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href="/blog/pages/d61b1cb4cdac1f63/#一、异步操作概述" class="sidebar-link">一、异步操作概述</a><ul class="sidebar-sub-headers"><li class="sidebar-sub-header level3"><a href="/blog/pages/d61b1cb4cdac1f63/#_1、单线程模型" class="sidebar-link">1、单线程模型</a></li><li class="sidebar-sub-header level3"><a href="/blog/pages/d61b1cb4cdac1f63/#_2、同步任务和异步任务" class="sidebar-link">2、同步任务和异步任务</a></li><li class="sidebar-sub-header level3"><a href="/blog/pages/d61b1cb4cdac1f63/#_3、任务队列和事件循环" class="sidebar-link">3、任务队列和事件循环</a></li><li class="sidebar-sub-header level3"><a href="/blog/pages/d61b1cb4cdac1f63/#_4、异步操作的模式" class="sidebar-link">4、异步操作的模式</a></li><li class="sidebar-sub-header level4"><a href="/blog/pages/d61b1cb4cdac1f63/#_1-回调函数" class="sidebar-link">（1）回调函数</a></li><li class="sidebar-sub-header level4"><a href="/blog/pages/d61b1cb4cdac1f63/#_2-事件监听" class="sidebar-link">（2）事件监听</a></li><li class="sidebar-sub-header level4"><a href="/blog/pages/d61b1cb4cdac1f63/#_3-发布-订阅" class="sidebar-link">（3）发布/订阅</a></li><li class="sidebar-sub-header level3"><a href="/blog/pages/d61b1cb4cdac1f63/#_5、异步操作的流程控制" class="sidebar-link">5、异步操作的流程控制</a></li><li class="sidebar-sub-header level4"><a href="/blog/pages/d61b1cb4cdac1f63/#_1-串行执行" class="sidebar-link">（1）串行执行</a></li><li class="sidebar-sub-header level4"><a href="/blog/pages/d61b1cb4cdac1f63/#_2-并行执行" class="sidebar-link">（2）并行执行</a></li><li class="sidebar-sub-header level4"><a href="/blog/pages/d61b1cb4cdac1f63/#_3-并行与串行的结合" class="sidebar-link">（3）并行与串行的结合</a></li></ul></li><li class="sidebar-sub-header level2"><a href="/blog/pages/d61b1cb4cdac1f63/#文档" class="sidebar-link">文档</a></li></ul></li><li><a href="/blog/pages/7d961b8030c6099e/" class="sidebar-link">DOM</a></li><li><a href="/blog/pages/10b2761db5a8e089/" class="sidebar-link">事件</a></li><li><a href="/blog/pages/bab4930124ad2c10/" class="sidebar-link">浏览器模型</a></li></ul> </aside> <div><main class="page"><div class="theme-vdoing-wrapper "><div class="articleInfo-wrap" data-v-06970110><div class="articleInfo" data-v-06970110><ul class="breadcrumbs" data-v-06970110><li data-v-06970110><a href="/blog/" title="首页" class="iconfont icon-home router-link-active" data-v-06970110></a></li> <li data-v-06970110><a href="/blog/note/javascript/#《JavaScript教程》笔记" data-v-06970110>《JavaScript教程》笔记</a></li></ul> <div class="info" data-v-06970110><div title="作者" class="author iconfont icon-touxiang" data-v-06970110><a href="https://github.com/heBody" target="_blank" title="作者" class="beLink" data-v-06970110>heBody</a></div> <div title="创建时间" class="date iconfont icon-riqi" data-v-06970110><a href="javascript:;" data-v-06970110>2020-01-12</a></div> <!----></div></div></div> <!----> <div class="content-wrapper"><div class="right-menu-wrapper"><div class="right-menu-margin"><div class="right-menu-title">目录</div> <div class="right-menu-content"></div></div></div> <h1><img src="data:image/png;base64,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">异步操作<!----></h1>  <div class="theme-vdoing-content content__default"><h1 id="异步操作"><a href="#异步操作" class="header-anchor">#</a> 异步操作</h1> <h2 id="一、异步操作概述"><a href="#一、异步操作概述" class="header-anchor">#</a> 一、异步操作概述</h2> <h3 id="_1、单线程模型"><a href="#_1、单线程模型" class="header-anchor">#</a> 1、单线程模型</h3> <p>单线程模型指的是，<strong>JavaScript 只在一个线程上运行</strong>。也就是说，<strong>JavaScript 同时只能执行一个任务</strong>，其他任务都必须在后面排队等待。</p> <p>注意，<strong>JavaScript 只在一个线程上运行，不代表 JavaScript 引擎只有一个线程</strong>。事实上，JavaScript 引擎有多个线程，单个脚本只能在一个线程上运行（称为主线程），其他线程都是在后台配合。</p> <p>JavaScript 之所以采用单线程，而不是多线程，跟历史有关系。JavaScript 从诞生起就是单线程，原因是不想让浏览器变得太复杂，因为多线程需要共享资源、且有可能修改彼此的运行结果，对于一种网页脚本语言来说，这就太复杂了。