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<meta property="og:description" content="生成器（ generator）是能返回一个迭代器的函数。生成器函数也是一种函数，最直观的表现就是比普通的function多了个星号*，在其函数体内可以使用yield关键字，有意思的是函数会在每个yield后暂停。 这里生活中有一个比较形象的例子。咱们到银行办理业务时候都得向大厅的机器取一张排队号。你拿到你的排队号，机器并不会自动为你再出下一张票。也就是说取票机“暂停”住了，直到下一个人再次唤起才会">
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2018-08-21
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<span class="post-meta-item-text">字数统计：</span>
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1,275 (字)
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5 (分钟)
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<p>生成器（ generator）是能返回一个迭代器的函数。生成器函数也是一种函数，最直观的表现就是比普通的function多了个星号*，在其函数体内可以使用yield关键字，有意思的是函数会在每个yield后暂停。</p>
<p>这里生活中有一个比较形象的例子。咱们到银行办理业务时候都得向大厅的机器取一张排队号。你拿到你的排队号，机器并不会自动为你再出下一张票。也就是说取票机“暂停”住了，直到下一个人再次唤起才会继续吐票。</p>
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<p>OK。说说迭代器。当你调用一个generator时，它将返回一个迭代器对象。这个迭代器对象拥有一个叫做next的方法来帮助你重启generator函数并得到下一个值。next方法不仅返回值，它返回的对象具有两个属性：done和value。value是你获得的值，done用来表明你的generator是否已经停止提供值。继续用刚刚取票的例子，每张排队号就是这里的value，打印票的纸是否用完就这是这里的done。</p>
<ul>
<li>function 关键字与函数名之间有一个星号；</li>
<li>函数体内部使用 yield表达式，定义不同的内部状态；</li>
<li>next 指针移向下一个状态</li>
</ul>
<figure class="highlight javascript"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br><span class="line">6</span><br><span class="line">7</span><br><span class="line">8</span><br><span class="line">9</span><br><span class="line">10</span><br><span class="line">11</span><br><span class="line">12</span><br></pre></td><td class="code"><pre><span class="line"><span class="comment">// 生成器 最大的特点是可以交出函数的执行权</span></span><br><span class="line"><span class="function"><span class="keyword">function</span> *<span class="title">createGenerator</span>(<span class="params"></span>)</span>{</span><br><span class="line"> <span class="keyword">yield</span> <span class="number">1</span>;</span><br><span class="line"> <span class="keyword">yield</span> <span class="number">2</span>;</span><br><span class="line"> <span class="keyword">yield</span> <span class="number">3</span>;</span><br><span class="line">}</span><br><span class="line"></span><br><span class="line"><span class="comment">// 迭代器</span></span><br><span class="line"><span class="keyword">let</span> iterator=createGenerator()</span><br><span class="line"><span class="built_in">console</span>.log(iterator.next().value) <span class="comment">// 1</span></span><br><span class="line"><span class="built_in">console</span>.log(iterator.next().value) <span class="comment">// 2</span></span><br><span class="line"><span class="built_in">console</span>.log(iterator.next().value) <span class="comment">// 3</span></span><br></pre></td></tr></table></figure>
<figure class="highlight js"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br><span class="line">6</span><br><span class="line">7</span><br><span class="line">8</span><br><span class="line">9</span><br></pre></td><td class="code"><pre><span class="line"><span class="function"><span class="keyword">function</span> *<span class="title">foo</span>(<span class="params">x</span>) </span>{</span><br><span class="line"> <span class="keyword">let</span> y = <span class="number">2</span> * (<span class="keyword">yield</span> (x + <span class="number">1</span>))</span><br><span class="line"> <span class="keyword">let</span> z = <span class="keyword">yield</span> (y / <span class="number">3</span>)</span><br><span class="line"> <span class="keyword">return</span> (x + y + z)</span><br><span class="line">}</span><br><span class="line"><span class="keyword">let</span> it = foo(<span class="number">5</span>)</span><br><span class="line"><span class="built_in">console</span>.log(it.next()) <span class="comment">// => {value: 6, done: false}</span></span><br><span class="line"><span class="built_in">console</span>.log(it.next(<span class="number">12</span>)) <span class="comment">// => {value: 8, done: false}</span></span><br><span class="line"><span class="built_in">console</span>.log(it.next(<span class="number">13</span>)) <span class="comment">// => {value: 42, done: true}</span></span><br></pre></td></tr></table></figure>
