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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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<h1 class="menu-title">Rust 程序设计语言 简体中文版</h1>
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<main>
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<h2 id="流streams"><a class="header" href="#流streams">流（Streams）</a></h2>
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<blockquote>
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<p><a href="https://github.com/rust-lang/book/blob/main/src/ch17-04-streams.md">ch17-04-streams.md</a>
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<br>
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commit f04d20fe8d1a49c3bffa10a3086c58e527ff0a90</p>
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</blockquote>
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<p>到本章的目前为止，我们大部分时间停留在独立的 future 上。一个重要的例外就是我们用过的异步信道。回忆一下在本章之前的 <a href="ch17-02-concurrency-with-async.html#%E6%B6%88%E6%81%AF%E4%BC%A0%E9%80%92">“消息传递”</a> 中我们如何使用异步信道接收端的。异步 <code>recv</code> 方法随着时间的推移产生一个序列的项。这是一个通用的多的模式的实例，通常被称为 <em>流</em>（<em>stream</em>）。</p>
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<p>一个序列的项是我们之前是见过的，回忆一下第十三章的 <code>Iterator</code> trait，不过迭代器和异步信道接收端有两个区别。第一个区别是时间的维度：迭代器是同步的，而信道接收端是异步的。第二个区别是 API。当直接处理 <code>Iterator</code> 时，我们会调用其同步 <code>next</code> 方法。对于这个特定的 <code>trpl::Receiver</code> 流，我们调用一个异步的 <code>recv</code> 方法。不过这两个 API 看起来非常相似。</p>
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<p>这种相似性并非巧合。流类似于一种异步形式的迭代器。不过鉴于 <code>trpl::Receiver</code> 专门等待接收消息，多用途的流 API 则更为通用：它像 <code>Iterator</code> 一样提供了下一个项，不过是异步版本的。Rust 中迭代器和流的相似性意味着我们实际上可以从任何迭代器上创建流。就迭代器而言，可以通过调用其 <code>next</code> 方法并 await 输出来使用流，如示例 17-30 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><code class="language-rust ignore does_not_compile"><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">fn main() {
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|
</span><span class="boring"> trpl::run(async {
|
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</span> let values = [1, 2, 3, 4, 5, 6, 7, 8, 9, 10];
|
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let iter = values.iter().map(|n| n * 2);
|
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let mut stream = trpl::stream_from_iter(iter);
|
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|
while let Some(value) = stream.next().await {
|
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|
|
println!("The value was: {value}");
|
|
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|
|
}
|
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<span class="boring"> });
|
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</span><span class="boring">}</span></code></pre>
|
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<figcaption>示例 17-30：从迭代器创建流并打印其值</figcaption>
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</figure>
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<p>我们以一组数字作为开始，将其转换为一个迭代器并接着调用 <code>map</code> 将其所有值翻倍。然后使用 <code>trpl::stream_from_iter</code> 函数将迭代器转换为流。再然后在 <code>while let</code> 循环中到达时循环处理流中的项。</p>
|
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<p>不幸的是当我们尝试运行代码时，代码无法编译。相反如果我们观察其输出，它会报告没有可用的 <code>next</code> 方法。</p>
|
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|
<!-- manual-regeneration
|
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|
|
cd listings/ch17-async-await/listing-17-30
|
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|
|
cargo build
|
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|
copy only the error output
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|
-->
|
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|
|
<pre><code class="language-console">error[E0599]: no method named `next` found for struct `Iter` in the current scope
|
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|
|
--> src/main.rs:10:40
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10 | while let Some(value) = stream.next().await {
|
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|
|
| ^^^^
|
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|
= note: the full type name has been written to 'file:///projects/async_await/target/debug/deps/async_await-9de943556a6001b8.long-type-1281356139287206597.txt'
|
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|
|
= note: consider using `--verbose` to print the full type name to the console
|
|
|
|
|
|
= help: items from traits can only be used if the trait is in scope
|
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|
|
help: the following traits which provide `next` are implemented but not in scope; perhaps you want to import one of them
|
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|
