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<ol class="chapter"><li class="chapter-item expanded affix "><a href="title-page.html">Rust 程序设计语言</a></li><li class="chapter-item expanded affix "><a href="foreword.html">前言</a></li><li class="chapter-item expanded affix "><a href="ch00-00-introduction.html">简介</a></li><li class="chapter-item expanded "><a href="ch01-00-getting-started.html"><strong aria-hidden="true">1.</strong> 入门指南</a></li><li><ol class="section"><li class="chapter-item expanded "><a href="ch01-01-installation.html"><strong aria-hidden="true">1.1.</strong> 安装</a></li><li class="chapter-item expanded "><a href="ch01-02-hello-world.html"><strong aria-hidden="true">1.2.</strong> Hello, World!</a></li><li class="chapter-item expanded "><a href="ch01-03-hello-cargo.html"><strong aria-hidden="true">1.3.</strong> Hello, Cargo!</a></li></ol></li><li class="chapter-item expanded "><a href="ch02-00-guessing-game-tutorial.html"><strong aria-hidden="true">2.</strong> 写个猜数字游戏</a></li><li class="chapter-item expanded "><a href="ch03-00-common-programming-concepts.html"><strong aria-hidden="true">3.</strong> 常见编程概念</a></li><li><ol class="section"><li class="chapter-item expanded "><a href="ch03-01-variables-and-mutability.html"><strong aria-hidden="true">3.1.</strong> 变量与可变性</a></li><li class="chapter-item expanded "><a href="ch03-02-data-types.html"><strong aria-hidden="true">3.2.</strong> 数据类型</a></li><li class="chapter-item expanded "><a href="ch03-03-how-functions-work.html"><strong aria-hidden="true">3.3.</strong> 函数</a></li><li class="chapter-item expanded "><a href="ch03-04-comments.html"><strong aria-hidden="true">3.4.</strong> 注释</a></li><li class="chapter-item expanded "><a href="ch03-05-control-flow.html"><strong aria-hidden="true">3.5.</strong> 控制流</a></li></ol></li><li class="chapter-item expanded "><a href="ch04-00-understanding-ownership.html"><strong aria-hidden="true">4.</strong> 认识所有权</a></li><li><ol class="section"><li class="chapter-item expanded "><a href="ch04-01-what-is-ownership.html"><strong aria-hidden="true">4.1.</strong> 什么是所有权？</a></li><li class="chapter-item expanded "><a href="ch04-02-references-and-borrowing.html"><strong aria-hidden="true">4.2.</strong> 引用与借用</a></li><li class="chapter-item expanded "><a href="ch04-03-slices.html"><strong aria-hidden="true">4.3.</strong> Slice 类型</a></li></ol></li><li class="chapter-item expanded "><a href="ch05-00-structs.html"><strong aria-hidden="true">5.</strong> 使用结构体组织相关联的数据</a></li><li><ol class="section"><li class="chapter-item expanded "><a href="ch05-01-defining-structs.html"><strong aria-hidden="true">5.1.</strong> 结构体的定义和实例化</a></li><li class="chapter-item expanded "><a href="ch05-02-example-structs.html"><strong aria-hidden="true">5.2.</strong> 结构体示例程序</a></li><li class="chapter-item expanded "><a href="ch05-03-method-syntax.html"><strong aria-hidden="true">5.3.</strong> 方法语法</a></li></ol></li><li class="chapter-item expanded "><a href="ch06-00-enums.html"><strong aria-hidden="true">6.</strong> 枚举和模式匹配</a></li><li><ol class="section"><li class="chapter-item expanded "><a href="ch06-01-defining-an-enum.html"><strong aria-hidden="true">6.1.</strong> 枚举的定义</a></li><li class="chapter-item expanded "><a href="ch06-02-match.html"><strong aria-hidden="true">6.2.</strong> match 控制流结构</a></li><li class="chapter-item expanded "><a href="ch06-03-if-let.html"><strong aria-hidden="true">6.3.</strong> if let 简洁控制流</a></li></ol></li><li class="chapter-item expanded "><a href="ch07-00-managing-growing-projects-with-packages-crates-and-modules.html"><strong aria-hidden="true">7.</strong> 使用包、Crate 和模块管理不断增长的项目</a></li><li><ol class="section"><li class="chapter-item expanded "><a href="ch07-01-packages-and-crates.html"><strong aria-hidden="true">7.1.