如果 JavaScript 同时有两个线程，一个线程在网页 DOM 节点上添加内容，另一个线程删除了这个节点，这时浏览器应该以哪个线程为准？是不是还要有锁机制？所以，<strong>为了避免复杂性</strong>，<strong>JavaScript 一开始就是单线程</strong>，这已经成了这门语言的核心特征，将来也不会改变。</p> <p>这种模式的<strong>好处</strong>是<strong>实现起来比较简单，执行环境相对单纯</strong>；<strong>坏处</strong>是<strong>只要有一个任务耗时很长，后面的任务都必须排队等着，会拖延整个程序的执行</strong>。常见的浏览器无响应（假死），往往就是因为某一段 JavaScript 代码长时间运行（比如死循环），导致整个页面卡在这个地方，其他任务无法执行。JavaScript 语言本身并不慢，慢的是读写外部数据，比如等待 Ajax 请求返回结果。这个时候，如果对方服务器迟迟没有响应，或者网络不通畅，就会导致脚本的长时间停滞。</p> <p>如果排队是因为计算量大，CPU 忙不过来，倒也算了，但是很多时候 CPU 是闲着的，因为 <strong>IO 操作（输入输出）<strong>很慢（比如 Ajax 操作从网络读取数据），不得不等着结果出来，再往下执行。JavaScript 语言的设计者意识到，这时 CPU 完全可以不管 IO 操作，<strong>挂起处于等待中的任务，先运行排在后面的任务。等到 IO 操作返回了结果，再回过头，把挂起的任务继续执行下去</strong>。这种机制就是 JavaScript 内部采用的</strong>“事件循环”机制（Event Loop）</strong>。</p> <p>单线程模型虽然对 JavaScript 构成了很大的限制，但也因此使它具备了其他语言不具备的优势。如果用得好，JavaScript 程序是不会出现堵塞的，这就是为什么 <strong>Node 可以用很少的资源，应付大流量访问</strong>的原因。</p> <p>为了利用多核 CPU 的计算能力，<strong>HTML5 提出 Web Worker 标准</strong>，允许 JavaScript 脚本创建多个线程，但是子线程完全受主线程控制，且不得操作 DOM。所以，这个新标准并没有改变 JavaScript 单线程的本质。</p> <h3 id="_2、同步任务和异步任务"><a href="#_2、同步任务和异步任务" class="header-anchor">#</a> 2、同步任务和异步任务</h3> <p>程序里面所有的任务，可以<strong>分成两类：同步任务（synchronous）和异步任务（asynchronous）</strong>。</p> <p><strong>同步任务是那些没有被引擎挂起、在主线程上排队执行的任务</strong>。只有前一个任务执行完毕，才能执行后一个任务。</p> <p><strong>异步任务是那些被引擎放在一边，不进入主线程、而进入任务队列的任务</strong>。只有引擎认为某个异步任务可以执行了（比如 Ajax 操作从服务器得到了结果），<strong>该任务（采用回调函数的形式）才会进入主线程执行</strong>。排在异步任务后面的代码，不用等待异步任务结束会马上运行，也就是说，异步任务不具有“堵塞”效应。</p> <p>举例来说，<strong>Ajax 操作可以当作同步任务处理，也可以当作异步任务处理，由开发者决定</strong>。如果是同步任务，主线程就等着 Ajax 操作返回结果，再往下执行；如果是异步任务，主线程在发出 Ajax 请求以后，就直接往下执行，等到 Ajax 操作有了结果，主线程再执行对应的回调函数。</p> <h3 id="_3、任务队列和事件循环"><a href="#_3、任务队列和事件循环" class="header-anchor">#</a> 3、任务队列和事件循环</h3> <p>JavaScript 运行时，除了一个正在运行的主线程，引擎还提供一个<strong>任务队列</strong>（task queue），<strong>里面是各种需要当前程序处理的异步任务</strong>。（实际上，根据异步任务的类型，存在多个任务队列。为了方便理解，这里假设只存在一个队列。）</p> <p>首先，主线程会去执行所有的同步任务。等到同步任务全部执行完，就会去看任务队列里面的异步任务。如果满足条件，那么异步任务就重新进入主线程开始执行，这时它就变成同步任务了。等到执行完，下一个异步任务再进入主线程开始执行。一旦任务队列清空，程序就结束执行。</p> <p><strong>异步任务的写法通常是回调函数</strong>。一旦异步任务重新进入主线程，就会执行对应的回调函数。如果一个异步任务没有回调函数，就不会进入任务队列，也就是说，不会重新进入主线程，因为没有用回调函数指定下一步的操作。</p> <p>JavaScript 引擎怎么知道异步任务有没有结果，能不能进入主线程呢？答案就是<strong>引擎在不停地检查，一遍又一遍，只要同步任务执行完了，引擎就会去检查那些挂起来的异步任务，是不是可以进入主线程了</strong>。这种循环检查的机制，就叫做<strong>事件循环（Event Loop）</strong>。<a href="https://en.wikipedia.org/wiki/Event_loop" target="_blank" rel="noopener noreferrer">维基百科<span><svg xmlns="http://www.w3.org/2000/svg" aria-hidden="true" focusable="false" x="0px" y="0px" viewBox="0 0 100 100" width="15" height="15" class="icon outbound"><path fill="currentColor" d="M18.8,85.1h56l0,0c2.2,0,4-1.8,4-4v-32h-8v28h-48v-48h28v-8h-32l0,0c-2.2,0-4,1.8-4,4v56C14.8,83.3,16.6,85.1,18.8,85.1z"></path> <polygon fill="currentColor" points="45.7,48.7 51.3,54.3 77.2,28.5 77.2,37.2 85.2,37.2 85.2,14.9 62.8,14.9 62.8,22.9 71.5,22.9"></polygon></svg> <span class="sr-only">(opens new window)</span></span></a>的定义是：“事件循环是一个程序结构，用于等待和发送消息和事件（a programming construct that waits for and dispatches events or messages in a program）”。</p> <h3 id="_4、异步操作的模式"><a href="#_4、异步操作的模式" class="header-anchor">#</a> 4、异步操作的模式</h3> <h4 id="_1-回调函数"><a href="#_1-回调函数" class="header-anchor">#</a> （1）回调函数</h4> <p>回调函数<strong>是异步操作最基本的方法。</strong></p> <p>下面是两个函数<code>f1</code>和<code>f2</code>，编程的意图是<code>f2</code>必须等到<code>f1</code>执行完成，才能执行。</p> <div class="language-js line-numbers-mode"><pre class="language-js"><code><span class="token keyword">function</span> <span class="token function">f1</span><span class="token punctuation">(</span><span class="token punctuation">)</span> <span class="token punctuation">{</span> <span class="token comment">// 此函数是异步操作</span>
    26. 

