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<h1 id="异步跟同步共存"><a class="header" href="#异步跟同步共存">异步跟同步共存</a></h1>
<p>一些异步程序例如 tokio 指南 章节中的绝大多数例子，它们整个程序都是异步的，包括程序入口 <code>main</code> 函数：</p>
<pre><pre class="playground"><code class="language-rust edition2021">#[tokio::main]
async fn main() {
println!(&quot;Hello world&quot;);
}</code></pre></pre>
<p>在一些场景中，你可能只想在异步程序中运行一小部分同步代码，这种需求可以考虑下 <a href="https://docs.rs/tokio/1.16.1/tokio/task/fn.spawn_blocking.html"><code>spawn_blocking</code></a>。</p>
<p>但是在很多场景中，我们只想让程序的某一个部分成为异步的，也许是因为同步代码更好实现，又或许是同步代码可读性、兼容性都更好。例如一个 <code>GUI</code> 应用可能想要让 <code>UI</code> 相关的代码在主线程中，然后通过另一个线程使用 <code>tokio</code> 的运行时来处理一些异步任务。</p>
<p>因此本章节的目标很纯粹：如何在同步代码中使用一小部分异步代码。</p>
<h2 id="tokiomain-的展开"><a class="header" href="#tokiomain-的展开"><code>#[tokio::main]</code> 的展开</a></h2>
<p>在 Rust 中， <code>main</code> 函数不能是异步的，有同学肯定不愿意了，我们在之前章节..不对，就在开头，你还用到了 <code>async fn main</code> 的声明方式，怎么就不能异步了呢？</p>
<p>其实，<code>#[tokio::main]</code> 该宏仅仅是提供语法糖，目的是让大家可以更简单、更一致的去写异步代码，它会将你写下的<code>async fn main</code> 函数替换为：</p>
<pre><pre class="playground"><code class="language-rust edition2021">fn main() {
tokio::runtime::Builder::new_multi_thread()
.enable_all()
.build()
.unwrap()
.block_on(async {
println!(&quot;Hello world&quot;);
})
}</code></pre></pre>
<p>注意到上面的 <code>block_on</code> 方法了嘛？在我们自己的同步代码中，可以使用它开启一个 <code>async/await</code> 世界。</p>
<h2 id="mini-redis-的同步接口"><a class="header" href="#mini-redis-的同步接口">mini-redis 的同步接口</a></h2>
<p>在下面，我们将一起构建一个同步的 <code>mini-redis</code> ，为了实现这一点，需要将 <code>Runtime</code> 对象存储起来，然后利用上面提到的 <code>block_on</code> 方法。</p>
<p>首先，创建一个文件 <code>src/blocking_client.rs</code>，然后使用下面代码将异步的 <code>Client</code> 结构体包裹起来:</p>
<pre><pre class="playground"><code class="language-rust edition2021"><span class="boring">#![allow(unused)]
</span><span class="boring">fn main() {
</span>use tokio::net::ToSocketAddrs;
use tokio::runtime::Runtime;
pub use crate::client::Message;
/// 建立到 redis 服务端的连接
pub struct BlockingClient {
/// 之前实现的异步客户端 `Client`
inner: crate::client::Client,
/// 一个 `current_thread` 模式的 `tokio` 运行时，
/// 使用阻塞的方式来执行异步客户端 `Client` 上的操作
rt: Runtime,
}
pub fn connect&lt;T: ToSocketAddrs&gt;(addr: T) -&gt; crate::Result&lt;BlockingClient&gt; {
// 构建一个 tokio 运行时： Runtime
let rt = tokio::runtime::Builder::new_current_thread()
.enable_all()
.build()?;
// 使用运行时来调用异步的连接方法
let inner = rt.block_on(crate::client::connect(addr))?;
Ok(BlockingClient { inner, rt })
}
<span class="boring">}</span></code></pre></pre>
<p>在这里，我们使用了一个构造器函数用于在同步代码中执行异步的方法：使用 <code>Runtime</code> 上的 <code>block_on</code> 方法来执行一个异步方法并返回结果。</p>
