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<h2><a href="./">Go by Example 中文版</a>: WaitGroup</h2>

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<p>想要等待多个协程完成，我们可以使用 <em>wait group</em> 。</p>

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<a href="https://play.studygolang.com/p/YnBi_jvjQzm"><img title="Run code" src="play.png" class="run" /></a><img title="Copy code" src="clipboard.png" class="copy" />

<pre class="chroma"><span class="kn">package</span> <span class="nx">main</span>

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<pre class="chroma"><span class="kn">import</span> <span class="p">(</span>

<span class="s">&#34;fmt&#34;</span>

<span class="s">&#34;sync&#34;</span>

<span class="s">&#34;time&#34;</span>

<span class="p">)</span>

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<p>每个协程都会运行该函数。</p>

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<span class="kd">func</span> <span class="nf">worker</span><span class="p">(</span><span class="nx">id</span> <span class="kt">int</span><span class="p">)</span> <span class="p">{</span>

<span class="nx">fmt</span><span class="p">.</span><span class="nf">Printf</span><span class="p">(</span><span class="s">&#34;Worker %d starting\n&#34;</span><span class="p">,</span> <span class="nx">id</span><span class="p">)</span>

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<p>睡眠一秒钟，以此来模拟耗时的任务。</p>

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<span class="nx">time</span><span class="p">.</span><span class="nf">Sleep</span><span class="p">(</span><span class="nx">time</span><span class="p">.</span><span class="nx">Second</span><span class="p">)</span>

<span class="nx">fmt</span><span class="p">.</span><span class="nf">Printf</span><span class="p">(</span><span class="s">&#34;Worker %d done\n&#34;</span><span class="p">,</span> <span class="nx">id</span><span class="p">)</span>

<span class="p">}</span>

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<pre class="chroma"><span class="kd">func</span> <span class="nf">main</span><span class="p">()</span> <span class="p">{</span>

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<p>这个 WaitGroup 用于等待这里启动的所有协程完成。

注意：如果 WaitGroup 显式传递到函数中，则应使用 <em>指针</em> 。</p>

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<span class="kd">var</span> <span class="nx">wg</span> <span class="nx">sync</span><span class="p">.</span><span class="nx">WaitGroup</span>

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<p>启动几个协程，并为其递增 WaitGroup 的计数器。</p>

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<span class="k">for</span> <span class="nx">i</span> <span class="o">:=</span> <span class="mi">1</span><span class="p">;</span> <span class="nx">i</span> <span class="o">&lt;=</span> <span class="mi">5</span><span class="p">;</span> <span class="nx">i</span><span class="o">++</span> <span class="p">{</span>

<span class="nx">wg</span><span class="p">.</span><span class="nf">Add</span><span class="p">(</span><span class="mi">1</span><span class="p">)</span>

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<p>避免在每个协程闭包中重复利用相同的 <code>i</code> 值

更多细节可以参考 <a href="https://golang.org/doc/faq#closures_and_goroutines">the FAQ</a></p>

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<span class="nx">i</span> <span class="o">:=</span> <span class="nx">i</span>

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<p>将 worker 调用包装在一个闭包中，可以确保通知 WaitGroup 此工作线程已完成。

这样，worker 线程本身就不必知道其执行中涉及的并发原语。</p>

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<span class="k">go</span> <span class="kd">func</span><span class="p">()</span> <span class="p">{</span>

<span class="k">defer</span> <span class="nx">wg</span><span class="p">.</span><span class="nf">Done</span><span class="p">()</span>

<span class="nf">worker</span><span class="p">(</span><span class="nx">i</span><span class="p">)</span>

<span class="p">}()</span>

<span class="p">}</span>

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<p>阻塞，直到 WaitGroup 计数器恢复为 0；

即所有协程的工作都已经完成。</p>

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<span class="nx">wg</span><span class="p">.</span><span class="nf">Wait</span><span class="p">()</span>

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<p>请注意，WaitGroup 这种使用方式没有直观的办法传递来自 worker 的错误。

更高级的用例，请参见 <a href="https://pkg.go.dev/golang.org/x/sync/errgroup">errgroup package</a></p>

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<span class="p">}</span>

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<pre class="chroma"><span class="gp">$</span> go run waitgroups.go

<span class="go">Worker 5 starting

</span><span class="go">Worker 3 starting

</span><span class="go">Worker 4 starting

</span><span class="go">Worker 1 starting

</span><span class="go">Worker 2 starting

</span><span class="go">Worker 4 done

</span><span class="go">Worker 1 done

</span><span class="go">Worker 2 done

</span><span class="go">Worker 5 done

</span><span class="go">Worker 3 done</span></pre>

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<p>每次运行，各个协程开启和完成的时间可能是不同的。</p>

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下一个例子: <a href="rate-limiting">速率限制</a>

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<a href="https://twitter.com/mmcgrana">@mmcgrana</a> 和<a href="https://eli.thegreenplace.net">Eli Bendersky</a>编写 | <a href="https://github.com/gobyexample-cn">gobyexample-cn</a> 翻译 | <a href="https://github.com/gobyexample-cn/gobyexample/issues">反馈</a> | <a href="https://github.com/gobyexample-cn/gobyexample">源码</a> | <a href="https://github.com/mmcgrana/gobyexample#license">license</a> </p>

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