tink
/
dive-into-go-v2
1
0
Fork 0
Code Issues Pull Requests Packages Projects Releases Wiki Activity
You cannot select more than 25 topics Topics must start with a letter or number, can include dashes ('-') and can be up to 35 characters long.
main
Branches Tags
${ item.name }
Create tag ${ searchTerm }
Create branch ${ searchTerm }
from '5c66d5bef3'
${ noResults }
dive-into-go-v2/public/feature/select/index.html

421 lines
20 KiB
HTML
Raw Blame History Unescape Escape

This file contains ambiguous Unicode characters!

This file contains ambiguous Unicode characters that may be confused with others in your current locale. If your use case is intentional and legitimate, you can safely ignore this warning. Use the Escape button to highlight these characters.

<!DOCTYPE html>
<html lang="zh-cn" dir="ltr">
<head>
<meta charset="UTF-8">
<meta name="viewport" content="width=device-width, initial-scale=1.0">
<meta name="description" content="通道选择器-select # Go 语言中select关键字结构跟switch结构类似，但是select结构的case语句都是跟通道操作相关的。Go 语言会从select结构中已经可读取或可以写入通道对应的case语句中随机选择一个执行，如果所有case语句中的通道都不能可读取或可写入且存在default语句的话，那么会执行default语句。
根据Go 官方语法指南指出select语句执行分为以下几个步骤：
For all the cases in the statement, the channel operands of receive operations and the channel and right-hand-side expressions of send statements are evaluated exactly once, in source order, upon entering the &ldquo;select&rdquo; statement. The result is a set of channels to receive from or send to, and the corresponding values to send. Any side effects in that evaluation will occur irrespective of which (if any) communication operation is selected to proceed.">
<meta name="theme-color" media="(prefers-color-scheme: light)" content="#ffffff">
<meta name="theme-color" media="(prefers-color-scheme: dark)" content="#343a40">
<meta name="color-scheme" content="light dark"><meta property="og:title" content="通道选择器-select" />
<meta property="og:description" content="通道选择器-select # Go 语言中select关键字结构跟switch结构类似，但是select结构的case语句都是跟通道操作相关的。Go 语言会从select结构中已经可读取或可以写入通道对应的case语句中随机选择一个执行，如果所有case语句中的通道都不能可读取或可写入且存在default语句的话，那么会执行default语句。
根据Go 官方语法指南指出select语句执行分为以下几个步骤：
For all the cases in the statement, the channel operands of receive operations and the channel and right-hand-side expressions of send statements are evaluated exactly once, in source order, upon entering the &ldquo;select&rdquo; statement. The result is a set of channels to receive from or send to, and the corresponding values to send. Any side effects in that evaluation will occur irrespective of which (if any) communication operation is selected to proceed." />
<meta property="og:type" content="article" />
<meta property="og:url" content="https://go.cyub.vip/feature/select/" /><meta property="article:section" content="feature" />
<title>通道选择器-select | 深入Go语言之旅</title>
<link rel="manifest" href="/manifest.json">
<link rel="icon" href="/favicon.png" >
<link rel="stylesheet" href="/book.min.f06572240ce28e67eb332ac5cf5d59a696c47ad4c6f700d5842c5ed93dd8ec77.css" integrity="sha256-8GVyJAzijmfrMyrFz11ZppbEetTG9wDVhCxe2T3Y7Hc=" crossorigin="anonymous">
<script defer src="/flexsearch.min.js"></script>
<script defer src="/en.search.min.93e09a692e83129c8b0cc95c521eab7d4f5256b35651cf852c4ad4185bdc55a5.js" integrity="sha256-k&#43;CaaS6DEpyLDMlcUh6rfU9SVrNWUc&#43;FLErUGFvcVaU=" crossorigin="anonymous"></script>
<script async src="https://www.googletagmanager.com/gtag/js?id=G-BQ229RRTTX"></script>
<script>
var doNotTrack = false;
if (!doNotTrack) {
window.dataLayer = window.dataLayer || [];
function gtag(){dataLayer.push(arguments);}
gtag('js', new Date());
