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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"><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" class="active"><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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<h2 id="使用任意数量的-futures"><a class="header" href="#使用任意数量的-futures">使用任意数量的 futures</a></h2>
|
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<blockquote>
|
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<p><a href="https://github.com/rust-lang/book/blob/main/src/ch17-03-more-futures.md">ch17-03-more-futures.md</a>
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<br>
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commit 9e85fcc9938e8f8c935d0ad8b4db7f45caaa2ca4</p>
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</blockquote>
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<p>当我们在上一部分从使用两个 future 到三个 future 的时候，我们也必须从使用 <code>join</code> 切换到 <code>join3</code>。每次我们想要改变 join 的 future 数量时都不得不调用一个不同的函数是很烦人的。令人高兴的是，我们有一个宏版本的 <code>join</code> 可以传递任意数量的参数。它还会自行处理 await 这些 future。因此，我们可以重写示例 17-13 中的代码来使用 <code>join!</code> 而不是 <code>join3</code>，如示例 17-14 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">use std::time::Duration;
|
||
</span><span class="boring">
|
||
</span><span class="boring">fn main() {
|
||
</span><span class="boring"> trpl::run(async {
|
||
</span><span class="boring"> let (tx, mut rx) = trpl::channel();
|
||
</span><span class="boring">
|
||
</span><span class="boring"> let tx1 = tx.clone();
|
||
</span><span class="boring"> let tx1_fut = async move {
|
||
</span><span class="boring"> let vals = vec![
|
||
</span><span class="boring"> String::from("hi"),
|
||
</span><span class="boring"> String::from("from"),
|
||
</span><span class="boring"> String::from("the"),
|
||
</span><span class="boring"> String::from("future"),
|
||
</span><span class="boring"> ];
|
||
</span><span class="boring">
|
||
</span><span class="boring"> for val in vals {
|
||
</span><span class="boring"> tx1.send(val).unwrap();
|
||
</span><span class="boring"> trpl::sleep(Duration::from_secs(1)).await;
|
||
</span><span class="boring"> }
|
||
</span><span class="boring"> };
|
||
</span><span class="boring">
|
||
</span><span class="boring"> let rx_fut = async {
|
||
</span><span class="boring"> while let Some(value) = rx.recv().await {
|
||
</span><span class="boring"> println!("received '{value}'");
|
||
</span><span class="boring"> }
|
||
</span><span class="boring"> };
|
||
</span><span class="boring">
|
||
</span><span class="boring"> let tx_fut = async move {
|
||
</span><span class="boring"> let vals = vec![
|
||
</span><span class="boring"> String::from("more"),
|
||
</span><span class="boring"> String::from("messages"),
|
||
</span><span class="boring"> String::from("for"),
|
||
</span><span class="boring"> String::from("you"),
|
||
</span><span class="boring"> ];
|
||
</span><span class="boring">
|
||
</span><span class="boring"> for val in vals {
|
||
</span><span class="boring"> tx.send(val).unwrap();
|
||
</span><span class="boring"> trpl::sleep(Duration::from_secs(1)).await;
|
||
</span><span class="boring"> }
|
||
</span><span class="boring"> };
|
||
</span><span class="boring">
|
||
</span> trpl::join!(tx1_fut, tx_fut, rx_fut);
|
||
<span class="boring"> });
|
||
</span><span class="boring">}</span></code></pre></pre>
|
||
<figcaption>示例 17-14：使用 `join!` 来等待多个 future</figcaption>
|
||
</figure>
|
||
<p>相比需要在 <code>join</code> 和 <code>join3</code> 和 <code>join4</code> 等等直接切换来说这绝对是一个进步！然而，即便是这个宏形式也只能用于我们提前知道 future 的数量的时候。不过，在现实世界的 Rust 中，将 futures 放进一个集合并接着等待集合中的一些或者全部 future 完成是一个常见的模式。</p>
|
||
<p>为了检查一些集合中的所有 future，我们需要遍历并 join 它们 <em>全部</em>。<code>trpl::join_all</code> 函数接受任何实现了 <code>Iterator</code> trait 的类型，我们在之前的第十三章中学习过它们，所以这正是我们需要的。让我们将 futures 放进一个向量，并将 <code>join!</code> 替换为 <code>join_all</code>。</p>
|
||
<figure class="listing">
|
||
<p><span class="file-name">文件名：src/main.rs</span></p>
|
||
<pre><code class="language-rust ignore does_not_compile"><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><span class="boring"> let (tx, mut rx) = trpl::channel();