  26.   <span class="token comment">// ...</span>
    27. 

  27. <span class="token punctuation">}</span>
    28. 

  28. 

  29. <span class="token keyword">function</span> <span class="token function">f2</span><span class="token punctuation">(</span><span class="token punctuation">)</span> <span class="token punctuation">{</span> <span class="token comment">// 期望是等f1执行完再执行f2，但是f1是异步操作，会先执行f2</span>
    30. 

  30.   <span class="token comment">// ...</span>
    31. 

  31. <span class="token punctuation">}</span>
    32. 

  32. 

  33. <span class="token function">f1</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
    34. 

  34. <span class="token function">f2</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
    35. 

  35. </code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br><span class="line-number">3</span><br><span class="line-number">4</span><br><span class="line-number">5</span><br><span class="line-number">6</span><br><span class="line-number">7</span><br><span class="line-number">8</span><br><span class="line-number">9</span><br><span class="line-number">10</span><br></div></div><p>上面代码的问题在于，如果<code>f1</code>是异步操作，<code>f2</code>会立即执行，不会等到<code>f1</code>结束再执行。</p> <p>这时，可以考虑改写<code>f1</code>，把<code>f2</code>写成<code>f1</code>的回调函数。</p> <div class="language-js line-numbers-mode"><pre class="language-js"><code><span class="token keyword">function</span> <span class="token function">f1</span><span class="token punctuation">(</span><span class="token parameter">callback</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
    36. 

  36.   <span class="token comment">// ...</span>
    37. 

  37.   <span class="token function">callback</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">;</span> <span class="token comment">// f2传入当成回调函数，等f1执行完再执行f2</span>
    38. 

  38. <span class="token punctuation">}</span>
    39. 

  39. 

  40. <span class="token keyword">function</span> <span class="token function">f2</span><span class="token punctuation">(</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
    41. 

  41.   <span class="token comment">// ...</span>
    42. 

  42. <span class="token punctuation">}</span>
    43. 

  43. 

  44. <span class="token function">f1</span><span class="token punctuation">(</span>f2<span class="token punctuation">)</span><span class="token punctuation">;</span>
    45. 

  45. 

  46. <span class="token comment">// 优点：简单、容易理解和实现</span>
    47. 

  47. <span class="token comment">// 缺点：不利于代码的阅读和维护，各个部分之间高度耦合，使得程序结构混乱、流程难以追踪（尤其是多个回调函数嵌套的情况），而且每个任务只能指定一个回调函数。</span>
    48. 

  48. </code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br><span class="line-number">3</span><br><span class="line-number">4</span><br><span class="line-number">5</span><br><span class="line-number">6</span><br><span class="line-number">7</span><br><span class="line-number">8</span><br><span class="line-number">9</span><br><span class="line-number">10</span><br><span class="line-number">11</span><br><span class="line-number">12</span><br><span class="line-number">13</span><br></div></div><p>回调函数的优点是简单、容易理解和实现，缺点是不利于代码的阅读和维护，各个部分之间高度<a href="https://en.wikipedia.org/wiki/Coupling_(computer_programming)" target="_blank" rel="noopener noreferrer">耦合<span><svg xmlns="http://www.w3.org/2000/svg" aria-hidden="true" focusable="false" x="0px" y="0px" viewBox="0 0 100 100" width="15" height="15" class="icon outbound"><path fill="currentColor" d="M18.8,85.1h56l0,0c2.2,0,4-1.8,4-4v-32h-8v28h-48v-48h28v-8h-32l0,0c-2.2,0-4,1.8-4,4v56C14.8,83.3,16.6,85.1,18.8,85.1z"></path> <polygon fill="currentColor" points="45.7,48.7 51.3,54.3 77.2,28.5 77.2,37.2 85.2,37.2 85.2,14.9 62.8,14.9 62.8,22.9 71.5,22.9"></polygon></svg> <span class="sr-only">(opens new window)</span></span></a>（coupling），使得程序结构混乱、流程难以追踪（尤其是多个回调函数嵌套的情况），而且每个任务只能指定一个回调函数。</p> <h4 id="_2-事件监听"><a href="#_2-事件监听" class="header-anchor">#</a> （2）事件监听</h4> <p>另一种思路是采用事件驱动模式。<strong>异步任务的执行不取决于代码的顺序，而取决于某个事件是否发生</strong>。</p> <p>还是以<code>f1</code>和<code>f2</code>为例。首先，为<code>f1</code>绑定一个事件（这里采用的 jQuery 的<a href="https://api.jquery.com/on/" target="_blank" rel="noopener noreferrer">写法<span><svg xmlns="http://www.w3.org/2000/svg" aria-hidden="true" focusable="false" x="0px" y="0px" viewBox="0 0 100 100" width="15" height="15" class="icon outbound"><path fill="currentColor" d="M18.8,85.1h56l0,0c2.2,0,4-1.8,4-4v-32h-8v28h-48v-48h28v-8h-32l0,0c-2.2,0-4,1.8-4,4v56C14.8,83.3,16.6,85.1,18.8,85.1z"></path> <polygon fill="currentColor" points="45.7,48.7 51.3,54.3 77.2,28.5 77.2,37.2 85.2,37.2 85.2,14.9 62.8,14.9 62.8,22.9 71.5,22.9"></polygon></svg> <span class="sr-only">(opens new window)</span></span></a>）。</p> <div class="language-js line-numbers-mode"><pre class="language-js"><code>f1<span class="token punctuation">.</span><span class="token function">on</span><span class="token punctuation">(</span><span class="token string">'done'</span><span class="token punctuation">,</span> f2<span class="token punctuation">)</span><span class="token punctuation">;</span><span class="token comment">// done事件被执行后，就执行f2</span>
    49. 

  49. </code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br></div></div><p>上面这行代码的意思是，当<code>f1</code>发生<code>done</code>事件，就执行<code>f2</code>。然后，对<code>f1</code>进行改写：</p> <div class="language-js line-numbers-mode"><pre class="language-js"><code><span class="token keyword">function</span> <span class="token function">f1</span><span class="token punctuation">(</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
    50. 

  50.   <span class="token function">setTimeout</span><span class="token punctuation">(</span><span class="token keyword">function</span> <span class="token punctuation">(</span><span class="token punctuation">)</span> <span class="token punctuation">{</span><span class="token comment">// 异步任务</span>
    51. 

  51.     <span class="token comment">// ...</span>
    52. 