</strong> 包和 Crate</a></li><li class="chapter-item expanded "><a href="ch07-02-defining-modules-to-control-scope-and-privacy.html"><strong aria-hidden="true">7.2.</strong> 定义模块来控制作用域与私有性</a></li><li class="chapter-item expanded "><a href="ch07-03-paths-for-referring-to-an-item-in-the-module-tree.html"><strong aria-hidden="true">7.3.</strong> 引用模块项目的路径</a></li><li class="chapter-item expanded "><a href="ch07-04-bringing-paths-into-scope-with-the-use-keyword.html"><strong aria-hidden="true">7.4.</strong> 使用 use 关键字将路径引入作用域</a></li><li class="chapter-item expanded "><a href="ch07-05-separating-modules-into-different-files.html"><strong aria-hidden="true">7.5.</strong> 将模块拆分成多个文件</a></li></ol></li><li class="chapter-item expanded "><a href="ch08-00-common-collections.html"><strong aria-hidden="true">8.</strong> 常见集合</a></li><li><ol class="section"><li class="chapter-item expanded "><a href="ch08-01-vectors.html"><strong aria-hidden="true">8.1.</strong> 使用 Vector 储存列表</a></li><li class="chapter-item expanded "><a href="ch08-02-strings.html"><strong aria-hidden="true">8.2.</strong> 使用字符串储存 UTF-8 编码的文本</a></li><li class="chapter-item expanded "><a href="ch08-03-hash-maps.html"><strong aria-hidden="true">8.3.</strong> 使用 Hash Map 储存键值对</a></li></ol></li><li class="chapter-item expanded "><a href="ch09-00-error-handling.html"><strong aria-hidden="true">9.</strong> 错误处理</a></li><li><ol class="section"><li class="chapter-item expanded "><a href="ch09-01-unrecoverable-errors-with-panic.html"><strong aria-hidden="true">9.1.</strong> 用 panic! 处理不可恢复的错误</a></li><li class="chapter-item expanded "><a href="ch09-02-recoverable-errors-with-result.html"><strong aria-hidden="true">9.2.</strong> 用 Result 处理可恢复的错误</a></li><li class="chapter-item expanded "><a href="ch09-03-to-panic-or-not-to-panic.html"><strong aria-hidden="true">9.3.</strong> 要不要 panic!</a></li></ol></li><li class="chapter-item expanded "><a href="ch10-00-generics.html"><strong aria-hidden="true">10.</strong> 泛型、Trait 和生命周期</a></li><li><ol class="section"><li class="chapter-item expanded "><a href="ch10-01-syntax.html"><strong aria-hidden="true">10.1.</strong> 泛型数据类型</a></li><li class="chapter-item expanded "><a href="ch10-02-traits.html"><strong aria-hidden="true">10.2.</strong> Trait：定义共同行为</a></li><li class="chapter-item expanded "><a href="ch10-03-lifetime-syntax.html"><strong aria-hidden="true">10.3.</strong> 生命周期确保引用有效</a></li></ol></li><li class="chapter-item expanded "><a href="ch11-00-testing.html"><strong aria-hidden="true">11.</strong> 编写自动化测试</a></li><li><ol class="section"><li class="chapter-item expanded "><a href="ch11-01-writing-tests.html"><strong aria-hidden="true">11.1.</strong> 如何编写测试</a></li><li class="chapter-item expanded "><a href="ch11-02-running-tests.html"><strong aria-hidden="true">11.2.</strong> 控制测试如何运行</a></li><li class="chapter-item expanded "><a href="ch11-03-test-organization.html"><strong aria-hidden="true">11.3.</strong> 测试的组织结构</a></li></ol></li><li class="chapter-item expanded "><a href="ch12-00-an-io-project.html"><strong aria-hidden="true">12.</strong> 一个 I/O 项目：构建命令行程序</a></li><li><ol class="section"><li class="chapter-item expanded "><a href="ch12-01-accepting-command-line-arguments.html"><strong aria-hidden="true">12.1.</strong> 接受命令行参数</a></li><li class="chapter-item expanded "><a href="ch12-02-reading-a-file.html"><strong aria-hidden="true">12.2.</strong> 读取文件</a></li><li class="chapter-item expanded "><a href="ch12-03-improving-error-handling-and-modularity.html"><strong aria-hidden="true">12.3.</strong> 重构以改进模块化与错误处理</a></li><li class="chapter-item expanded "><a href="ch12-04-testing-the-librarys-functionality.html"><strong aria-hidden="true">12.4.