182 lines
7.1 KiB
Plaintext
182 lines
7.1 KiB
Plaintext
<!DOCTYPE html>
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<html>
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<head>
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<meta http-eqiv="content-type" content="text/html;charset=utf-8">
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<title>Go by Example: Select</title>
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<div class="example" id="select">
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<h2><a href="./">Go by Example</a>: Select</h2>
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<table>
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<tr>
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<td class="docs">
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<p>Go’s <em>select</em> lets you wait on multiple channel
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operations. Combining goroutines and channels with
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select is powerful feature of Go.</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/NGSkDrli6L"><img title="Run code" src="play.png" class="run" /></a>
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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">"time"</span>
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<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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</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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</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>For our example we’ll select across two channels.</p>
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</td>
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<td class="code leading">
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<div class="highlight"><pre> <span class="nx">c1</span> <span class="o">:=</span> <span class="nb">make</span><span class="p">(</span><span class="kd">chan</span> <span class="kt">string</span><span class="p">)</span>
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<span class="nx">c2</span> <span class="o">:=</span> <span class="nb">make</span><span class="p">(</span><span class="kd">chan</span> <span class="kt">string</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>Each channel will receive a value after some amount
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of time, to simulate e.g. blocking RPC operations
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executing in concurrent goroutines.</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="nx">time</span><span class="p">.</span><span class="nx">Sleep</span><span class="p">(</span><span class="nx">time</span><span class="p">.</span><span class="nx">Second</span> <span class="o">*</span> <span class="mi">1</span><span class="p">)</span>
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<span class="nx">c1</span> <span class="o"><-</span> <span class="s">"one"</span>
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<span class="p">}()</span>
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<span class="k">go</span> <span class="kd">func</span><span class="p">()</span> <span class="p">{</span>
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<span class="nx">time</span><span class="p">.</span><span class="nx">Sleep</span><span class="p">(</span><span class="nx">time</span><span class="p">.</span><span class="nx">Second</span> <span class="o">*</span> <span class="mi">2</span><span class="p">)</span>
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<span class="nx">c2</span> <span class="o"><-</span> <span class="s">"two"</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>We’ll use <code>select</code> to await both of these values
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simultaneously, printing each one as it arrives.</p>
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</td>
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<td class="code">
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<div class="highlight"><pre> <span class="k">for</span> <span class="nx">i</span> <span class="o">:=</span> <span class="mi">0</span><span class="p">;</span> <span class="nx">i</span> <span class="p"><</span> <span class="mi">2</span><span class="p">;</span> <span class="nx">i</span><span class="o">++</span> <span class="p">{</span>
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<span class="k">select</span> <span class="p">{</span>
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<span class="k">case</span> <span class="nx">msg1</span> <span class="o">:=</span> <span class="o"><-</span><span class="nx">c1</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"</span><span class="p">,</span> <span class="nx">msg1</span><span class="p">)</span>
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<span class="k">case</span> <span class="nx">msg2</span> <span class="o">:=</span> <span class="o"><-</span><span class="nx">c2</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"</span><span class="p">,</span> <span class="nx">msg2</span><span class="p">)</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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</table>
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<table>
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<tr>
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<td class="docs">
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<p>We receive the values <code>"one"</code> and then <code>"two"</code> as
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expected.</p>
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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> <span class="nb">time </span>go run <span class="k">select</span>.go
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<span class="go">received one</span>
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<span class="go">received two</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>Note that the total execution time is only ~2 seconds
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since both the 1 and 2 second <code>Sleeps</code> execute
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concurrently.</p>
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</td>
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<td class="code">
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<div class="highlight"><pre><span class="go">real 0m2.245s</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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<p class="next">
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Next example: <a href="timeouts">Timeouts</a>.
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</p>
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<p class="footer">
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by <a href="https://twitter.com/mmcgrana">@mmcgrana</a> | <a href="mailto:mmcgrana@gmail.com">feedback</a> | <a href="https://github.com/mmcgrana/gobyexample/blob/master/examples/select">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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