|
1 + use crate::trpl::StreamExt;
|
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|
|
1 + use futures_util::stream::stream::StreamExt;
|
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|
|
1 + use std::iter::Iterator;
|
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|
|
1 + use std::str::pattern::Searcher;
|
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|
|
help: there is a method `try_next` with a similar name
|
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|
|
10 | while let Some(value) = stream.try_next().await {
|
|
|
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|
|
| ~~~~~~~~
|
|
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|
|
</code></pre>
|
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|
|
<p>正如输出中所建议的，编译器错误的原因是我们需要在作用域中有正确的 trait 以便能够使用 <code>next</code> 方法。鉴于目前为止的讨论，你可能会合理地推测是 <code>Stream</code>，不过这里需要的 trait 实际上是 <code>StreamExt</code>。这里的 <code>Ext</code> 是 “extension”：在 Rust 社区中这是用另一个 trait 扩展 trait 的常见模式。</p>
|
|
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|
|
<p>为什么我们需要 <code>StreamExt</code> 而不是 <code>Stream</code>，而 <code>Stream</code> trait 本身又是做什么的呢？简单来说，答案是贯穿整个 Rust 生态系统，<code>Stream</code> trait 定义了一个底层接口用于有效地组合 <code>Iterator</code> 和 <code>Future</code> trait。<code>StreamExt</code> trait 在 <code>Stream</code> 之上提供了一组高层 API，这包括 <code>next</code> 和其它类似于 <code>Iterator</code> trait 提供的工具方法。在本章的最后我们会回到 <code>Stream</code> 和 <code>StreamExt</code> 并介绍更多细节。现在这已经足够我们继续了。</p>
|
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|
|
<p>对编译器错误的修复是增加一个 <code>trpl::StreamExt</code> 的 <code>use</code> 语句，如示例 17-31 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 trpl::StreamExt;
|
|
|
|
|
|
|
|
|
|
|
|
fn main() {
|
|
|
|
|
|
trpl::run(async {
|
|
|
|
|
|
let values = [1, 2, 3, 4, 5, 6, 7, 8, 9, 10];
|
|
|
|
|
|
let iter = values.iter().map(|n| n * 2);
|
|
|
|
|
|
let mut stream = trpl::stream_from_iter(iter);
|
|
|
|
|
|
|
|
|
|
|
|
while let Some(value) = stream.next().await {
|
|
|
|
|
|
println!("The value was: {value}");
|
|
|
|
|
|
}
|
|
|
|
|
|
});
|
|
|
|
|
|
}</code></pre></pre>
|
|
|
|
|
|
<figcaption>示例 17-31：成功使用迭代器作为流的基础</figcaption>
|
|
|
|
|
|
</figure>
|
|
|
|
|
|
<p>将所有这些代码片段拼凑在一起，这段代码如我们预期般运行！更重要的是，现在我们在作用域中拥有 <code>StreamExt</code>，就可以使用所有其工具方法。例如在示例 17-32 中，我们使用 <code>filter</code> 方法来过滤掉所有不是 3 或者 5 的倍数的项。</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 trpl::StreamExt;
|
|
|
|
|
|
|
|
|
|
|
|
fn main() {
|
|
|
|
|
|
trpl::run(async {
|
|
|
|
|
|
let values = 1..101;
|
|
|
|
|
|
let iter = values.map(|n| n * 2);
|
|
|
|
|
|
let stream = trpl::stream_from_iter(iter);
|
|
|
|
|
|
|
|
|
|
|
|
let mut filtered =
|
|
|
|
|
|
stream.filter(|value| value % 3 == 0 || value % 5 == 0);
|
|
|
|
|
|
|
|
|
|
|
|
while let Some(value) = filtered.next().await {
|
|
|
|
|
|
println!("The value was: {value}");
|
|
|
|
|
|
}
|
|
|
|
|
|
});
|
|
|
|
|
|
}</code></pre></pre>
|
|
|
|
|
|
<figcaption>示例 17-32：使用 `StreamExt::filter` 方法来过滤 `Stream`</figcaption>
|
|
|
|
|
|
</figure>
|
|
|
|
|
|
<p>当然这并不是非常的有趣。我们完全可以使用普通的迭代器而不用任何异步操作来做到这些。所以让我们看看一些其它的只能用流的内容。</p>
|
|
|
|
|
|
<h3 id="组合流"><a class="header" href="#组合流">组合流</a></h3>
|
|
|
|
|
|
<p>很多概念天然地可以用流来代表：队列中可用的项；或者处理超过计算机内存的数据，一次只能从文件系统拉取一个块；或者通过网络随着时间推移到达的数据。因为流是 future，我们也可以将其用于任何其它类型的 future，并且我们可以用一种非常有趣的方式来组合它们。例如，我们可以批量处理事件来避免触发过多的网络调用，为一系列的长时间运行的任务设置超时，或者对用户接口事件限速来避免进行无尽的工作。</p>
|
|
|
|
|
|
<p>让我们构建一个小的消息流作为开始，将其作为一个可能从 WebSocket 或者其它现实世界中的通信协议中遇到的数据流的替代。在示例 17-33 中，我们创建了一个返回 <code>impl Stream<Item = String></code> 的 <code>get_messages</code> 函数。作为其实现，我们创建了一个异步信道，循环英文字母表的前是个字符，并通过信道发送它们。</p>
|
|
|
|
|
|
<p>我们还使用了一个新类型：<code>ReceiverStream</code>，它将 <code>trpl::channel</code> 的 <code>rx</code> 接收端转换为一个带有带有 <code>next</code> 方法的 <code>Stream</code>。回到 <code>main</code>，我们使用了一个 <code>while let</code> 循环来打印来自流中的所有消息。</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 trpl::{ReceiverStream, Stream, StreamExt};
|
|
|
|
|
|
|
|
|
|
|
|
fn main() {
|
|
|
|
|
|
trpl::run(async {
|
|
|
|
|
|
let mut messages = get_messages();
|
|
|
|
|
|
|
|
|
|
|
|
while let Some(message) = messages.next().await {
|
|
|
|
|
|
println!("{message}");
|
|
|
|
|
|
}
|
|
|
|
|
|
});
|
|
|
|
|
|
}
|
|
|
|
|
|
|
|
|
|
|
|
fn get_messages() -> impl Stream<Item = String> {
|
|
|
|
|
|
let (tx, rx) = trpl::channel();
|
|
|
|
|
|
|
|
|
|
|
|
let messages = ["a", "b", "c", "d", "e", "f", "g", "h", "i", "j"];
|
|
|
|
|
|
for message in messages {
|
|
|
|
|
|
tx.send(format!("Message: '{message}'")).unwrap();
|
|
|
|
|
|
}
|
|
|
|
|
|
|
|
|
|
|
|
ReceiverStream::new(rx)
|
|
|
|
|
|
}</code></pre></pre>
|
|
|
|
|
|
<figcaption>示例 17-33：使用 `rx` 接收端作为一个 `ReceiverStream`</figcaption>
|
|
|
|
|
|
</figure>
|
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