</strong> 采用测试驱动开发完善库的功能</a></li><li class="chapter-item expanded "><a href="ch12-05-working-with-environment-variables.html"><strong aria-hidden="true">12.5.</strong> 处理环境变量</a></li><li class="chapter-item expanded "><a href="ch12-06-writing-to-stderr-instead-of-stdout.html"><strong aria-hidden="true">12.6.</strong> 将错误信息输出到标准错误而不是标准输出</a></li></ol></li><li class="chapter-item expanded "><a href="ch13-00-functional-features.html"><strong aria-hidden="true">13.</strong> Rust 中的函数式语言功能：迭代器与闭包</a></li><li><ol class="section"><li class="chapter-item expanded "><a href="ch13-01-closures.html"><strong aria-hidden="true">13.1.</strong> 闭包：可以捕获其环境的匿名函数</a></li><li class="chapter-item expanded "><a href="ch13-02-iterators.html"><strong aria-hidden="true">13.2.</strong> 使用迭代器处理元素序列</a></li><li class="chapter-item expanded "><a href="ch13-03-improving-our-io-project.html"><strong aria-hidden="true">13.3.</strong> 改进之前的 I/O 项目</a></li><li class="chapter-item expanded "><a href="ch13-04-performance.html"><strong aria-hidden="true">13.4.</strong> 性能比较：循环对迭代器</a></li></ol></li><li class="chapter-item expanded "><a href="ch14-00-more-about-cargo.html"><strong aria-hidden="true">14.</strong> 更多关于 Cargo 和 Crates.io 的内容</a></li><li><ol class="section"><li class="chapter-item expanded "><a href="ch14-01-release-profiles.html"><strong aria-hidden="true">14.1.</strong> 采用发布配置自定义构建</a></li><li class="chapter-item expanded "><a href="ch14-02-publishing-to-crates-io.html"><strong aria-hidden="true">14.2.</strong> 将 crate 发布到 Crates.io</a></li><li class="chapter-item expanded "><a href="ch14-03-cargo-workspaces.html"><strong aria-hidden="true">14.3.</strong> Cargo 工作空间</a></li><li class="chapter-item expanded "><a href="ch14-04-installing-binaries.html"><strong aria-hidden="true">14.4.</strong> 使用 cargo install 安装二进制文件</a></li><li class="chapter-item expanded "><a href="ch14-05-extending-cargo.html"><strong aria-hidden="true">14.5.</strong> Cargo 自定义扩展命令</a></li></ol></li><li class="chapter-item expanded "><a href="ch15-00-smart-pointers.html"><strong aria-hidden="true">15.</strong> 智能指针</a></li><li><ol class="section"><li class="chapter-item expanded "><a href="ch15-01-box.html"><strong aria-hidden="true">15.1.</strong> 使用 Box<T> 指向堆上数据</a></li><li class="chapter-item expanded "><a href="ch15-02-deref.html"><strong aria-hidden="true">15.2.</strong> 使用 Deref Trait 将智能指针当作常规引用处理</a></li><li class="chapter-item expanded "><a href="ch15-03-drop.html"><strong aria-hidden="true">15.3.</strong> 使用 Drop Trait 运行清理代码</a></li><li class="chapter-item expanded "><a href="ch15-04-rc.html"><strong aria-hidden="true">15.4.</strong> Rc<T> 引用计数智能指针</a></li><li class="chapter-item expanded "><a href="ch15-05-interior-mutability.html"><strong aria-hidden="true">15.5.</strong> RefCell<T> 与内部可变性模式</a></li><li class="chapter-item expanded "><a href="ch15-06-reference-cycles.html"><strong aria-hidden="true">15.6.</strong> 引用循环会导致内存泄漏</a></li></ol></li><li class="chapter-item expanded "><a href="ch16-00-concurrency.html"><strong aria-hidden="true">16.</strong> 无畏并发</a></li><li><ol class="section"><li class="chapter-item expanded "><a href="ch16-01-threads.html"><strong aria-hidden="true">16.1.</strong> 使用线程同时地运行代码</a></li><li class="chapter-item expanded "><a href="ch16-02-message-passing.html"><strong aria-hidden="true">16.2.</strong> 使用消息传递在线程间通信</a></li><li class="chapter-item expanded "><a href="ch16-03-shared-state.html"><strong aria-hidden="true">16.3.</strong> 共享状态并发</a></li><li class="chapter-item expanded "><a href="ch16-04-extensible-concurrency-sync-and-send.html"><strong aria-hidden="true">16.4.</strong> 使用 Sync 与 Send Traits 的可扩展并发</a></li></ol></li><li class="chapter-item expanded "><a href="ch17-00-async-await.html"><strong aria-hidden="true">17.