<ol>
<li>首先 Generator 函数调用时它会返回一个迭代器</li>
<li>当执行第一次 next 时，传参会被忽略，并且函数暂停在 yield (x + 1) 处，所以返回 5 + 1 = 6</li>
<li>当执行第二次 next 时，传入的参数等于上一个 yield 的返回值，如果你不传参，yield 永远返回 undefined。此时 let y = 2 <em> 12，所以第二个
yield 等于 2 </em> 12 / 3 = 8</li>
<li>当执行第三次 next 时，传入的参数会传递给 z，所以 z = 13, x = 5, y = 24，相加等于 42</li>
</ol>
<p>那生成器和迭代器又有什么用处呢？</p>
<p>围绕着生成器的许多兴奋点都与异步编程直接相关。异步调用对于我们来说是很困难的事，我们的函数并不会等待异步调用完再执行，你可能会想到用回调函数，（当然还有其他方案比如Promise比如Async/await）。</p>
<p>生成器可以让我们的代码进行等待。就不用嵌套的回调函数。使用generator可以确保当异步调用在我们的generator函数运行一下行代码之前完成时暂停函数的执行。</p>
<p>那么问题来了，咱们也不能手动一直调用next()方法，你需要一个能够调用生成器并启动迭代器的方法。就像这样子的：</p>
<figure class="highlight javascript"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br><span class="line">6</span><br><span class="line">7</span><br><span class="line">8</span><br><span class="line">9</span><br><span class="line">10</span><br><span class="line">11</span><br><span class="line">12</span><br><span class="line">13</span><br><span class="line">14</span><br><span class="line">15</span><br><span class="line">16</span><br><span class="line">17</span><br><span class="line">18</span><br><span class="line">19</span><br></pre></td><td class="code"><pre><span class="line"><span class="function"><span class="keyword">function</span> <span class="title">run</span>(<span class="params">taskDef</span>)</span>{</span><br><span class="line"> <span class="comment">// 创建迭代器，让它在别处可用</span></span><br><span class="line"> <span class="keyword">let</span> task=taskDef();</span><br><span class="line"></span><br><span class="line"> <span class="comment">// 启动任务</span></span><br><span class="line"> <span class="keyword">let</span> result=task.next();</span><br><span class="line"></span><br><span class="line"> <span class="comment">// 递归使用函数来保持对next()的调用</span></span><br><span class="line"> <span class="function"><span class="keyword">function</span> <span class="title">step</span>(<span class="params"></span>)</span>{</span><br><span class="line"> <span class="comment">// 如果还有更多要做的</span></span><br><span class="line"> <span class="keyword">if</span>(!result.done){</span><br><span class="line"> result=task.next();</span><br><span class="line"> step();</span><br><span class="line"> }</span><br><span class="line"> }</span><br><span class="line"></span><br><span class="line"> <span class="comment">// 开始处理过程</span></span><br><span class="line"> step();</span><br><span class="line">}</span><br></pre></td></tr></table></figure>
<p>生成器与迭代器最有趣、最令人激动的方面，或许就是可创建外观清晰的异步操作代码。可以不必到处使用回调函数，而是可以建立貌似同步的代码，但实际上却使用
<code>yield</code> 来等待异步操作结束。</p>
<p>es6引入了 async 函数，使得异步操作变得更加方便。</p>
<p>async 函数是什么？一句话，它就是 Generator 函数的语法糖。</p>
<figure class="highlight javascript"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br><span class="line">6</span><br><span class="line">7</span><br><span class="line">8</span><br><span class="line">9</span><br><span class="line">10</span><br><span class="line">11</span><br><span class="line">12</span><br></pre></td><td class="code"><pre><span class="line"><span class="function"><span class="keyword">function</span> <span class="title">timeout</span>(<span class="params">ms</span>)</span>{</span><br><span class="line"> <span class="keyword">return</span> <span class="keyword">new</span> <span class="built_in">Promise</span>(<span class="function">(<span class="params">resolve</span>)=></span>{</span><br><span