gtag('config', 'G-BQ229RRTTX', { 'anonymize_ip': false });
}
</script>
<!--
Made with Book Theme
https://github.com/alex-shpak/hugo-book
-->
</head>
<body dir="ltr">
<input type="checkbox" class="hidden toggle" id="menu-control" />
<input type="checkbox" class="hidden toggle" id="toc-control" />
<main class="container flex">
<aside class="book-menu">
<div class="book-menu-content">
<nav>
<h2 class="book-brand">
<a class="flex align-center" href="/"><img src="https://static.cyub.vip/images/202310/golang-480.png" alt="Logo" /><span>深入Go语言之旅</span>
</a>
</h2>
<div class="book-search">
<input type="text" id="book-search-input" placeholder="Search" aria-label="Search" maxlength="64" data-hotkeys="s/" />
<div class="book-search-spinner hidden"></div>
<ul id="book-search-results"></ul>
</div>
<ul>
<li>
<a href="https://www.cyub.vip/" target="_blank" rel="noopener">
个人博客
</a>
</li>
<li>
<a href="https://github.com/cyub" target="_blank" rel="noopener">
Github主页
</a>
</li>
<li>
<a href="https://www.topgoer.cn/?ref=go.cyub.vip" target="_blank" rel="noopener">
地鼠文档
</a>
</li>
</ul>
<ul>
<li>
<p><strong>
<a href="/">深入Go语言之旅</a></strong></p>
</li>
<li>
<p><strong>准备篇</strong></p>
<ul>
<li>
<a href="/compiler/">编译流程</a></li>
<li>
<a href="/analysis-tools/">分析工具</a>
<ul>
<li>
<a href="/analysis-tools/gdb/">GDB</a></li>
<li>
<a href="/analysis-tools/dlv/">Delve</a></li>
<li>
<a href="/analysis-tools/go-buildin-tools/">Go 内置工具</a></li>
</ul>
</li>
<li>
<a href="/go-assembly/">Go汇编</a></li>
</ul>
</li>
<li>
<p><strong>基础篇</strong></p>
<ul>
<li>
<a href="/type/">数据类型与数据结构</a>
<ul>
<li>
<a href="/type/string/">字符串</a></li>
<li>
<a href="/type/array/">数组</a></li>
<li>
<a href="/type/slice/">切片</a></li>
<li>
<a href="/type/nil/">nil</a></li>
<li>
<a href="/type/empty_struct/">空结构体</a></li>
<li>
<a href="/type/pointer/">指针</a></li>
<li>
<a href="/type/map/">映射</a></li>
</ul>
</li>
<li>
<a href="/function/">函数</a>
<ul>
<li>
<a href="/function/first-class/">一等公民</a></li>
<li>
<a href="/function/call-stack/">函数调用栈</a></li>
<li>
<a href="/function/pass-by-value/">值传递</a></li>
<li>
<a href="/function/closure/">闭包</a></li>
<li>
<a href="/function/method/">方法</a></li>
</ul>
</li>
<li>
<a href="/feature/">语言特性</a>
<ul>
<li>
<a href="/feature/comma-ok/">逗号ok模式</a></li>
<li>
<a href="/feature/for-range/">遍历 - for-range语法</a></li>
<li>
<a href="/feature/defer/">延迟执行 - defer语法</a></li>
<li>
<a href="/feature/select/"class=active>通道选择器 - select语法</a></li>
<li>
<a href="/feature/panic-recover/">恐慌与恢复 - panic/recover</a></li>
</ul>
</li>
</ul>
</li>
<li>
<p><strong>运行时篇</strong></p>
<ul>
<li>
<a href="/concurrency/">并发编程</a>
<ul>
<li>
<a href="/concurrency/memory-model/">内存模型</a></li>
<li>
<a href="/concurrency/context/">上下文 - context</a></li>
<li>
<a href="/concurrency/channel/">通道 - channel</a></li>
<li>
<a href="/concurrency/atomic/">原子操作 - atomic</a></li>
<li>
<a href="/concurrency/sync-map/">并发Map - sync.Map</a></li>
<li>
<a href="/concurrency/sync-waitgroup/">等待组 - sync.WaitGroup</a></li>
<li>
<a href="/concurrency/sync-once/">一次性操作 - sync.Once</a></li>
<li>
<a href="/concurrency/sync-pool/">缓冲池 - sync.Pool</a></li>
<li>
<a href="/concurrency/sync-cond/">条件变量 - sync.Cond</a></li>
<li>
<a href="/concurrency/sync-mutex/">互斥锁 - sync.Mutex</a></li>
<li>
<a href="/concurrency/sync-rwmutex/">读写锁 - sync.RWMutex</a></li>
</ul>
</li>
<li>
<a href="/gmp/">G-M-P调度机制</a>
<ul>
<li>
<a href="/gmp/gmp-model/">调度机制概述</a></li>
<li>
<a href="/gmp/scheduler/">调度器</a></li>
</ul>
</li>
<li>
<a href="/memory/">内存管理</a>
<ul>
<li>
<a href="/memory/allocator/">内存分配器</a></li>
<li>
<a href="/memory/gc/">GC</a></li>
</ul>
</li>
<li>