  52.     f1<span class="token punctuation">.</span><span class="token function">trigger</span><span class="token punctuation">(</span><span class="token string">'done'</span><span class="token punctuation">)</span><span class="token punctuation">;</span> <span class="token comment">// trigger表示触发事件done</span>
    53. 

  53.   <span class="token punctuation">}</span><span class="token punctuation">,</span> <span class="token number">1000</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
    54. 

  54. <span class="token punctuation">}</span>
    55. 

  55. 

  56. <span class="token comment">// 优点：比较容易理解，可以绑定多个事件，每个事件可以指定多个回调函数，而且可以去耦合，有利于实现模块化。</span>
    57. 

  57. <span class="token comment">// 缺点：整个程序都要变成事件驱动型，运行流程会变得很不清晰。阅读代码的时候，很难看出主流程。</span>
    58. 

  58. </code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br><span class="line-number">3</span><br><span class="line-number">4</span><br><span class="line-number">5</span><br><span class="line-number">6</span><br><span class="line-number">7</span><br><span class="line-number">8</span><br><span class="line-number">9</span><br></div></div><p>上面代码中，<code>f1.trigger('done')</code>表示，执行完成后，立即触发<code>done</code>事件，从而开始执行<code>f2</code>。</p> <p>这种方法的优点是比较容易理解，可以绑定多个事件，每个事件可以指定多个回调函数，而且可以“<a href="https://en.wikipedia.org/wiki/Decoupling" target="_blank" rel="noopener noreferrer">去耦合<span><svg xmlns="http://www.w3.org/2000/svg" aria-hidden="true" focusable="false" x="0px" y="0px" viewBox="0 0 100 100" width="15" height="15" class="icon outbound"><path fill="currentColor" d="M18.8,85.1h56l0,0c2.2,0,4-1.8,4-4v-32h-8v28h-48v-48h28v-8h-32l0,0c-2.2,0-4,1.8-4,4v56C14.8,83.3,16.6,85.1,18.8,85.1z"></path> <polygon fill="currentColor" points="45.7,48.7 51.3,54.3 77.2,28.5 77.2,37.2 85.2,37.2 85.2,14.9 62.8,14.9 62.8,22.9 71.5,22.9"></polygon></svg> <span class="sr-only">(opens new window)</span></span></a>”（decoupling），有利于实现模块化。缺点是整个程序都要变成事件驱动型，运行流程会变得很不清晰。阅读代码的时候，很难看出主流程。</p> <h4 id="_3-发布-订阅"><a href="#_3-发布-订阅" class="header-anchor">#</a> （3）发布/订阅</h4> <p>事件完全可以理解成“信号”，如果存在一个“信号中心”，某个任务执行完成，就向信号中心“发布”（publish）一个信号，其他任务可以向信号中心“订阅”（subscribe）这个信号，从而知道什么时候自己可以开始执行。这就叫做”<a href="https://en.wikipedia.org/wiki/Publish-subscribe_pattern" target="_blank" rel="noopener noreferrer">发布/订阅模式<span><svg xmlns="http://www.w3.org/2000/svg" aria-hidden="true" focusable="false" x="0px" y="0px" viewBox="0 0 100 100" width="15" height="15" class="icon outbound"><path fill="currentColor" d="M18.8,85.1h56l0,0c2.2,0,4-1.8,4-4v-32h-8v28h-48v-48h28v-8h-32l0,0c-2.2,0-4,1.8-4,4v56C14.8,83.3,16.6,85.1,18.8,85.1z"></path> <polygon fill="currentColor" points="45.7,48.7 51.3,54.3 77.2,28.5 77.2,37.2 85.2,37.2 85.2,14.9 62.8,14.9 62.8,22.9 71.5,22.9"></polygon></svg> <span class="sr-only">(opens new window)</span></span></a>”（publish-subscribe pattern），又称“<a href="https://en.wikipedia.org/wiki/Observer_pattern" target="_blank" rel="noopener noreferrer">观察者模式<span><svg xmlns="http://www.w3.org/2000/svg" aria-hidden="true" focusable="false" x="0px" y="0px" viewBox="0 0 100 100" width="15" height="15" class="icon outbound"><path fill="currentColor" d="M18.8,85.1h56l0,0c2.2,0,4-1.8,4-4v-32h-8v28h-48v-48h28v-8h-32l0,0c-2.2,0-4,1.8-4,4v56C14.8,83.3,16.6,85.1,18.8,85.1z"></path> <polygon fill="currentColor" points="45.7,48.7 51.3,54.3 77.2,28.5 77.2,37.2 85.2,37.2 85.2,14.9 62.8,14.9 62.8,22.9 71.5,22.9"></polygon></svg> <span class="sr-only">(opens new window)</span></span></a>”（observer pattern）。</p> <p>这个模式有多种<a href="https://msdn.microsoft.com/en-us/magazine/hh201955.aspx" target="_blank" rel="noopener noreferrer">实现<span><svg xmlns="http://www.w3.org/2000/svg" aria-hidden="true" focusable="false" x="0px" y="0px" viewBox="0 0 100 100" width="15" height="15" class="icon outbound"><path fill="currentColor" d="M18.8,85.1h56l0,0c2.2,0,4-1.8,4-4v-32h-8v28h-48v-48h28v-8h-32l0,0c-2.2,0-4,1.8-4,4v56C14.8,83.3,16.6,85.1,18.8,85.1z"></path> <polygon fill="currentColor" points="45.7,48.7 51.3,54.3 77.2,28.5 77.2,37.2 85.2,37.2 85.2,14.9 62.8,14.9 62.8,22.9 71.5,22.9"></polygon></svg> <span class="sr-only">(opens new window)</span></span></a>，下面采用的是 Ben Alman 的 <a href="https://gist.github.com/661855" target="_blank" rel="noopener noreferrer">Tiny Pub/Sub<span><svg xmlns="http://www.w3.org/2000/svg" aria-hidden="true" focusable="false" x="0px" y="0px" viewBox="0 0 100 100" width="15" height="15" class="icon outbound"><path fill="currentColor" d="M18.8,85.1h56l0,0c2.2,0,4-1.8,4-4v-32h-8v28h-48v-48h28v-8h-32l0,0c-2.2,0-4,1.8-4,4v56C14.8,83.3,16.6,85.1,18.8,85.1z"></path> <polygon fill="currentColor" points="45.7,48.7 51.3,54.3 77.2,28.5 77.2,37.2 85.2,37.2 85.2,14.9 62.8,14.9 62.8,22.9 71.5,22.9"></polygon></svg> <span class="sr-only">(opens new window)</span></span></a>，这是 jQuery 的一个插件。</p> <p>首先，<code>f2</code>向信号中心<code>jQuery</code>订阅<code>done</code>信号。</p> <div class="language-js line-numbers-mode"><pre class="language-js"><code>jQuery<span class="token punctuation">.</span><span class="token function">subscribe</span><span class="token punctuation">(</span><span class="token string">'done'</span><span class="token punctuation">,</span> f2<span class="token punctuation">)</span><span class="token punctuation">;</span><span class="token comment">// 订阅done信号，收到信号执行f2</span>
    59. 

