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<h2 id="深入理解-async-相关的-traits"><a class="header" href="#深入理解-async-相关的-traits">深入理解 async 相关的 traits</a></h2>
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<blockquote>
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<p><a href="https://github.com/rust-lang/book/blob/main/src/ch17-05-traits-for-async.md">ch17-05-traits-for-async.md</a>
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<br>
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commit 56ec353290429e6547109e88afea4de027b0f1a9</p>
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</blockquote>
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<p>贯穿本章，我们通过多种方式使用了 <code>Future</code>、<code>Pin</code>、<code>Unpin</code>、<code>Stream</code> 和 <code>StreamExt</code> trait。但是直到目前为止，我们避免过多地了解它们如何工作或者如何组合在一起的细节，这对你日常的 Rust 开发而言通常是没问题的。不过有时你会遇到需要了解更多细节的场景。在本小节，我们会足够深入以便理解这些场景，并仍会将 <em>真正</em> 有深度的内容留给其它文档。</p>
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<h3 id="future-trait"><a class="header" href="#future-trait"><code>Future</code> trait</a></h3>
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<p>让我们以更深入地了解 <code>Future</code> trait 作为开始。这里是 Rust 中其如何定义的：</p>
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<pre><pre class="playground"><code class="language-rust edition2021"><span class="boring">#![allow(unused)]
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</span><span class="boring">fn main() {
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</span>use std::pin::Pin;
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use std::task::{Context, Poll};
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pub trait Future {
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type Output;
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fn poll(self: Pin<&mut Self>, cx: &mut Context<'_>) -> Poll<Self::Output>;
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}
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<span class="boring">}</span></code></pre></pre>
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<p>trait 定义中包含一些的新类型和我们之前没有见过新语法，所以让我们逐步详细地解析一下这个定义。</p>
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<p>首先， <code>Future</code> 的关联类型 <code>Output</code> 表明 future 最终解析出的类型。这类似于 <code>Iterator</code> trait 的关联类型 <code>Item</code>。其次，<code>Future</code> 还有一个 <code>poll</code> 方法，其有一个特殊的 <code>self</code> 参数的 <code>Pin</code> 引用和一个 <code>Context</code> 类型的可变引用，并返回一个 <code>Poll<Self::Output></code>。稍后我们再细说 <code>Pin</code> 和 <code>Context</code>。现在让我们专注于方法返回的 <code>Poll</code> 类型：</p>
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<pre><pre class="playground"><code class="language-rust edition2021"><span class="boring">#![allow(unused)]
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</span><span class="boring">fn main() {
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</span>enum Poll<T> {
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Ready(T),
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Pending,
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}
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<span class="boring">}</span></code></pre></pre>
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<p><code>Poll</code> 类型类似于一个 <code>Option</code>。它有一个包含值的变体 <code>Ready(T)</code>，和一个没有值的变体 <code>Pending</code>。不过 <code>Poll</code> 所代表的意义与 <code>Option</code> 非常不同！<code>Pending</code> 变体表明 future 仍然还有工作要进行，所有调用者稍后需要再次检查。<code>Ready</code> 变体表明 future 已经完成了其工作并且 <code>T</code> 的值是可用的。</p>
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<blockquote>
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<p>注意：对于大部分功能，调用者不应在 future 返回 <code>Ready</code> 后再次调用 <code>poll</code>。很多 future 在完成后再次轮询会 panic。可以安全地再次轮询的 future 会在文档中显示地说明。这类似于 <code>Iterator::next</code> 的行为。</p>
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</blockquote>
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<p>当你见到使用 <code>await</code> 的代码时，Rust 会在底层将其编译为调用 <code>poll</code> 的代码。如果你回头看下示例 17-4，其在一个单个 URL 解析完成后打印出页面标题，Rust 将其编译为一些类似（虽然不完全是）这样的代码：</p>
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<pre><code class="language-rust ignore">match page_title(url).poll() {
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Ready(page_title) => match page_title {
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Some(title) => println!("The title for {url} was {title}"),
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None => println!("{url} had no title"),
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}
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Pending => {
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// But what goes here?
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}
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}</code></pre>
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<p>如果 future 仍然是 <code>Pending</code> 的话我们应该做什么呢？我们需要某种方式不断重试，直到 future 最终准备好。换句话说，我们需要一个循环：</p>
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<pre><code class="language-rust ignore">let mut page_title_fut = page_title(url);
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loop {
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match page_title_fut.poll() {
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Ready(value) => match page_title {
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Some(title) => println!("The title for {url} was {title}"),
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None => println!("{url} had no title"),
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}
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Pending => {
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// continue
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}
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}
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}</code></pre>
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<p>但是如何 Rust 将其编译为正好如此的代码的话，每一个 <code>await</code> 都会阻塞 -- 这与我们期望的完全不同！相反，Rust 确保循环可以将控制权交给一些可以暂停当前 future 并处理其它 future 并在之后再次检查的内容。如你所见，它就是异步运行时，这种安排和协调的工作是其主要工作之一。</p>
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<p>在本章的开头，我们描述了等待 <code>rx.recv</code>。<code>recv</code> 调用返回一个 future，并等待轮询它的 future。我们注意到当信道关闭时运行时会暂停 future 直到它就绪并返回 <code>Some(message)</code> 或 <code>None</code>。随着我们对 <code>Future</code> trait，尤其是 <code>Future::poll</code> 的理解的深入，我们可以看出其是如何工作的。运行时知道 future 返回 <code>Poll::Pending</code> 时没有完成。反过来说，当 <code>poll</code> 返回 <code>Poll::Ready(Some(message))</code> 或 <code>Poll::Ready(None)</code> 时运行时知道 future <strong>已经</strong>完成了并继续运行。</p>
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<p>运行时如何工作的具体细节超出了本书的范畴。不过关键在于理解 future 的基本机制：运行时<strong>轮询</strong>其所负责的每一个 future，在它们还没有完成时使其休眠。</p>
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<h3 id="pin-和-unpin-traits"><a class="header" href="#pin-和-unpin-traits"><code>Pin</code> 和 <code>Unpin</code> traits</a></h3>
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