<a href="/type-system/">类型系统</a>
<ul>
<li>
<a href="/type-system/type/">类型系统</a></li>
<li>
<a href="/type-system/interface/">接口</a></li>
<li>
<a href="/type-system/reflect/">反射</a></li>
</ul>
</li>
</ul>
</li>
</ul>
</nav>
<script>(function(){var e=document.querySelector("aside .book-menu-content");addEventListener("beforeunload",function(){localStorage.setItem("menu.scrollTop",e.scrollTop)}),e.scrollTop=localStorage.getItem("menu.scrollTop")})()</script>
</div>
</aside>
<div class="book-page">
<header class="book-header">
<div class="flex align-center justify-between">
<label for="menu-control">
<img src="/svg/menu.svg" class="book-icon" alt="Menu" />
</label>
<strong>通道选择器-select</strong>
<label for="toc-control">
</label>
</div>
</header>
<article class="markdown"><h1 id="通道选择器-select">
通道选择器-select
<a class="anchor" href="#%e9%80%9a%e9%81%93%e9%80%89%e6%8b%a9%e5%99%a8-select">#</a>
</h1>
<p>Go 语言中<code>select</code>关键字结构跟<code>switch</code>结构类似，但是<code>select</code>结构的case语句都是跟通道操作相关的。Go 语言会从<code>select</code>结构中已经可读取或可以写入通道对应的case语句中随机选择一个执行，如果所有case语句中的通道都不能可读取或可写入且存在default语句的话，那么会执行default语句。</p>
<p>根据Go 官方语法指南指出select语句执行分为以下几个步骤：</p>
<ol>
<li>
<p>For all the cases in the statement, the channel operands of receive operations and the channel and right-hand-side expressions of send statements are evaluated exactly once, in source order, upon entering the &ldquo;select&rdquo; statement. The result is a set of channels to receive from or send to, and the corresponding values to send. Any side effects in that evaluation will occur irrespective of which (if any) communication operation is selected to proceed. Expressions on the left-hand side of a RecvStmt with a short variable declaration or assignment are not yet evaluated.</p>
<p>对于case分支语句中写入通道的右侧表达式都会先执行，执行顺序是按照代码中case分支顺序，由上到下执行。case分支语句中读取通道的左右表达式不会先执行的。</p>
</li>
<li>
<p>If one or more of the communications can proceed, a single one that can proceed is chosen via a uniform pseudo-random selection. Otherwise, if there is a default case, that case is chosen. If there is no default case, the &ldquo;select&rdquo; statement blocks until at least one of the communications can proceed.</p>
<p>如果有一个或者多个case分支的通道可以通信（读取或写入），那么会随机选择一个case分支执行。否则如果存在default分支，那么执行default分支，若没有default分支，那么select语句会阻塞，直到某一个case分支的通道可以通信。</p>
</li>
<li>
<p>Unless the selected case is the default case, the respective communication operation is executed.</p>
<p>除非选择的case分支是default分支，否则将执行相应case分支的通道读写操作。</p>
</li>
<li>
<p>If the selected case is a RecvStmt with a short variable declaration or an assignment, the left-hand side expressions are evaluated and the received value (or values) are assigned.</p>
</li>
<li>
<p>The statement list of the selected case is executed.</p>
<p>执行所选case中的语句。</p>
</li>
</ol>
<p>上面介绍的执行顺序第一步骤，我们可以从下面代码输出结果可以看出来：</p>
<div class="highlight"><pre tabindex="0" style="color:#f8f8f2;background-color:#272822;-moz-tab-size:4;-o-tab-size:4;tab-size:4;"><code class="language-go" data-lang="go"><span style="display:flex;"><span><span style="color:#66d9ef">func</span> <span style="color:#a6e22e">main</span>() {
</span></span><span style="display:flex;"><span> <span style="color:#a6e22e">ch</span> <span style="color:#f92672">:=</span> make(<span style="color:#66d9ef">chan</span> <span style="color:#66d9ef">int</span>, <span style="color:#ae81ff">1</span>)
</span></span><span style="display:flex;"><span> <span style="color:#66d9ef">select</span> {
</span></span><span style="display:flex;"><span> <span style="color:#66d9ef">case</span> <span style="color:#a6e22e">ch</span> <span style="color:#f92672">&lt;-</span> <span style="color:#a6e22e">getVal</span>(<span style="color:#ae81ff">1</span>):