  59. </code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br></div></div><p>然后，<code>f1</code>进行如下改写。</p> <div class="language-js line-numbers-mode"><pre class="language-js"><code><span class="token keyword">function</span> <span class="token function">f1</span><span class="token punctuation">(</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
    60. 

  60.   <span class="token function">setTimeout</span><span class="token punctuation">(</span><span class="token keyword">function</span> <span class="token punctuation">(</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
    61. 

  61.     <span class="token comment">// ...</span>
    62. 

  62.     jQuery<span class="token punctuation">.</span><span class="token function">publish</span><span class="token punctuation">(</span><span class="token string">'done'</span><span class="token punctuation">)</span><span class="token punctuation">;</span> <span class="token comment">// 发布done信号</span>
    63. 

  63.   <span class="token punctuation">}</span><span class="token punctuation">,</span> <span class="token number">1000</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
    64. 

  64. <span class="token punctuation">}</span>
    65. 

  65. </code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br><span class="line-number">3</span><br><span class="line-number">4</span><br><span class="line-number">5</span><br><span class="line-number">6</span><br></div></div><p>上面代码中，<code>jQuery.publish('done')</code>的意思是，<code>f1</code>执行完成后，向信号中心<code>jQuery</code>发布<code>done</code>信号，从而引发<code>f2</code>的执行。</p> <p><code>f2</code>完成执行后，可以取消订阅（unsubscribe）。</p> <div class="language-js line-numbers-mode"><pre class="language-js"><code>jQuery<span class="token punctuation">.</span><span class="token function">unsubscribe</span><span class="token punctuation">(</span><span class="token string">'done'</span><span class="token punctuation">,</span> f2<span class="token punctuation">)</span><span class="token punctuation">;</span> <span class="token comment">// 取消订阅done信号</span>
    66. 

  66. </code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br></div></div><p>这种方法的性质与“事件监听”类似，但是明显优于后者。因为可以通过查看“消息中心”，了解存在多少信号、每个信号有多少订阅者，从而监控程序的运行。</p> <h3 id="_5、异步操作的流程控制"><a href="#_5、异步操作的流程控制" class="header-anchor">#</a> 5、异步操作的流程控制</h3> <p>如果有<strong>多个异步操作，就存在一个流程控制的问题</strong>：如何确定异步操作执行的顺序，以及如何保证遵守这种顺序。</p> <div class="language-js line-numbers-mode"><pre class="language-js"><code><span class="token keyword">function</span> <span class="token keyword">async</span><span class="token punctuation">(</span><span class="token parameter">arg<span class="token punctuation">,</span> callback</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
    67. 

  67.   console<span class="token punctuation">.</span><span class="token function">log</span><span class="token punctuation">(</span><span class="token string">'参数为 '</span> <span class="token operator">+</span> arg <span class="token operator">+</span><span class="token string">' , 1秒后返回结果'</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
    68. 

  68.   <span class="token function">setTimeout</span><span class="token punctuation">(</span><span class="token keyword">function</span> <span class="token punctuation">(</span><span class="token punctuation">)</span> <span class="token punctuation">{</span> <span class="token function">callback</span><span class="token punctuation">(</span>arg <span class="token operator">*</span> <span class="token number">2</span><span class="token punctuation">)</span><span class="token punctuation">;</span> <span class="token punctuation">}</span><span class="token punctuation">,</span> <span class="token number">1000</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
    69. 

  69. <span class="token punctuation">}</span>
    70. 

  70. </code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br><span class="line-number">3</span><br><span class="line-number">4</span><br></div></div><p>上面代码的<code>async</code>函数是一个异步任务，非常耗时，每次执行需要1秒才能完成，然后再调用回调函数。</p> <p>如果有六个这样的异步任务，需要全部完成后，才能执行最后的<code>final</code>函数。请问应该如何安排操作流程？</p> <div class="language-js line-numbers-mode"><pre class="language-js"><code><span class="token keyword">function</span> <span class="token function">final</span><span class="token punctuation">(</span><span class="token parameter">value</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
    71. 

  71.   console<span class="token punctuation">.</span><span class="token function">log</span><span class="token punctuation">(</span><span class="token string">'完成: '</span><span class="token punctuation">,</span> value<span class="token punctuation">)</span><span class="token punctuation">;</span>
    72. 

  72. <span class="token punctuation">}</span>
    73. 

  73. 

  74. <span class="token keyword">async</span><span class="token punctuation">(</span><span class="token number">1</span><span class="token punctuation">,</span> <span class="token keyword">function</span> <span class="token punctuation">(</span><span class="token parameter">value</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
    75. 

  75.   <span class="token keyword">async</span><span class="token punctuation">(</span><span class="token number">2</span><span class="token punctuation">,</span> <span class="token keyword">function</span> <span class="token punctuation">(</span><span class="token parameter">value</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
    76. 

  76.     <span class="token keyword">async</span><span class="token punctuation">(</span><span class="token number">3</span><span class="token punctuation">,</span> <span class="token keyword">function</span> <span class="token punctuation">(</span><span class="token parameter">value</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
    77. 