</strong> Async 和 await</a></li><li><ol class="section"><li class="chapter-item expanded "><a href="ch17-01-futures-and-syntax.html"><strong aria-hidden="true">17.1.</strong> Futures 和 async 语法</a></li><li class="chapter-item expanded "><a href="ch17-02-concurrency-with-async.html" class="active"><strong aria-hidden="true">17.2.</strong> 并发与 async</a></li><li class="chapter-item expanded "><a href="ch17-03-more-futures.html"><strong aria-hidden="true">17.3.</strong> 使用任意数量的 futures</a></li><li class="chapter-item expanded "><a href="ch17-04-streams.html"><strong aria-hidden="true">17.4.</strong> 流（Streams）</a></li><li class="chapter-item expanded "><a href="ch17-05-traits-for-async.html"><strong aria-hidden="true">17.5.</strong> 深入理解 async 相关的 traits</a></li><li class="chapter-item expanded "><a href="ch17-06-futures-tasks-threads.html"><strong aria-hidden="true">17.6.</strong> Futures，任务（tasks）和线程（threads）</a></li></ol></li><li class="chapter-item expanded "><a href="ch18-00-oop.html"><strong aria-hidden="true">18.</strong> Rust 的面向对象编程特性</a></li><li><ol class="section"><li class="chapter-item expanded "><a href="ch18-01-what-is-oo.html"><strong aria-hidden="true">18.1.</strong> 面向对象语言的特点</a></li><li class="chapter-item expanded "><a href="ch18-02-trait-objects.html"><strong aria-hidden="true">18.2.</strong> 顾及不同类型值的 trait 对象</a></li><li class="chapter-item expanded "><a href="ch18-03-oo-design-patterns.html"><strong aria-hidden="true">18.3.</strong> 面向对象设计模式的实现</a></li></ol></li><li class="chapter-item expanded "><a href="ch19-00-patterns.html"><strong aria-hidden="true">19.</strong> 模式与模式匹配</a></li><li><ol class="section"><li class="chapter-item expanded "><a href="ch19-01-all-the-places-for-patterns.html"><strong aria-hidden="true">19.1.</strong> 所有可能会用到模式的位置</a></li><li class="chapter-item expanded "><a href="ch19-02-refutability.html"><strong aria-hidden="true">19.2.</strong> Refutability（可反驳性）: 模式是否会匹配失效</a></li><li class="chapter-item expanded "><a href="ch19-03-pattern-syntax.html"><strong aria-hidden="true">19.3.</strong> 模式语法</a></li></ol></li><li class="chapter-item expanded "><a href="ch20-00-advanced-features.html"><strong aria-hidden="true">20.</strong> 高级特征</a></li><li><ol class="section"><li class="chapter-item expanded "><a href="ch20-01-unsafe-rust.html"><strong aria-hidden="true">20.1.</strong> 不安全的 Rust</a></li><li class="chapter-item expanded "><a href="ch20-03-advanced-traits.html"><strong aria-hidden="true">20.2.</strong> 高级 trait</a></li><li class="chapter-item expanded "><a href="ch20-04-advanced-types.html"><strong aria-hidden="true">20.3.</strong> 高级类型</a></li><li class="chapter-item expanded "><a href="ch20-05-advanced-functions-and-closures.html"><strong aria-hidden="true">20.4.</strong> 高级函数与闭包</a></li><li class="chapter-item expanded "><a href="ch20-06-macros.html"><strong aria-hidden="true">20.5.</strong> 宏</a></li></ol></li><li class="chapter-item expanded "><a href="ch21-00-final-project-a-web-server.html"><strong aria-hidden="true">21.</strong> 最后的项目：构建多线程 web server</a></li><li><ol class="section"><li class="chapter-item expanded "><a href="ch21-01-single-threaded.html"><strong aria-hidden="true">21.1.</strong> 建立单线程 web server</a></li><li class="chapter-item expanded "><a href="ch21-02-multithreaded.html"><strong aria-hidden="true">21.2.</strong> 将单线程 server 变为多线程 server</a></li><li class="chapter-item expanded "><a href="ch21-03-graceful-shutdown-and-cleanup.html"><strong aria-hidden="true">21.3.</strong> 优雅停机与清理</a></li></ol></li><li class="chapter-item expanded "><a href="appendix-00.html"><strong aria-hidden="true">22.</strong> 附录</a></li><li><ol class="section"><li class="chapter-item expanded "><a href="appendix-01-keywords.html"><strong aria-hidden="true">22.1.