class="line"> setTimeout(resolve,ms)</span><br><span class="line"> });</span><br><span class="line">}</span><br><span class="line"></span><br><span class="line"><span class="keyword">async</span> <span class="function"><span class="keyword">function</span> <span class="title">asyncPrint</span>(<span class="params">value,ms</span>)</span>{</span><br><span class="line"> <span class="keyword">await</span> timeout(ms);</span><br><span class="line"> <span class="built_in">console</span>.log(value);</span><br><span class="line">}</span><br><span class="line"></span><br><span class="line">asyncPrint(<span class="string">'hello world'</span>,<span class="number">50</span>)</span><br></pre></td></tr></table></figure>
<p>一比较就会发现，async函数就是将 Generator 函数的星号（*）替换成async，将yield替换成await，仅此而已。
async函数对 Generator 函数的改进，体现在以下四点：</p>
<ul>
<li>内置执行器</li>
<li>更好的语义</li>
<li>更广的适用性</li>
<li>返回值是Promise</li>
</ul>
<h2 id="可迭代对象的特点"><a href="#可迭代对象的特点" class="headerlink" title="可迭代对象的特点"></a>可迭代对象的特点</h2><ol>
<li>知道如何每次访问集合中的一项，并跟踪该序列中的当前位置</li>
<li>提供而next方法，来返回序列中的下一项(value, done)</li>
<li>迭代器一旦被创建，可以反复调用next</li>
</ol>
<figure class="highlight js"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br><span class="line">6</span><br><span class="line">7</span><br><span class="line">8</span><br><span class="line">9</span><br><span class="line">10</span><br><span class="line">11</span><br><span class="line">12</span><br><span class="line">13</span><br><span class="line">14</span><br><span class="line">15</span><br><span class="line">16</span><br><span class="line">17</span><br></pre></td><td class="code"><pre><span class="line"><span class="function"><span class="keyword">function</span> <span class="title">makeIterator</span>(<span class="params">array</span>)</span>{</span><br><span class="line"> <span class="keyword">var</span> nextIndex = <span class="number">0</span>;</span><br><span class="line"> <span class="keyword">return</span> {</span><br><span class="line"> next: <span class="function"><span class="keyword">function</span>(<span class="params"></span>)</span>{</span><br><span class="line"> <span class="keyword">return</span> nextIndex < array.length ?</span><br><span class="line"> {<span class="attr">value</span>: array[nextIndex++], <span class="attr">done</span>: <span class="literal">false</span>} :</span><br><span class="line"> {<span class="attr">done</span>: <span class="literal">true</span>};</span><br><span class="line"> }</span><br><span class="line"> };</span><br><span class="line">}</span><br><span class="line"><span class="comment">// 使用 next 方法依次访问对象中的键值</span></span><br><span class="line"><span class="keyword">var</span> it = makeIterator([<span class="string">'step'</span>, <span class="string">'by'</span>,<span class="string">'step'</span>]);</span><br><span class="line"><span class="built_in">console</span>.log(it.next().value); <span class="comment">// 'step'</span></span><br><span class="line"><span class="built_in">console</span>.log(it.next().value); <span class="comment">// 'by'</span></span><br><span class="line"><span class="built_in">console</span>.log(it.next().value); <span class="comment">// 'step'</span></span><br><span class="line"><span class="built_in">console</span>.log(it.next().value); <span class="comment">// undefined</span></span><br><span class="line"><span class="built_in">console</span>.log(it.next().done); <span class="comment">// true</span></span><br></pre></td></tr></table></figure>
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<p><span>本文标题:</span><a href="/es6generator.html">generator生成器</a></p>
<p><span>文章作者:</span><a href="/" title="访问 陈宇（cosyer） 的个人博客">陈宇（cosyer）</a></p>
<p><span>发布时间:</span>2018年08月21日 - 11:08</p>
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