  77.       <span class="token keyword">async</span><span class="token punctuation">(</span><span class="token number">4</span><span class="token punctuation">,</span> <span class="token keyword">function</span> <span class="token punctuation">(</span><span class="token parameter">value</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
    78. 

  78.         <span class="token keyword">async</span><span class="token punctuation">(</span><span class="token number">5</span><span class="token punctuation">,</span> <span class="token keyword">function</span> <span class="token punctuation">(</span><span class="token parameter">value</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
    79. 

  79.           <span class="token keyword">async</span><span class="token punctuation">(</span><span class="token number">6</span><span class="token punctuation">,</span> final<span class="token punctuation">)</span><span class="token punctuation">;</span>
    80. 

  80.         <span class="token punctuation">}</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
    81. 

  81.       <span class="token punctuation">}</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
    82. 

  82.     <span class="token punctuation">}</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
    83. 

  83.   <span class="token punctuation">}</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
    84. 

  84. <span class="token punctuation">}</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
    85. 

  85. <span class="token comment">// 参数为 1 , 1秒后返回结果</span>
    86. 

  86. <span class="token comment">// 参数为 2 , 1秒后返回结果</span>
    87. 

  87. <span class="token comment">// 参数为 3 , 1秒后返回结果</span>
    88. 

  88. <span class="token comment">// 参数为 4 , 1秒后返回结果</span>
    89. 

  89. <span class="token comment">// 参数为 5 , 1秒后返回结果</span>
    90. 

  90. <span class="token comment">// 参数为 6 , 1秒后返回结果</span>
    91. 

  91. <span class="token comment">// 完成:  12</span>
    92. 

  92. </code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br><span class="line-number">3</span><br><span class="line-number">4</span><br><span class="line-number">5</span><br><span class="line-number">6</span><br><span class="line-number">7</span><br><span class="line-number">8</span><br><span class="line-number">9</span><br><span class="line-number">10</span><br><span class="line-number">11</span><br><span class="line-number">12</span><br><span class="line-number">13</span><br><span class="line-number">14</span><br><span class="line-number">15</span><br><span class="line-number">16</span><br><span class="line-number">17</span><br><span class="line-number">18</span><br><span class="line-number">19</span><br><span class="line-number">20</span><br><span class="line-number">21</span><br><span class="line-number">22</span><br></div></div><p>上面代码中，六个回调函数的嵌套，不仅写起来麻烦，容易出错，而且难以维护。</p> <h4 id="_1-串行执行"><a href="#_1-串行执行" class="header-anchor">#</a> （1）串行执行</h4> <p>我们可以编写一个流程控制函数，让它来控制异步任务，一个任务完成以后，再执行另一个。这就叫串行执行。</p> <div class="language-js line-numbers-mode"><pre class="language-js"><code><span class="token keyword">var</span> items <span class="token operator">=</span> <span class="token punctuation">[</span> <span class="token number">1</span><span class="token punctuation">,</span> <span class="token number">2</span><span class="token punctuation">,</span> <span class="token number">3</span><span class="token punctuation">,</span> <span class="token number">4</span><span class="token punctuation">,</span> <span class="token number">5</span><span class="token punctuation">,</span> <span class="token number">6</span> <span class="token punctuation">]</span><span class="token punctuation">;</span>
    93. 

  93. <span class="token keyword">var</span> results <span class="token operator">=</span> <span class="token punctuation">[</span><span class="token punctuation">]</span><span class="token punctuation">;</span>
    94. 

  94. 

  95. <span class="token keyword">function</span> <span class="token keyword">async</span><span class="token punctuation">(</span><span class="token parameter">arg<span class="token punctuation">,</span> callback</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
    96. 

  96.   console<span class="token punctuation">.</span><span class="token function">log</span><span class="token punctuation">(</span><span class="token string">'参数为 '</span> <span class="token operator">+</span> arg <span class="token operator">+</span><span class="token string">' , 1秒后返回结果'</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
    97. 

  97.   <span class="token function">setTimeout</span><span class="token punctuation">(</span><span class="token keyword">function</span> <span class="token punctuation">(</span><span class="token punctuation">)</span> <span class="token punctuation">{</span> <span class="token function">callback</span><span class="token punctuation">(</span>arg <span class="token operator">*</span> <span class="token number">2</span><span class="token punctuation">)</span><span class="token punctuation">;</span> <span class="token punctuation">}</span><span class="token punctuation">,</span> <span class="token number">1000</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
    98. 

  98. <span class="token punctuation">}</span>
    99. 

  99. 

  100. <span class="token keyword">function</span> <span class="token function">final</span><span class="token punctuation">(</span><span class="token parameter">value</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
    101. 

  101.   console<span class="token punctuation">.</span><span class="token function">log</span><span class="token punctuation">(</span><span class="token string">'完成: '</span><span class="token punctuation">,</span> value<span class="token punctuation">)</span><span class="token punctuation">;</span>
    102. 

  102. <span class="token punctuation">}</span>
    103. 

  103. 

  104. <span class="token keyword">function</span> <span class="token function">series</span><span class="token punctuation">(</span><span class="token parameter">item</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
    105. 

  105.   <span class="token keyword">if</span><span class="token punctuation">(</span>item<span class="token punctuation">)</span> <span class="token punctuation">{</span>
    106. 

  106.     <span class="token keyword">async</span><span class="token punctuation">(</span> item<span class="token punctuation">,</span> <span class="token keyword">function</span><span class="token punctuation">(</span><span class="token parameter">result</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
    107. 

  107.       results<span class="token punctuation">.</span><span class="token function">push</span><span class="token punctuation">(</span>result<span class="token punctuation">)</span><span class="token punctuation">;</span>
    108. 

  108.       <span class="token keyword">return</span> <span class="token function">series</span><span class="token punctuation">(</span>items<span class="token punctuation">.</span><span class="token function">shift</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
    109. 

  109.     <span class="token punctuation">}</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
    110. 

  110.   <span class="token punctuation">}</span> <span class="token keyword">else</span> <span class="token punctuation">{</span>
    111. 

  111.       <span class="token comment">// results =&gt; [2,4,6,8,10,12]</span>
    112. 

