如 <code>map</code><code>filter</code></li>
<li>消费者适配器,会消费掉一个迭代器,最终生成一个值,例如 <code>collect</code> 可以将迭代器收集成一个集合</li>
</ul>
<p>与迭代器类似,<code>stream</code> 也有适配器,例如一个 <code>stream</code> 适配器可以将一个 <code>stream</code> 转变成另一个 <code>stream</code> ,例如 <code>map</code><code>take</code><code>filter</code></p>
<p>在之前的客户端中,<code>subscribe</code> 订阅一直持续下去,直到程序被关闭。现在,让我们来升级下,让它在收到三条消息后就停止迭代,最终结束。</p>
<pre><pre class="playground"><code class="language-rust edition2021"><span class="boring">#![allow(unused)]
</span><span class="boring">fn main() {
</span>let messages = subscriber
.into_stream()
.take(3);
<span class="boring">}</span></code></pre></pre>
<p>这里关键就在于 <code>take</code> 适配器,它会限制 <code>stream</code> 只能生成最多 <code>n</code> 条消息。运行下看看结果:</p>
<pre><code class="language-console">got = Ok(Message { channel: &quot;numbers&quot;, content: b&quot;1&quot; })
got = Ok(Message { channel: &quot;numbers&quot;, content: b&quot;two&quot; })
got = Ok(Message { channel: &quot;numbers&quot;, content: b&quot;3&quot; })
</code></pre>
<p>程序终于可以正常结束了。现在,让我们过滤 <code>stream</code> 中的消息,只保留数字类型的值:</p>
<pre><pre class="playground"><code class="language-rust edition2021"><span class="boring">#![allow(unused)]
</span><span class="boring">fn main() {
</span>let messages = subscriber
.into_stream()
.filter(|msg| match msg {
Ok(msg) if msg.content.len() == 1 =&gt; true,
_ =&gt; false,
})
.take(3);
<span class="boring">}</span></code></pre></pre>
<p>运行后输出:</p>
<pre><code class="language-console">got = Ok(Message { channel: &quot;numbers&quot;, content: b&quot;1&quot; })
got = Ok(Message { channel: &quot;numbers&quot;, content: b&quot;3&quot; })
got = Ok(Message { channel: &quot;numbers&quot;, content: b&quot;6&quot; })
</code></pre>
<p>需要注意的是,适配器的顺序非常重要,<code>.filter(...).take(3)</code><code>.take(3).filter(...)</code> 的结果可能大相径庭,大家可以自己尝试下。</p>
<p>现在,还有一件事要做,咱们的消息被不太好看的 <code>Ok(...)</code> 所包裹,现在通过 <code>map</code> 适配器来简化下:</p>
<pre><pre class="playground"><code class="language-rust edition2021"><span class="boring">#![allow(unused)]
</span><span class="boring">fn main() {
</span>let messages = subscriber
.into_stream()
.filter(|msg| match msg {
Ok(msg) if msg.content.len() == 1 =&gt; true,
_ =&gt; false,
})
.map(|msg| msg.unwrap().content)
.take(3);
<span class="boring">}</span></code></pre></pre>
<p>注意到 <code>msg.unwrap</code> 了吗?大家可能会以为我们是出于示例的目的才这么用,实际上并不是,由于 <code>filter</code> 的先执行, <code>map</code> 中的 <code>msg</code> 只能是 <code>Ok(...)</code>,因此 <code>unwrap</code> 非常安全。</p>
<pre><code class="language-console">got = b&quot;1&quot;
got = b&quot;3&quot;
got = b&quot;6&quot;
</code></pre>
<p>还有一点可以改进的地方:当 <code>filter</code><code>map</code> 一起使用时,你往往可以用一个统一的方法来实现 <a href="https://docs.rs/tokio-stream/0.1.8/tokio_stream/trait.StreamExt.html#method.filter_map"><code>filter_map</code></a></p>
<pre><pre class="playground"><code class="language-rust edition2021"><span class="boring">#![allow(unused)]
</span><span class="boring">fn main() {
</span>let messages = subscriber
.into_stream()
.filter_map(|msg| match msg {
Ok(msg) if msg.content.len() == 1 =&gt; Some(msg.content),
_ =&gt; None,
})
.take(3);
<span class="boring">}</span></code></pre></pre>