|
||
</span><span class="boring">
|
||
</span><span class="boring"> let tx1 = tx.clone();
|
||
</span><span class="boring"> let tx1_fut = async move {
|
||
</span><span class="boring"> let vals = vec![
|
||
</span><span class="boring"> String::from("hi"),
|
||
</span><span class="boring"> String::from("from"),
|
||
</span><span class="boring"> String::from("the"),
|
||
</span><span class="boring"> String::from("future"),
|
||
</span><span class="boring"> ];
|
||
</span><span class="boring">
|
||
</span><span class="boring"> for val in vals {
|
||
</span><span class="boring"> tx1.send(val).unwrap();
|
||
</span><span class="boring"> trpl::sleep(Duration::from_secs(1)).await;
|
||
</span><span class="boring"> }
|
||
</span><span class="boring"> };
|
||
</span><span class="boring">
|
||
</span><span class="boring"> let rx_fut = async {
|
||
</span><span class="boring"> while let Some(value) = rx.recv().await {
|
||
</span><span class="boring"> println!("received '{value}'");
|
||
</span><span class="boring"> }
|
||
</span><span class="boring"> };
|
||
</span><span class="boring">
|
||
</span><span class="boring"> let tx_fut = async move {
|
||
</span><span class="boring"> let vals = vec![
|
||
</span><span class="boring"> String::from("more"),
|
||
</span><span class="boring"> String::from("messages"),
|
||
</span><span class="boring"> String::from("for"),
|
||
</span><span class="boring"> String::from("you"),
|
||
</span><span class="boring"> ];
|
||
</span><span class="boring">
|
||
</span><span class="boring"> for val in vals {
|
||
</span><span class="boring"> tx.send(val).unwrap();
|
||
</span><span class="boring"> trpl::sleep(Duration::from_secs(1)).await;
|
||
</span><span class="boring"> }
|
||
</span><span class="boring"> };
|
||
</span><span class="boring">
|
||
</span> let futures = vec![tx1_fut, rx_fut, tx_fut];
|
||
|
||
trpl::join_all(futures).await;
|
||
<span class="boring"> });
|
||
</span><span class="boring">}</span></code></pre>
|
||
<figcaption>示例 17-15：将匿名 futures 储存在一个向量中并调用 `join_all`</figcaption>
|
||
</figure>
|
||
<p>不幸的是这还不能编译。相反我们会得到这个错误：</p>
|
||
<!-- manual-regeneration
|
||
cd listings/ch17-async-await/listing-17-16/
|
||
cargo build
|
||
copy just the compiler error
|
||
-->
|
||
<pre><code class="language-text">error[E0308]: mismatched types
|
||
--> src/main.rs:43:37
|
||
|
|
||
8 | let tx1_fut = async move {
|
||
| _______________________-
|
||
9 | | let vals = vec![
|
||
10 | | String::from("hi"),
|
||
11 | | String::from("from"),
|
||
... |
|
||
19 | | }
|
||
20 | | };
|
||
| |_________- the expected `async` block
|
||
21 |
|
||
22 | let rx_fut = async {
|
||
| ______________________-
|
||
23 | | while let Some(value) = rx.recv().await {
|
||
24 | | println!("received '{value}'");
|
||
25 | | }
|
||
26 | | };
|
||
| |_________- the found `async` block
|
||
...
|
||
43 | let futures = vec![tx1_fut, rx_fut, tx_fut];
|
||
| ^^^^^^ expected `async` block, found a different `async` block
|
||
|
|
||
= note: expected `async` block `{async block@src/main.rs:8:23: 20:10}`
|
||
found `async` block `{async block@src/main.rs:22:22: 26:10}`
|
||
= note: no two async blocks, even if identical, have the same type
|
||
= help: consider pinning your async block and and casting it to a trait object
|
||
</code></pre>
|
||
<p>这可能有点令人惊讶。毕竟没有一个 future 返回了任何值，所以每个代码块都会产生一个 <code>Future<Output = ()></code>。然而，<code>Future</code> 是一个 trait，不是一个具体类型。其具体类型是编译器为各个异步代码块生成的（不同的）数据结构。你不能将两个不同的手写的 struct 放进同一个 <code>Vec</code>，同样的原理也适用于编译器生成的不同 struct。</p>
|
||
<p>为了使代码能够正常工作，我们需要使用 <em>trait objects</em>，正如我们在第十二章的 <a href="ch12-03-improving-error-handling-and-modularity.html">“从 <code>run</code> 函数中返回错误”</a> 中做的那样。（第十八章会详细介绍 trait objects。）使用 trait objects 允许我们将这些类型所产生的不同的匿名 future 看作相同的类型，因为它们都实现了 <code>Future</code> trait。</p>
|
||
<blockquote>
|
||
<p>注意：在第八章中，我们讨论过另一种将多种类型包含进一个 <code>Vec</code> 的方式：使用一个枚举来代表每个可以出现在向量中的不同类型。不过这里我们不能这么做。首先，没有方法来命名这些不同的类型，因为它们是匿名的。</p>
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