  112.     <span class="token keyword">return</span> <span class="token function">final</span><span class="token punctuation">(</span>results<span class="token punctuation">[</span>results<span class="token punctuation">.</span>length <span class="token operator">-</span> <span class="token number">1</span><span class="token punctuation">]</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
    113. 

  113.   <span class="token punctuation">}</span>
    114. 

  114. <span class="token punctuation">}</span>
    115. 

  115. 

  116. <span class="token function">series</span><span class="token punctuation">(</span>items<span class="token punctuation">.</span><span class="token function">shift</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
    117. 

  117. </code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br><span class="line-number">3</span><br><span class="line-number">4</span><br><span class="line-number">5</span><br><span class="line-number">6</span><br><span class="line-number">7</span><br><span class="line-number">8</span><br><span class="line-number">9</span><br><span class="line-number">10</span><br><span class="line-number">11</span><br><span class="line-number">12</span><br><span class="line-number">13</span><br><span class="line-number">14</span><br><span class="line-number">15</span><br><span class="line-number">16</span><br><span class="line-number">17</span><br><span class="line-number">18</span><br><span class="line-number">19</span><br><span class="line-number">20</span><br><span class="line-number">21</span><br><span class="line-number">22</span><br><span class="line-number">23</span><br><span class="line-number">24</span><br><span class="line-number">25</span><br></div></div><p>上面代码中，函数<code>series</code>就是串行函数，它会依次执行异步任务，所有任务都完成后，才会执行<code>final</code>函数。<code>items</code>数组保存每一个异步任务的参数，<code>results</code>数组保存每一个异步任务的运行结果。</p> <p>注意，<strong>上面的写法需要六秒，才能完成整个脚本</strong>。</p> <h4 id="_2-并行执行"><a href="#_2-并行执行" class="header-anchor">#</a> （2）并行执行</h4> <p>流程控制函数也可以是并行执行，即所有异步任务同时执行，等到全部完成以后，才执行<code>final</code>函数。</p> <div class="language-js line-numbers-mode"><pre class="language-js"><code><span class="token keyword">var</span> items <span class="token operator">=</span> <span class="token punctuation">[</span> <span class="token number">1</span><span class="token punctuation">,</span> <span class="token number">2</span><span class="token punctuation">,</span> <span class="token number">3</span><span class="token punctuation">,</span> <span class="token number">4</span><span class="token punctuation">,</span> <span class="token number">5</span><span class="token punctuation">,</span> <span class="token number">6</span> <span class="token punctuation">]</span><span class="token punctuation">;</span>
    118. 

  118. <span class="token keyword">var</span> results <span class="token operator">=</span> <span class="token punctuation">[</span><span class="token punctuation">]</span><span class="token punctuation">;</span>
    119. 

  119. 

  120. <span class="token keyword">function</span> <span class="token keyword">async</span><span class="token punctuation">(</span><span class="token parameter">arg<span class="token punctuation">,</span> callback</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
    121. 

  121.   console<span class="token punctuation">.</span><span class="token function">log</span><span class="token punctuation">(</span><span class="token string">'参数为 '</span> <span class="token operator">+</span> arg <span class="token operator">+</span><span class="token string">' , 1秒后返回结果'</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
    122. 

  122.   <span class="token function">setTimeout</span><span class="token punctuation">(</span><span class="token keyword">function</span> <span class="token punctuation">(</span><span class="token punctuation">)</span> <span class="token punctuation">{</span> <span class="token function">callback</span><span class="token punctuation">(</span>arg <span class="token operator">*</span> <span class="token number">2</span><span class="token punctuation">)</span><span class="token punctuation">;</span> <span class="token punctuation">}</span><span class="token punctuation">,</span> <span class="token number">1000</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
    123. 

  123. <span class="token punctuation">}</span>
    124. 

  124. 

  125. <span class="token keyword">function</span> <span class="token function">final</span><span class="token punctuation">(</span><span class="token parameter">value</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
    126. 

  126.   console<span class="token punctuation">.</span><span class="token function">log</span><span class="token punctuation">(</span><span class="token string">'完成: '</span><span class="token punctuation">,</span> value<span class="token punctuation">)</span><span class="token punctuation">;</span>
    127. 

  127. <span class="token punctuation">}</span>
    128. 

  128. 

  129. items<span class="token punctuation">.</span><span class="token function">forEach</span><span class="token punctuation">(</span><span class="token keyword">function</span><span class="token punctuation">(</span><span class="token parameter">item</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
    130. 

  130.   <span class="token keyword">async</span><span class="token punctuation">(</span>item<span class="token punctuation">,</span> <span class="token keyword">function</span><span class="token punctuation">(</span><span class="token parameter">result</span><span class="token punctuation">)</span><span class="token punctuation">{</span>
    131. 

  131.     results<span class="token punctuation">.</span><span class="token function">push</span><span class="token punctuation">(</span>result<span class="token punctuation">)</span><span class="token punctuation">;</span>
    132. 

  132.     <span class="token keyword">if</span><span class="token punctuation">(</span>results<span class="token punctuation">.</span>length <span class="token operator">===</span> items<span class="token punctuation">.</span>length<span class="token punctuation">)</span> <span class="token punctuation">{</span>
    133. 

  133.       <span class="token function">final</span><span class="token punctuation">(</span>results<span class="token punctuation">[</span>results<span class="token punctuation">.</span>length <span class="token operator">-</span> <span class="token number">1</span><span class="token punctuation">]</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
    134. 

  134.     <span class="token punctuation">}</span>
    135. 

  135.   <span class="token punctuation">}</span><span class="token punctuation">)</span>
    136. 

  136. <span class="token punctuation">}</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
    137. 