</span></span><span style="display:flex;"><span> println(<span style="color:#e6db74">&#34;recv: &#34;</span>, <span style="color:#f92672">&lt;-</span><span style="color:#a6e22e">ch</span>)
</span></span><span style="display:flex;"><span> <span style="color:#66d9ef">case</span> <span style="color:#a6e22e">ch</span> <span style="color:#f92672">&lt;-</span> <span style="color:#a6e22e">getVal</span>(<span style="color:#ae81ff">2</span>):
</span></span><span style="display:flex;"><span> println(<span style="color:#e6db74">&#34;recv: &#34;</span>, <span style="color:#f92672">&lt;-</span><span style="color:#a6e22e">ch</span>)
</span></span><span style="display:flex;"><span> }
</span></span><span style="display:flex;"><span>}
</span></span><span style="display:flex;"><span>
</span></span><span style="display:flex;"><span><span style="color:#66d9ef">func</span> <span style="color:#a6e22e">getVal</span>(<span style="color:#a6e22e">n</span> <span style="color:#66d9ef">int</span>) <span style="color:#66d9ef">int</span> {
</span></span><span style="display:flex;"><span> println(<span style="color:#e6db74">&#34;getVal: &#34;</span>, <span style="color:#a6e22e">n</span>)
</span></span><span style="display:flex;"><span> <span style="color:#66d9ef">return</span> <span style="color:#a6e22e">n</span>
</span></span><span style="display:flex;"><span>}
</span></span></code></pre></div><p>上面代码输出结果可能如下：</p>
<pre tabindex="0"><code>getVal: 1
getVal: 2
recv: 2
</code></pre><p>可以看到通道写入的右侧表达式<code>getVal(1)</code>和<code>getVal(2)</code>都会立马执行，执行顺序跟case语句顺序一样。</p>
<p>接下来我们来看看第二步骤：</p>
<div class="highlight"><pre tabindex="0" style="color:#f8f8f2;background-color:#272822;-moz-tab-size:4;-o-tab-size:4;tab-size:4;"><code class="language-go" data-lang="go"><span style="display:flex;"><span><span style="color:#66d9ef">func</span> <span style="color:#a6e22e">main</span>() {
</span></span><span style="display:flex;"><span> <span style="color:#a6e22e">ch</span> <span style="color:#f92672">:=</span> make(<span style="color:#66d9ef">chan</span> <span style="color:#66d9ef">int</span>, <span style="color:#ae81ff">1</span>)
</span></span><span style="display:flex;"><span> <span style="color:#a6e22e">ch</span> <span style="color:#f92672">&lt;-</span> <span style="color:#ae81ff">100</span>
</span></span><span style="display:flex;"><span>
</span></span><span style="display:flex;"><span> <span style="color:#66d9ef">select</span> {
</span></span><span style="display:flex;"><span> <span style="color:#66d9ef">case</span> <span style="color:#a6e22e">i</span> <span style="color:#f92672">:=</span> <span style="color:#f92672">&lt;-</span><span style="color:#a6e22e">ch</span>:
</span></span><span style="display:flex;"><span> println(<span style="color:#e6db74">&#34;case1 recv: &#34;</span>, <span style="color:#a6e22e">i</span>)
</span></span><span style="display:flex;"><span> <span style="color:#66d9ef">case</span> <span style="color:#a6e22e">i</span> <span style="color:#f92672">:=</span> <span style="color:#f92672">&lt;-</span><span style="color:#a6e22e">ch</span>:
</span></span><span style="display:flex;"><span> println(<span style="color:#e6db74">&#34;case2 recv: &#34;</span>, <span style="color:#a6e22e">i</span>)
</span></span><span style="display:flex;"><span> }
</span></span><span style="display:flex;"><span>}
</span></span></code></pre></div><p>上面代码中case1 和case2分支的通道都是可以通信状态，那么Go会随机选择一个分支执行，我们执行代码后打印出来的结果可以证明这一点。</p>
<p>我们接下来再看看下面的代码：</p>