</strong> A - 关键字</a></li><li class="chapter-item expanded "><a href="appendix-02-operators.html"><strong aria-hidden="true">22.2.</strong> B - 运算符与符号</a></li><li class="chapter-item expanded "><a href="appendix-03-derivable-traits.html"><strong aria-hidden="true">22.3.</strong> C - 可派生的 trait</a></li><li class="chapter-item expanded "><a href="appendix-04-useful-development-tools.html"><strong aria-hidden="true">22.4.</strong> D - 实用开发工具</a></li><li class="chapter-item expanded "><a href="appendix-05-editions.html"><strong aria-hidden="true">22.5.</strong> E - 版本</a></li><li class="chapter-item expanded "><a href="appendix-06-translation.html"><strong aria-hidden="true">22.6.</strong> F - 本书译本</a></li><li class="chapter-item expanded "><a href="appendix-07-nightly-rust.html"><strong aria-hidden="true">22.7.</strong> G - Rust 是如何开发的与 “Nightly Rust”</a></li></ol></li></ol>
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<main>
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<h2 id="并发与-async"><a class="header" href="#并发与-async">并发与 async</a></h2>
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<blockquote>
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<p><a href="https://github.com/rust-lang/book/blob/main/src/ch17-02-concurrency-with-async.md">ch17-02-concurrency-with-async.md</a>
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<br>
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commit 62d441060d66f9a1c3d3cdfffa8eed40f817d1aa</p>
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</blockquote>
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<p>在这一部分，我们将使用异步来应对一些与第十六章中通过线程解决的相同的并发问题。因为之前我们已经讨论了很多关键理念了，这一部分我们会专注于线程与 future 的区别。</p>
|
||
<p>在很多情况下，使用异步处理并发的 API 与使用线程的非常相似。在其它的一些情况，它们则非常不同。即便线程与异步的的 API <em>看起来</em> 很类似，通常它们有着不同的行为，同时它们几乎总是有着不同的性能特点。</p>
|
||
<h3 id="计数"><a class="header" href="#计数">计数</a></h3>
|
||
<p>第十六章中我们应付的第一个任务是在两个不同的线程中计数。让我们用异步来完成相同的任务。<code>trpl</code> crate 提供了一个 <code>spawn_task</code> 函数，它看起来非常像 <code>thread::spawn</code> API，和一个 <code>sleep</code> 函数，这是 <code>thread::sleep</code> API 的异步版本。我们可以将它们结合使用，实现与线程示例相同的计数功能，如示例 17-6 所示。</p>
|
||
<figure class="listing">
|
||
<p><span class="file-name">文件名：src/main.rs</span></p>
|
||
<pre><pre class="playground"><code class="language-rust edition2021"><span class="boring">extern crate trpl; // required for mdbook test
|
||
</span><span class="boring">
|
||
</span>use std::time::Duration;
|
||
|
||
fn main() {
|
||
trpl::run(async {
|
||
trpl::spawn_task(async {
|
||
for i in 1..10 {
|
||
println!("hi number {i} from the first task!");
|
||
trpl::sleep(Duration::from_millis(500)).await;
|
||
}
|
||
});
|
||
|
||
for i in 1..5 {
|
||
println!("hi number {i} from the second task!");
|
||
trpl::sleep(Duration::from_millis(500)).await;
|
||
}
|
||
});
|
||
}</code></pre></pre>
|
||
<figcaption>示例 17-6：使用 `spawn_task` 启动两个计数任务</figcaption>
|
||
</figure>
|
||
<p>作为开始，我们在 <code>main</code> 函数中使用 <code>trpl::run</code>，这样我们的顶层函数可以是异步的。</p>
|
||
<blockquote>
|
||
<p>注意：本章从现在开始，每一个示例的 <code>main</code> 中都会包含几乎相同的 <code>trpl::run</code> 封装代码，所以我们经常会连同 <code>main</code> 一同省略。别忘了在你的代码中加入它们！</p>
|
||
</blockquote>
|
||
<p>接着我们在代码块中编写了两个循环，每个其中都有一个 <code>trpl::sleep</code> 调用，每一个都在发送下一个信息之前等待半秒（500 毫秒）。我们将一个循环放到 <code>trpl::spawn_task</code> 中并将另一个放在顶层的 <code>for</code> 循环中。我们也在 <code>sleep</code> 调用之后加入了一个 <code>await</code>。</p>
|
||
<p>这个实现与基于线程的版本类似，包括在运行时，你可能会在终端中看到消息以不同顺序出现的情况。</p>
|
||
<!-- Not extracting output because changes to this output aren't significant;
|
||
the changes are likely to be due to the threads running differently rather than
|
||
changes in the compiler -->
|
||
<pre><code class="language-text">hi number 1 from the second task!