<p>想要学习更多的适配器,可以看看 <a href="https://docs.rs/tokio-stream/0.1.8/tokio_stream/trait.StreamExt.html"><code>StreamExt</code></a> 特征。</p>
<h2 id="实现-stream-特征"><a class="header" href="#实现-stream-特征">实现 Stream 特征</a></h2>
<p>如果大家还没忘记 <code>Future</code> 特征,那 <code>Stream</code> 特征相信你也会很快记住,因为它们非常类似:</p>
<pre><pre class="playground"><code class="language-rust edition2021"><span class="boring">#![allow(unused)]
</span><span class="boring">fn main() {
</span>use std::pin::Pin;
use std::task::{Context, Poll};
pub trait Stream {
type Item;
fn poll_next(
self: Pin&lt;&amp;mut Self&gt;,
cx: &amp;mut Context&lt;'_&gt;
) -&gt; Poll&lt;Option&lt;Self::Item&gt;&gt;;
fn size_hint(&amp;self) -&gt; (usize, Option&lt;usize&gt;) {
(0, None)
}
}
<span class="boring">}</span></code></pre></pre>
<p><code>Stream::poll_next()</code> 函数跟 <code>Future::poll</code> 很相似,区别就是前者为了从 <code>stream</code> 收到多个值需要重复的进行调用。 就像在 <a href="https://course.rs/tokio/async.html"><code>深入async</code></a> 章节提到的那样,当一个 <code>stream</code> 没有做好返回一个值的准备时,它将返回一个 <code>Poll::Pending</code> ,同时将任务的 <code>waker</code> 进行注册。一旦 <code>stream</code> 准备好后, <code>waker</code> 将被调用。</p>
<p>通常来说,如果想要手动实现一个 <code>Stream</code>,需要组合 <code>Future</code> 和其它 <code>Stream</code>。下面,还记得在<a href="https://course.rs/tokio/async.html"><code>深入async</code></a> 中构建的 <code>Delay Future</code> 吗?现在让我们来更进一步,将它转换成一个 <code>stream</code>,每 10 毫秒生成一个值,总共生成 3 次:</p>
<pre><pre class="playground"><code class="language-rust edition2021"><span class="boring">#![allow(unused)]
</span><span class="boring">fn main() {
</span>use tokio_stream::Stream;
use std::pin::Pin;
use std::task::{Context, Poll};
use std::time::Duration;
struct Interval {
rem: usize,
delay: Delay,
}
impl Stream for Interval {
type Item = ();
fn poll_next(mut self: Pin&lt;&amp;mut Self&gt;, cx: &amp;mut Context&lt;'_&gt;)
-&gt; Poll&lt;Option&lt;()&gt;&gt;
{
if self.rem == 0 {
// 去除计时器实现
return Poll::Ready(None);
}
match Pin::new(&amp;mut self.delay).poll(cx) {
Poll::Ready(_) =&gt; {
let when = self.delay.when + Duration::from_millis(10);
self.delay = Delay { when };
self.rem -= 1;
Poll::Ready(Some(()))
}
Poll::Pending =&gt; Poll::Pending,
}
}
}
<span class="boring">}</span></code></pre></pre>
<h4 id="async-stream"><a class="header" href="#async-stream">async-stream</a></h4>
<p>手动实现 <code>Stream</code> 特征实际上是相当麻烦的事不幸地是Rust 语言的 <code>async/await</code> 语法目前还不能用于定义 <code>stream</code>,虽然相关的工作已经在进行中。</p>
<p>作为替代方案,<a href="https://docs.rs/async-stream/latest/async_stream/"><code>async-stream</code></a> 包提供了一个 <code>stream!</code> 宏,它可以将一个输入转换成 <code>stream</code>,使用这个包,上面的代码可以这样实现:</p>
<pre><pre class="playground"><code class="language-rust edition2021"><span class="boring">#![allow(unused)]
</span><span class="boring">fn main() {
</span>use async_stream::stream;
use std::time::{Duration, Instant};
stream! {
let mut when = Instant::now();
for _ in 0..3 {
let delay = Delay { when };
delay.await;
yield ();
when += Duration::from_millis(10);
}
}
<span class="boring">}</span></code></pre></pre>
<p>嗯,看上去还是相当不错的,代码可读性大幅提升!</p>
<!-- todo generators -->
<p>是不是发现了一个关键字 <code>yield</code> ,他是用来配合生成器使用的。详见<a href="https://doc.rust-lang.org/beta/unstable-book/language-features/generators.html">原文</a></p>
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