<p>有一个很重要的点，就是我们还使用了 <a href="https://docs.rs/tokio/1.16.1/tokio/runtime/struct.Builder.html#method.new_current_thread"><code>current_thread</code></a> 模式的运行时。这个可不常见，原因是异步程序往往要利用多线程的威力来实现更高的吞吐性能，相对应的模式就是 <a href="https://docs.rs/tokio/1.16.1/tokio/runtime/struct.Builder.html#method.new_multi_thread"><code>multi_thread</code></a>，该模式会生成多个运行在后台的线程，它们可以高效的实现多个任务的同时并行处理。</p>
<p>但是对于我们的使用场景来说，在同一时间点只需要做一件事，无需并行处理，多个线程并不能帮助到任何事情，因此 <code>current_thread</code> 此时成为了最佳的选择。</p>
<p>在构建 <code>Runtime</code> 的过程中还有一个 <a href="https://docs.rs/tokio/1.16.1/tokio/runtime/struct.Builder.html#method.enable_all"><code>enable_all</code></a> 方法调用，它可以开启 <code>Tokio</code> 运行时提供的 IO 和定时器服务。</p>
<blockquote>
<p>由于 <code>current_thread</code> 运行时并不生成新的线程，只是运行在已有的主线程上，因此只有当 <code>block_on</code> 被调用后，该运行时才能执行相应的操作。一旦 <code>block_on</code> 返回，那运行时上所有生成的任务将再次冻结，直到 <code>block_on</code> 的再次调用。</p>
<p>如果这种模式不符合使用场景的需求，那大家还是需要用 <code>multi_thread</code> 运行时来代替。事实上，在 tokio 之前的章节中，我们默认使用的就是 <code>multi_thread</code> 模式。</p>
</blockquote>
<pre><pre class="playground"><code class="language-rust edition2021"><span class="boring">#![allow(unused)]
</span><span class="boring">fn main() {
</span>use bytes::Bytes;
use std::time::Duration;
impl BlockingClient {
pub fn get(&amp;mut self, key: &amp;str) -&gt; crate::Result&lt;Option&lt;Bytes&gt;&gt; {
self.rt.block_on(self.inner.get(key))
}
pub fn set(&amp;mut self, key: &amp;str, value: Bytes) -&gt; crate::Result&lt;()&gt; {
self.rt.block_on(self.inner.set(key, value))
}
pub fn set_expires(
&amp;mut self,
key: &amp;str,
value: Bytes,
expiration: Duration,
) -&gt; crate::Result&lt;()&gt; {
self.rt.block_on(self.inner.set_expires(key, value, expiration))
}
pub fn publish(&amp;mut self, channel: &amp;str, message: Bytes) -&gt; crate::Result&lt;u64&gt; {
self.rt.block_on(self.inner.publish(channel, message))
}
}
<span class="boring">}</span></code></pre></pre>
<p>这代码看上去挺长，实际上很简单，通过 <code>block_on</code> 将异步形式的 <code>Client</code> 的方法变成同步调用的形式。例如 <code>BlockingClient</code> 的 <code>get</code> 方法实际上是对内部的异步 <code>get</code> 方法的同步调用。</p>
<p>与上面的平平无奇相比，下面的代码将更有趣，因为它将 <code>Client</code> 转变成一个 <code>Subscriber</code> 对象:</p>
<pre><pre class="playground"><code class="language-rust edition2021"><span class="boring">#![allow(unused)]
</span><span class="boring">fn main() {
</span>/// 下面的客户端可以进入 pub/sub (发布/订阅) 模式
///
/// 一旦客户端订阅了某个消息通道，那就只能执行 pub/sub 相关的命令。
/// 将`BlockingClient` 类型转换成 `BlockingSubscriber` 是为了防止非 `pub/sub` 方法被调用
pub struct BlockingSubscriber {
/// 异步版本的 `Subscriber`
inner: crate::client::Subscriber,
/// 一个 `current_thread` 模式的 `tokio` 运行时，
