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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 h
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<main>
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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>
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<figure class="listing">
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<p><span class="file-name">文件名：src/main.rs</span></p>
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<pre><pre class="playground"><code class="language-rust edition2021"><span class="boring">extern crate trpl; // required for mdbook test
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</span><span class="boring">
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</span><span class="boring">use std::time::Duration;
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</span><span class="boring">
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</span><span class="boring">fn main() {
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</span><span class="boring"> trpl::run(async {
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</span><span class="boring"> let (tx, mut rx) = trpl::channel();
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</span><span class="boring">
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</span><span class="boring"> let tx1 = tx.clone();
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</span><span class="boring"> let tx1_fut = async move {
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</span><span class="boring"> let vals = vec![
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</span><span class="boring"> String::from("hi"),
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</span><span class="boring"> String::from("from"),
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</span><span class="boring"> String::from("the"),
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</span><span class="boring"> String::from("future"),
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</span><span class="boring"> ];
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</span><span class="boring">
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</span><span class="boring"> for val in vals {
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</span><span class="boring"> tx1.send(val).unwrap();
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</span><span class="boring"> trpl::sleep(Duration::from_secs(1)).await;
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</span><span class="boring"> }
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</span><span class="boring"> };
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</span><span class="boring">
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</span><span class="boring"> let rx_fut = async {
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</span><span class="boring"> while let Some(value) = rx.recv().await {
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</span><span class="boring"> println!("received '{value}'");
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</span><span class="boring"> }
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</span><span class="boring"> };
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</span><span class="boring">
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</span><span class="boring"> let tx_fut = async move {
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</span><span class="boring"> let vals = vec![
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</span><span class="boring"> String::from("more"),
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</span><span class="boring"> String::from("messages"),
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</span><span class="boring"> String::from("for"),
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</span><span class="boring"> String::from("you"),
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</span><span class="boring"> ];
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</span><span class="boring">
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</span><span class="boring"> for val in vals {
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</span><span class="boring"> tx.send(val).unwrap();
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</span><span class="boring"> trpl::sleep(Duration::from_secs(1)).await;
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</span><span class="boring"> }
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</span><span class="boring"> };
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</span><span class="boring">
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</span> trpl::join!(tx1_fut, tx_fut, rx_fut);
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<span class="boring"> });
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</span><span class="boring">}</span></code></pre></pre>
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<figcaption>示例 17-14：使用 `join!` 来等待多个 future</figcaption>
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</figure>
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<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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