<div class="highlight"><pre tabindex="0" style="color:#f8f8f2;background-color:#272822;-moz-tab-size:4;-o-tab-size:4;tab-size:4;"><code class="language-go" data-lang="go"><span style="display:flex;"><span><span style="color:#66d9ef">func</span> <span style="color:#a6e22e">main</span>() {
</span></span><span style="display:flex;"><span> <span style="color:#a6e22e">ch</span> <span style="color:#f92672">:=</span> make(<span style="color:#66d9ef">chan</span> <span style="color:#66d9ef">int</span>, <span style="color:#ae81ff">1</span>)
</span></span><span style="display:flex;"><span> <span style="color:#66d9ef">go</span> <span style="color:#66d9ef">func</span>() {
</span></span><span style="display:flex;"><span> <span style="color:#a6e22e">time</span>.<span style="color:#a6e22e">Sleep</span>(<span style="color:#a6e22e">time</span>.<span style="color:#a6e22e">Second</span>)
</span></span><span style="display:flex;"><span> <span style="color:#a6e22e">ch</span> <span style="color:#f92672">&lt;-</span> <span style="color:#ae81ff">100</span>
</span></span><span style="display:flex;"><span> }()
</span></span><span style="display:flex;"><span>
</span></span><span style="display:flex;"><span> <span style="color:#66d9ef">select</span> {
</span></span><span style="display:flex;"><span> <span style="color:#66d9ef">case</span> <span style="color:#a6e22e">i</span> <span style="color:#f92672">:=</span> <span style="color:#f92672">&lt;-</span><span style="color:#a6e22e">ch</span>:
</span></span><span style="display:flex;"><span> println(<span style="color:#e6db74">&#34;case1 recv: &#34;</span>, <span style="color:#a6e22e">i</span>)
</span></span><span style="display:flex;"><span> <span style="color:#66d9ef">case</span> <span style="color:#a6e22e">i</span> <span style="color:#f92672">:=</span> <span style="color:#f92672">&lt;-</span><span style="color:#a6e22e">ch</span>:
</span></span><span style="display:flex;"><span> println(<span style="color:#e6db74">&#34;case2 recv: &#34;</span>, <span style="color:#a6e22e">i</span>)
</span></span><span style="display:flex;"><span> <span style="color:#66d9ef">default</span>:
</span></span><span style="display:flex;"><span> println(<span style="color:#e6db74">&#34;default case&#34;</span>)
</span></span><span style="display:flex;"><span> }
</span></span><span style="display:flex;"><span>}
</span></span></code></pre></div><p>上面代码中case1 和case2语句中的ch是未可以通信状态，由于存在default分支，那么Go会执行default分支，进而打印出<code>default case</code>。</p>
<p>如果我们注释掉default分支，我们可以发现select会阻塞，直到1秒之后ch通道是可以通信状态，此时case1或case2中某个分支会执行。</p>
</article>
<footer class="book-footer">
<div class="flex flex-wrap justify-between">
</div>
<script>(function(){function e(e){const t=window.getSelection(),n=document.createRange();n.selectNodeContents(e),t.removeAllRanges(),t.addRange(n)}document.querySelectorAll("pre code").forEach(t=>{t.addEventListener("click",function(){if(window.getSelection().toString())return;e(t.parentElement),navigator.clipboard&&navigator.clipboard.writeText(t.parentElement.textContent)})})})()</script>
</footer>
<div class="book-comments">
<script src="https://giscus.app/client.js" data-repo="cyub/go-1.14.13"
data-repo-id="MDEwOlJlcG9zaXRvcnkzMzc2ODUyMzQ=" data-category="Announcements"
data-category-id="DIC_kwDOFCCq8s4CZ3BC"
data-mapping="pathname" data-strict="0" data-emit-metadata="0"
data-input-position="bottom" data-reactions-enabled="0"
data-lang="zh-CN" data-theme="preferred_color_scheme" crossorigin="anonymous" async>
</script>
<noscript>Please enable JavaScript to view the comments powered by giscus.</noscript>
</div>
<label for="menu-control" class="hidden book-menu-overlay"></label>
</div>
</main>
</body>
</html>
Reference in New Issue View Git Blame Copy Permalink