/// 使用阻塞的方式来执行异步客户端 `Client` 上的操作
rt: Runtime,
}
impl BlockingClient {
pub fn subscribe(self, channels: Vec&lt;String&gt;) -&gt; crate::Result&lt;BlockingSubscriber&gt; {
let subscriber = self.rt.block_on(self.inner.subscribe(channels))?;
Ok(BlockingSubscriber {
inner: subscriber,
rt: self.rt,
})
}
}
impl BlockingSubscriber {
pub fn get_subscribed(&amp;self) -&gt; &amp;[String] {
self.inner.get_subscribed()
}
pub fn next_message(&amp;mut self) -&gt; crate::Result&lt;Option&lt;Message&gt;&gt; {
self.rt.block_on(self.inner.next_message())
}
pub fn subscribe(&amp;mut self, channels: &amp;[String]) -&gt; crate::Result&lt;()&gt; {
self.rt.block_on(self.inner.subscribe(channels))
}
pub fn unsubscribe(&amp;mut self, channels: &amp;[String]) -&gt; crate::Result&lt;()&gt; {
self.rt.block_on(self.inner.unsubscribe(channels))
}
}
<span class="boring">}</span></code></pre></pre>
<p>由上可知，<code>subscribe</code> 方法会使用运行时将一个异步的 <code>Client</code> 转变成一个异步的 <code>Subscriber</code>，此外，<code>Subscriber</code> 结构体有一个非异步的方法 <code>get_subscribed</code>，对于这种方法，只需直接调用即可，而无需使用运行时。</p>
<h2 id="其它方法"><a class="header" href="#其它方法">其它方法</a></h2>
<p>上面介绍的是最简单的方法，但是，如果只有这一种， tokio 也不会如此大名鼎鼎。</p>
<h4 id="runtimespawn"><a class="header" href="#runtimespawn">runtime.spawn</a></h4>
<p>可以通过 <code>Runtime</code> 的 <code>spawn</code> 方法来创建一个基于该运行时的后台任务：</p>
<pre><pre class="playground"><code class="language-rust edition2021">use tokio::runtime::Builder;
use tokio::time::{sleep, Duration};
fn main() {
let runtime = Builder::new_multi_thread()
.worker_threads(1)
.enable_all()
.build()
.unwrap();
let mut handles = Vec::with_capacity(10);
for i in 0..10 {
handles.push(runtime.spawn(my_bg_task(i)));
}
// 在后台任务运行的同时做一些耗费时间的事情
std::thread::sleep(Duration::from_millis(750));
println!(&quot;Finished time-consuming task.&quot;);
// 等待这些后台任务的完成
for handle in handles {
// `spawn` 方法返回一个 `JoinHandle`，它是一个 `Future`，因此可以通过 `block_on` 来等待它完成
runtime.block_on(handle).unwrap();
}
}
async fn my_bg_task(i: u64) {
let millis = 1000 - 50 * i;
println!(&quot;Task {} sleeping for {} ms.&quot;, i, millis);
sleep(Duration::from_millis(millis)).await;
println!(&quot;Task {} stopping.&quot;, i);
}</code></pre></pre>
<p>运行该程序，输出如下:</p>
<pre><code class="language-console">Task 0 sleeping for 1000 ms.
Task 1 sleeping for 950 ms.
Task 2 sleeping for 900 ms.
Task 3 sleeping for 850 ms.
Task 4 sleeping for 800 ms.
Task 5 sleeping for 750 ms.
Task 6 sleeping for 700 ms.
Task 7 sleeping for 650 ms.
Task 8 sleeping for 600 ms.
Task 9 sleeping for 550 ms.
Task 9 stopping.
Task 8 stopping.
Task 7 stopping.
Task 6 stopping.
Finished time-consuming task.
