202 lines
8.3 KiB
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Generated
202 lines
8.3 KiB
Plaintext
Generated
<!DOCTYPE html>
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<html>
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<head>
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<meta charset="utf-8">
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<title>Go by Example: Closing Channels</title>
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<link rel=stylesheet href="site.css">
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window.location.href = 'non-blocking-channel-operations';
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window.location.href = 'range-over-channels';
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<div class="example" id="closing-channels">
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<h2><a href="./">Go by Example</a>: Closing Channels</h2>
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<table>
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<tr>
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<td class="docs">
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<p><em>Closing</em> a channel indicates that no more values
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will be sent on it. This can be useful to communicate
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completion to the channel’s receivers.</p>
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</td>
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<td class="code empty leading">
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</td>
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</tr>
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<tr>
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<td class="docs">
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</td>
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<td class="code leading">
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<a href="http://play.golang.org/p/vCvRjcMq7p3"><img title="Run code" src="play.png" class="run" /></a><img title="Copy code" src="clipboard.png" class="copy" />
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<div class="highlight"><pre><span class="kn">package</span> <span class="nx">main</span>
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</pre></div>
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</td>
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</tr>
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<tr>
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<td class="docs">
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</td>
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<td class="code leading">
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<div class="highlight"><pre><span class="kn">import</span> <span class="s">"fmt"</span>
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</pre></div>
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</td>
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</tr>
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<tr>
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<td class="docs">
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<p>In this example we’ll use a <code>jobs</code> channel to
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communicate work to be done from the <code>main()</code> goroutine
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to a worker goroutine. When we have no more jobs for
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the worker we’ll <code>close</code> the <code>jobs</code> channel.</p>
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</td>
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<td class="code leading">
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<div class="highlight"><pre><span class="kd">func</span> <span class="nx">main</span><span class="p">()</span> <span class="p">{</span>
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<span class="nx">jobs</span> <span class="o">:=</span> <span class="nb">make</span><span class="p">(</span><span class="kd">chan</span> <span class="kt">int</span><span class="p">,</span> <span class="mi">5</span><span class="p">)</span>
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<span class="nx">done</span> <span class="o">:=</span> <span class="nb">make</span><span class="p">(</span><span class="kd">chan</span> <span class="kt">bool</span><span class="p">)</span>
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</pre></div>
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</td>
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</tr>
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<tr>
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<td class="docs">
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<p>Here’s the worker goroutine. It repeatedly receives
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from <code>jobs</code> with <code>j, more := <-jobs</code>. In this
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special 2-value form of receive, the <code>more</code> value
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will be <code>false</code> if <code>jobs</code> has been <code>close</code>d and all
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values in the channel have already been received.
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We use this to notify on <code>done</code> when we’ve worked
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all our jobs.</p>
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</td>
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<td class="code leading">
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<div class="highlight"><pre> <span class="k">go</span> <span class="kd">func</span><span class="p">()</span> <span class="p">{</span>
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<span class="k">for</span> <span class="p">{</span>
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<span class="nx">j</span><span class="p">,</span> <span class="nx">more</span> <span class="o">:=</span> <span class="o"><-</span><span class="nx">jobs</span>
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<span class="k">if</span> <span class="nx">more</span> <span class="p">{</span>
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<span class="nx">fmt</span><span class="p">.</span><span class="nx">Println</span><span class="p">(</span><span class="s">"received job"</span><span class="p">,</span> <span class="nx">j</span><span class="p">)</span>
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<span class="p">}</span> <span class="k">else</span> <span class="p">{</span>
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<span class="nx">fmt</span><span class="p">.</span><span class="nx">Println</span><span class="p">(</span><span class="s">"received all jobs"</span><span class="p">)</span>
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<span class="nx">done</span> <span class="o"><-</span> <span class="kc">true</span>
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<span class="k">return</span>
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<span class="p">}</span>
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<span class="p">}</span>
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<span class="p">}()</span>
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</pre></div>
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</td>
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</tr>
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<tr>
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<td class="docs">
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<p>This sends 3 jobs to the worker over the <code>jobs</code>
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channel, then closes it.</p>
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</td>
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<td class="code leading">
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<div class="highlight"><pre> <span class="k">for</span> <span class="nx">j</span> <span class="o">:=</span> <span class="mi">1</span><span class="p">;</span> <span class="nx">j</span> <span class="o"><=</span> <span class="mi">3</span><span class="p">;</span> <span class="nx">j</span><span class="o">++</span> <span class="p">{</span>
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<span class="nx">jobs</span> <span class="o"><-</span> <span class="nx">j</span>
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<span class="nx">fmt</span><span class="p">.</span><span class="nx">Println</span><span class="p">(</span><span class="s">"sent job"</span><span class="p">,</span> <span class="nx">j</span><span class="p">)</span>
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<span class="p">}</span>
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<span class="nb">close</span><span class="p">(</span><span class="nx">jobs</span><span class="p">)</span>
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<span class="nx">fmt</span><span class="p">.</span><span class="nx">Println</span><span class="p">(</span><span class="s">"sent all jobs"</span><span class="p">)</span>
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</pre></div>
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</td>
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</tr>
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<tr>
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<td class="docs">
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<p>We await the worker using the
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<a href="channel-synchronization">synchronization</a> approach
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we saw earlier.</p>
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</td>
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<td class="code">
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<div class="highlight"><pre> <span class="o"><-</span><span class="nx">done</span>
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<span class="p">}</span>
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</pre></div>
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</td>
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</tr>
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</table>
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<table>
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<tr>
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<td class="docs">
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</td>
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<td class="code leading">
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<div class="highlight"><pre><span class="gp">$</span> go run closing-channels.go
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<span class="go">sent job 1</span>
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<span class="go">received job 1</span>
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<span class="go">sent job 2</span>
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<span class="go">received job 2</span>
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<span class="go">sent job 3</span>
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<span class="go">received job 3</span>
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<span class="go">sent all jobs</span>
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<span class="go">received all jobs</span>
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</pre></div>
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</td>
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</tr>
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<tr>
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<td class="docs">
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<p>The idea of closed channels leads naturally to our next
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example: <code>range</code> over channels.</p>
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</td>
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<td class="code empty">
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</td>
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</tr>
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</table>
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<p class="next">
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Next example: <a href="range-over-channels">Range over Channels</a>.
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</p>
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<p class="footer">
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by <a href="https://markmcgranaghan.com">Mark McGranaghan</a> | <a href="https://github.com/mmcgrana/gobyexample/blob/master/examples/closing-channels">source</a> | <a href="https://github.com/mmcgrana/gobyexample#license">license</a>
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</p>
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</div>
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<script>
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var codeLines = [];
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codeLines.push('');codeLines.push('package main\u000A');codeLines.push('import \"fmt\"\u000A');codeLines.push('func main() {\u000A jobs :\x3D make(chan int, 5)\u000A done :\x3D make(chan bool)\u000A');codeLines.push(' go func() {\u000A for {\u000A j, more :\x3D \x3C-jobs\u000A if more {\u000A fmt.Println(\"received job\", j)\u000A } else {\u000A fmt.Println(\"received all jobs\")\u000A done \x3C- true\u000A return\u000A }\u000A }\u000A }()\u000A');codeLines.push(' for j :\x3D 1; j \x3C\x3D 3; j++ {\u000A jobs \x3C- j\u000A fmt.Println(\"sent job\", j)\u000A }\u000A close(jobs)\u000A fmt.Println(\"sent all jobs\")\u000A');codeLines.push(' \x3C-done\u000A}\u000A');codeLines.push('');codeLines.push('');
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