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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 class = "playground" > < code class = "language-rust edition2021" > #[tokio::main]
async fn main() {
println!("Hello world");
}< / code > < / 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 class = "playground" > < code class = "language-rust edition2021" > fn main() {
tokio::runtime::Builder::new_multi_thread()
.enable_all()
.build()
.unwrap()
.block_on(async {
println!("Hello world");
})
}< / code > < / 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 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< T: ToSocketAddrs> (addr: T) -> crate::Result< BlockingClient> {
// 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 >
< 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 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(& mut self, key: & str) -> crate::Result< Option< Bytes> > {
self.rt.block_on(self.inner.get(key))
}
pub fn set(& mut self, key: & str, value: Bytes) -> crate::Result< ()> {
self.rt.block_on(self.inner.set(key, value))
}
pub fn set_expires(
& mut self,
key: & str,
value: Bytes,
expiration: Duration,
) -> crate::Result< ()> {
self.rt.block_on(self.inner.set_expires(key, value, expiration))
}
pub fn publish(& mut self, channel: & str, message: Bytes) -> crate::Result< u64> {
self.rt.block_on(self.inner.publish(channel, message))
}
}
< span class = "boring" > }< / span > < / code > < / 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 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< String> ) -> crate::Result< BlockingSubscriber> {
let subscriber = self.rt.block_on(self.inner.subscribe(channels))?;
Ok(BlockingSubscriber {
inner: subscriber,
rt: self.rt,
})
}
}
impl BlockingSubscriber {
pub fn get_subscribed(& self) -> & [String] {
self.inner.get_subscribed()
}
pub fn next_message(& mut self) -> crate::Result< Option< Message> > {
self.rt.block_on(self.inner.next_message())
}
pub fn subscribe(& mut self, channels: & [String]) -> crate::Result< ()> {
self.rt.block_on(self.inner.subscribe(channels))
}
pub fn unsubscribe(& mut self, channels: & [String]) -> crate::Result< ()> {
self.rt.block_on(self.inner.unsubscribe(channels))
}
}
< span class = "boring" > }< / span > < / code > < / 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 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!("Finished time-consuming task.");
// 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!("Task {} sleeping for {} ms.", i, millis);
sleep(Duration::from_millis(millis)).await;
println!("Task {} stopping.", i);
}< / code > < / 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 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!("Got task {}", task.name);
}
#[derive(Clone)]
pub struct TaskSpawner {
spawn: mpsc::Sender< Task> ,
}
impl TaskSpawner {
pub fn new() -> 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，
});
});
TaskSpawner {
spawn: send,
}
}
pub fn spawn_task(& self, task: Task) {
match self.spawn.blocking_send(task) {
Ok(()) => {},
Err(_) => panic!("The shared runtime has shut down."),
}
}
}
< span class = "boring" > }< / span > < / code > < / 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 >
< / main >
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