Task 5 stopping.
Task 4 stopping.
Task 3 stopping.
Task 2 stopping.
Task 1 stopping.
Task 0 stopping.
</code></pre>
<p>在此例中，我们生成了 10 个后台任务在运行时中运行，然后等待它们的完成。作为一个例子，想象一下在图形渲染应用( GUI )中，有时候需要通过网络访问远程服务来获取一些数据，那上面的这种模式就非常适合，因为这些网络访问比较耗时，而且不会影响图形的主体渲染，因此可以在主线程中渲染图形，然后使用其它线程来运行 Tokio 的运行时，并通过该运行时使用异步的方式完成网络访问，最后将这些网络访问的结果发送到 GUI 进行数据渲染，例如一个进度条。</p>
<p>还有一点很重要，在本例子中只能使用 <code>multi_thread</code> 运行时。如果我们使用了 <code>current_thread</code>，你会发现主线程的耗时任务会在后台任务开始之前就完成了。因为在 <code>current_thread</code> 模式下，生成的任务只会在 <code>block_on</code> 期间才执行。</p>
<p>在 <code>multi_thread</code> 模式下，我们并不需要通过 <code>block_on</code> 来触发任务的运行，这里仅仅是用来阻塞并等待最终的结果。而除了通过 <code>block_on</code> 等待结果外，你还可以：</p>
<ul>
<li>使用消息传递的方式，例如 <code>tokio::sync::mpsc</code>，让异步任务将结果发送到主线程，然后主线程通过 <code>.recv</code>方法等待这些结果</li>
<li>通过共享变量的方式，例如 <code>Mutex</code>，这种方式非常适合实现 GUI 的进度条: GUI 在每个渲染帧读取该变量即可。</li>
</ul>
<h4 id="发送消息"><a class="header" href="#发送消息">发送消息</a></h4>
<p>在同步代码中使用异步的另一个方法就是生成一个运行时，然后使用消息传递的方式跟它进行交互。这个方法虽然更啰嗦一些，但是相对于之前的两种方法更加灵活：</p>
<pre><pre class="playground"><code class="language-rust edition2021"><span class="boring">#![allow(unused)]
</span><span class="boring">fn main() {
</span>use tokio::runtime::Builder;
use tokio::sync::mpsc;
pub struct Task {
name: String,
// 一些信息用于描述该任务
}
async fn handle_task(task: Task) {
println!(&quot;Got task {}&quot;, task.name);
}
#[derive(Clone)]
pub struct TaskSpawner {
spawn: mpsc::Sender&lt;Task&gt;,
}
impl TaskSpawner {
pub fn new() -&gt; TaskSpawner {
// 创建一个消息通道用于通信
let (send, mut recv) = mpsc::channel(16);
let rt = Builder::new_current_thread()
.enable_all()
.build()
.unwrap();
std::thread::spawn(move || {
rt.block_on(async move {
while let Some(task) = recv.recv().await {
tokio::spawn(handle_task(task));
}
// 一旦所有的发送端超出作用域被 drop 后，`.recv()` 方法会返回 None，同时 while 循环会退出，然后线程结束
});
});
TaskSpawner {
spawn: send,
}
}
pub fn spawn_task(&amp;self, task: Task) {
match self.spawn.blocking_send(task) {
Ok(()) =&gt; {},
Err(_) =&gt; panic!(&quot;The shared runtime has shut down.&quot;),
}
}
}
<span class="boring">}</span></code></pre></pre>
<p>为何说这种方法比较灵活呢？以上面代码为例，它可以在很多方面进行配置。例如，可以使用信号量 <a href="https://docs.rs/tokio/1.16.1/tokio/sync/struct.Semaphore.html"><code>Semaphore</code></a>来限制当前正在进行的任务数，或者你还可以使用一个消息通道将消息反向发送回任务生成器 <code>spawner</code>。</p>
<p>抛开细节，抽象来看，这是不是很像一个 Actor ？</p>
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