|
||
hi number 1 from the first task!
|
||
hi number 2 from the first task!
|
||
hi number 2 from the second task!
|
||
hi number 3 from the first task!
|
||
hi number 3 from the second task!
|
||
hi number 4 from the first task!
|
||
hi number 4 from the second task!
|
||
hi number 5 from the first task!
|
||
</code></pre>
|
||
<p>这个版本在 main 中的异步代码块中 for 循环结束后就停止了，因为当 main 函数结束时 <code>spawn_task</code> 产生的任务就会关闭。如果运行该任务直到结束，你需要使用一个 join 句柄（join handle）来等待第一个任务完成。对于线程来说，可以使用 <code>join</code> 方法来 “阻塞” 直到线程结束运行。在示例 17-7 中，我们可以使用 <code>await</code> 来实现相同的效果，因为任务句柄本身是一个 future。它的 <code>Output</code> 类型是一个 <code>Result</code>，所以我们还需要 unwrap 来 await 它。</p>
|
||
<figure class="listing">
|
||
<p><span class="file-name">文件名：src/main.rs</span></p>
|
||
<pre><pre class="playground"><code class="language-rust edition2021"><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::run(async {
|
||
</span> let handle = trpl::spawn_task(async {
|
||
for i in 1..10 {
|
||
println!("hi number {i} from the first task!");
|
||
trpl::sleep(Duration::from_millis(500)).await;
|
||
}
|
||
});
|
||
|
||
for i in 1..5 {
|
||
println!("hi number {i} from the second task!");
|
||
trpl::sleep(Duration::from_millis(500)).await;
|
||
}
|
||
|
||
handle.await.unwrap();
|
||
<span class="boring"> });
|
||
</span><span class="boring">}</span></code></pre></pre>
|
||
<figcaption>示例 17-7：在一个 join 句柄上使用 `await` 使得任务运行直到结束</figcaption>
|
||
</figure>
|
||
<p>更新后的版本会运行 <em>两个</em> 循环直到结束。</p>
|
||
<!-- Not extracting output because changes to this output aren't significant;
|
||
the changes are likely to be due to the threads running differently rather than
|
||
changes in the compiler -->
|
||
<pre><code class="language-text">hi number 1 from the second task!
|
||
hi number 1 from the first task!
|
||
hi number 2 from the first task!
|
||
hi number 2 from the second task!
|
||
hi number 3 from the first task!
|
||
hi number 3 from the second task!
|
||
hi number 4 from the first task!
|
||
hi number 4 from the second task!
|
||
hi number 5 from the first task!
|
||
hi number 6 from the first task!
|
||
hi number 7 from the first task!
|
||
hi number 8 from the first task!
|
||
hi number 9 from the first task!
|
||
</code></pre>
|
||
<p>目前为止，看起来异步和线程版本给出了基本一样的输出，它们只是使用了不同的语法：在 join 句柄上使用 <code>await</code> 而不是调用 <code>join</code>，和 await <code>sleep</code> 调用。</p>
|
||
<p>最大的区别在于无需再产生另一个操作系统线程来进行工作。事实上，我们甚至不需要产生一个任务。因为异步代码块会编译为匿名 future，我们可以将每一个循环放进一个异步代码块并使用 <code>trpl::join</code> 方法来让运行时将它们两个都运行至完成。</p>
|
||
<p>在第十六章中，我们展示了如何在 <code>std::thread::spawn</code> 调用返回的 <code>JoinHandle</code> 类型上调用 <code>join</code> 方法。<code>trpl::join</code> 函数也类似，不过它作用于 future。当你传递两个 future，它会产生单独一个 future 但它的输出是一个元组，当 <em>两者</em> 都完成时其中有每一个传递给它的 future 的输出。因此，在示例 17-8 中，我们使用 <code>trpl::join</code> 来等待 <code>fut1</code> 和 <code>fut2</code> 都结束。我们 <em>没有</em> await <code>fut1</code> 和 <code>fut2</code>，而是等待 <code>trpl::join</code> 新产生的 future。我们忽略其输出，因为它只是一个包含两个单元值（unit value）的元组。</p>
|
||
<figure class="listing">
|
||
<p><span class="file-name">文件名：src/main.rs</span></p>
|
||
<pre><pre class="playground"><code class="language-rust edition2021"><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::run(async {
|
||
</span> let fut1 = async {
|
||
for i in 1..10 {
|
||
println!("hi number {i} from the first task!");
|
||
trpl::sleep(Duration::from_millis(500)).await;
|
||
}
|
||
};
|
||
|
||
let fut2 = async {
|
||
for i in 1..5 {
|
||
println!("hi number {i} from the second task!");
|
||
trpl::sleep(Duration::from_millis(500)).await;
|
||
}
|
||
};
|
||
|
||
trpl::join(fut1, fut2).await;
|
||
<span class="boring"> });
|
||
</span><span class="boring">}</span></code></pre></pre>
|
||
<figcaption>示例 17-8：使用 `trpl::join` 来 await 两个匿名 future</figcaption>
|
||
</figure>
|
||
<p>当运行代码我们会看到两个 future 会运行至结束：</p>
|
||
<!-- Not extracting output because changes to this output aren't significant;
|
||
the changes are likely to be due to the threads running differently rather than
|
||
changes in the compiler -->
|
||
<pre><code class="language-text">hi number 1 from the first task!
|
||
hi number 1 from the second task!
|
||
hi number 2 from the first task!
|
||
hi number 2 from the second task!
|
||
hi number 3 from the first task!
|
||
hi number 3 from the second task!
|
||
hi number 4 from the first task!
|
||
hi number 4 from the second task!
|
||
hi number 5 from the first task!
|
||
hi number 6 from the first task!
|
||
hi number 7 from the first task!
|
||
hi number 8 from the first task!
|
||
hi number 9 from the first task!
|
||
</code></pre>
|
||
<p>这里，你每次都会看到完全相同的顺序，这与我们在线程中看到的情况非常不同。这是因为 <code>trpl::join</code> 函数是 <em>公平的</em>（<em>fair</em>），这意味着它以相同的频率检查每一个 future，使它们交替执行，绝不会让一个任务在另一个任务准备好时抢先执行。对于线程来说，操作系统会决定该检查哪个线程和会让它运行多长时间。对于异步 Rust 来说，运行时决定检查哪一个任务。（在实践中，细节会更为复杂，因为异步运行时可能在底层使用操作系统线程来作为其并发管理的一部分，因此要保证公平性可能会增加运行时的工作量，但这仍然是可行的！）运行时无需为任何操作保证公平性，同时运行时也经常提供不同的 API 来让你选择是否需要公平性。</p>
|
||
<p>尝试这些不同的 await future 的变体来观察它们的效果：</p>
|
||
<ul>
|
||
<li>去掉一个或者两个循环外的异步代码块。</li>
|
||
<li>在定义两个异步代码块后立刻 await 它们。</li>
|
||
<li>直将第一个循环封装进异步代码块，并在第二个循环体之后 await 作为结果的 future。</li>
|
||
</ul>
|
||
<p>作为额外的挑战，看看你能否在运行代码 <em>之前</em> 想出每个情况下的输出！</p>
|
||
<h3 id="消息传递"><a class="header" href="#消息传递">消息传递</a></h3>
|
||
<p>在 future 之间共享数据也与线程类似：我们会再次使用消息传递，不过这次使用的是异步版本的类型和函数。我们会采用与之前第十六章中使用的稍微不同的方法，来展示一些基于线程的并发与基于 future 的并发之间的关键差异。在示例 17-9 中，我们会从仅有一个异步代码块开始，<em>不像</em> 之前产生独立线程那样产生一个独立的任务。</p>
|
||
<figure class="listing">
|
||
<p><span class="file-name">文件名：src/main.rs</span></p>
|
||
<pre><pre class="playground"><code class="language-rust edition2021"><span class="boring">extern crate trpl; // required for mdbook test
|
||
</span><span class="boring">
|
||
</span><span class="boring">fn main() {
|
||
</span><span class="boring"> trpl::run(async {
|
||
</span> let (tx, mut rx) = trpl::channel();
|
||
|
||
let val = String::from("hi");
|
||
tx.send(val).unwrap();
|
||
|
||
let received = rx.recv().await.unwrap();
|
||
println!("Got: {received}");
|
||
<span class="boring"> });
|
||
</span><span class="boring">}</span></code></pre></pre>
|
||
<figcaption>示例 17-9：创建一个异步信道（async channel）并赋值其两端为 `tx` 和 `rx`</figcaption>
|
||
</figure>
|
||
<p>这里我们使用了 <code>trpl::channel</code>，一个第十六章用于线程的多生产者、单消费者信道 API 的异步版本。异步版本的 API 与基于线程的版本只有一点微小的区别：它使用一个可变的而不是不可变的 <code>rx</code>，并且它的 <code>recv</code> 方法产生一个需要 await 的 future 而不是直接返回值。现在我们可以发送端向接收端发送消息了。注意我们无需产生一个独立的线程或者任务；只需等待（await） <code>rx.recv</code> 调用。</p>
|
||
<p><code>std::mpsc::channel</code> 中的同步 <code>Receiver::recv</code> 方法阻塞执行直到它接收一个消息。<code>trpl::Receiver::recv</code> 则不会阻塞，因为它是异步的。不同于阻塞，它将控制权交还给运行时，直到接收到一个消息或者信道的发送端关闭。相比之下，我们不用 await <code>send</code>，因为它不会阻塞。也无需阻塞，因为信道的发送端的数量是没有限制的。</p>
|
||
<blockquote>
|
||
<p>注意：因为所有这些异步代码都运行在一个 <code>trpl::run</code> 调用的异步代码块中，其中的所有代码可以避免阻塞。然而，<em>外面</em> 的代码会阻塞到 <code>run</code> 函数返回。这正是 <code>trpl::run</code> 函数的全部意义：它允许你 <em>选择</em> 在何处阻塞一部分异步代码，也就是在何处进行同步和异步代码的转换。这正是在大部分运行时中 <code>run</code> 实际上被命名为 <code>block_on</code> 的原因。</p>
|
||
</blockquote>
|
||
<p>请注意这个示例中的两个地方：首先，消息立刻就会到达！其次，虽然我们使用了 future，但是这里还没有并发。示例中的所有事情都是顺序发生的，就像没涉及到 future 时一样。</p>
|
||
<p>让我们通过发送一系列消息并在之间休眠来解决第一个问题，如示例 17-10 所示：</p>
|
||
<figure class="listing">
|
||
<p><span class="file-name">文件名：src/main.rs</span></p>
|
||
<pre><pre class="playground"><code class="language-rust edition2021"><span class="boring">extern crate trpl; // required for mdbook test
|
||
</span><span class="boring">
|
||
</span><span class="boring">fn main() {
|
||
</span><span class="boring"> trpl::run(async {
|
||
</span> let (tx, mut rx) = trpl::channel();
|
||
|
||
let val = String::from("hi");
|
||
tx.send(val).unwrap();
|
||
|
||
let received = rx.recv().await.unwrap();
|
||
println!("Got: {received}");
|
||
<span class="boring"> });
|
||
</span><span class="boring">}</span></code></pre></pre>
|
||
<figcaption>示例 17-10：通过异步信道发送和接收多个消息并在每个消息之间通过 `await` 休眠</figcaption>
|
||
</figure>
|
||
<p>除了发送消息之外，我们需要接收它们。</p>
|
||
|
||
</main>
|
||
|
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