  137. </code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br><span class="line-number">3</span><br><span class="line-number">4</span><br><span class="line-number">5</span><br><span class="line-number">6</span><br><span class="line-number">7</span><br><span class="line-number">8</span><br><span class="line-number">9</span><br><span class="line-number">10</span><br><span class="line-number">11</span><br><span class="line-number">12</span><br><span class="line-number">13</span><br><span class="line-number">14</span><br><span class="line-number">15</span><br><span class="line-number">16</span><br><span class="line-number">17</span><br><span class="line-number">18</span><br><span class="line-number">19</span><br><span class="line-number">20</span><br></div></div><p>上面代码中，<code>forEach</code>方法会同时发起六个异步任务，等到它们全部完成以后，才会执行<code>final</code>函数。</p> <p>相比而言，<strong>上面的写法只要一秒</strong>，就能完成整个脚本。这就是说，并行执行的效率较高，比起串行执行一次只能执行一个任务，较为节约时间。但是问题在于如果并行的<strong>任务较多，很容易耗尽系统资源</strong>，拖慢运行速度。因此有了第三种流程控制方式。</p> <h4 id="_3-并行与串行的结合"><a href="#_3-并行与串行的结合" class="header-anchor">#</a> （3）并行与串行的结合</h4> <p>所谓并行与串行的结合，就是设置一个门槛，每次最多只能并行执行<code>n</code>个异步任务，这样就避免了过分占用系统资源。</p> <div class="language-js line-numbers-mode"><pre class="language-js"><code><span class="token keyword">var</span> items <span class="token operator">=</span> <span class="token punctuation">[</span> <span class="token number">1</span><span class="token punctuation">,</span> <span class="token number">2</span><span class="token punctuation">,</span> <span class="token number">3</span><span class="token punctuation">,</span> <span class="token number">4</span><span class="token punctuation">,</span> <span class="token number">5</span><span class="token punctuation">,</span> <span class="token number">6</span> <span class="token punctuation">]</span><span class="token punctuation">;</span>
    138. 

  138. <span class="token keyword">var</span> results <span class="token operator">=</span> <span class="token punctuation">[</span><span class="token punctuation">]</span><span class="token punctuation">;</span>
    139. 

  139. <span class="token keyword">var</span> running <span class="token operator">=</span> <span class="token number">0</span><span class="token punctuation">;</span> <span class="token comment">// 用于计算正在执行的任务数</span>
    140. 

  140. <span class="token keyword">var</span> limit <span class="token operator">=</span> <span class="token number">2</span><span class="token punctuation">;</span> <span class="token comment">// 最多可以有几个任务执行</span>
    141. 

  141. 

  142. <span class="token keyword">function</span> <span class="token keyword">async</span><span class="token punctuation">(</span><span class="token parameter">arg<span class="token punctuation">,</span> callback</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
    143. 

  143.   console<span class="token punctuation">.</span><span class="token function">log</span><span class="token punctuation">(</span><span class="token string">'参数为 '</span> <span class="token operator">+</span> arg <span class="token operator">+</span><span class="token string">' , 1秒后返回结果'</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
    144. 

  144.   <span class="token function">setTimeout</span><span class="token punctuation">(</span><span class="token keyword">function</span> <span class="token punctuation">(</span><span class="token punctuation">)</span> <span class="token punctuation">{</span> <span class="token function">callback</span><span class="token punctuation">(</span>arg <span class="token operator">*</span> <span class="token number">2</span><span class="token punctuation">)</span><span class="token punctuation">;</span> <span class="token punctuation">}</span><span class="token punctuation">,</span> <span class="token number">1000</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
    145. 

  145. <span class="token punctuation">}</span>
    146. 

  146. 

  147. <span class="token keyword">function</span> <span class="token function">final</span><span class="token punctuation">(</span><span class="token parameter">value</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
    148. 

  148.   console<span class="token punctuation">.</span><span class="token function">log</span><span class="token punctuation">(</span><span class="token string">'完成: '</span><span class="token punctuation">,</span> value<span class="token punctuation">)</span><span class="token punctuation">;</span>
    149. 

  149. <span class="token punctuation">}</span>
    150. 

  150. 

  151. <span class="token keyword">function</span> <span class="token function">launcher</span><span class="token punctuation">(</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
    152. 

  152.   <span class="token keyword">while</span><span class="token punctuation">(</span>running <span class="token operator">&lt;</span> limit <span class="token operator">&amp;&amp;</span> items<span class="token punctuation">.</span>length <span class="token operator">&gt;</span> <span class="token number">0</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
    153. 

  153.     <span class="token keyword">var</span> item <span class="token operator">=</span> items<span class="token punctuation">.</span><span class="token function">shift</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
    154. 

  154.     <span class="token keyword">async</span><span class="token punctuation">(</span>item<span class="token punctuation">,</span> <span class="token keyword">function</span><span class="token punctuation">(</span><span class="token parameter">result</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
    155. 

  155.       results<span class="token punctuation">.</span><span class="token function">push</span><span class="token punctuation">(</span>result<span class="token punctuation">)</span><span class="token punctuation">;</span>
    156. 

  156.       running<span class="token operator">--</span><span class="token punctuation">;</span>
    157. 

  157.       <span class="token keyword">if</span><span class="token punctuation">(</span>items<span class="token punctuation">.</span>length <span class="token operator">&gt;</span> <span class="token number">0</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
    158. 

  158.         <span class="token function">launcher</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
    159. 

  159.       <span class="token punctuation">}</span> <span class="token keyword">else</span> <span class="token keyword">if</span><span class="token punctuation">(</span>running <span class="token operator">==</span> <span class="token number">0</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
    160. 

  160.         <span class="token function">final</span><span class="token punctuation">(</span>results<span class="token punctuation">)</span><span class="token punctuation">;</span>
    161. 

  161.       <span class="token punctuation">}</span>
    162. 

  162.     <span class="token punctuation">}</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
    163. 

  163.     running<span class="token operator">++</span><span class="token punctuation">;</span>
    164. 

  164.   <span class="token punctuation">}</span>
    165. 

  165. <span class="token punctuation">}</span>
    166. 

  166. 

  167. <span class="token function">launcher</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
    168. 

  168. </code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br><span class="line-number">3</span><br><span class="line-number">4</span><br><span class="line-number">5</span><br><span class="line-number">6</span><br><span class="line-number">7</span><br><span class="line-number">8</span><br><span class="line-number">9</span><br><span class="line-number">10</span><br><span class="line-number">11</span><br><span class="line-number">12</span><br><span class="line-number">13</span><br><span class="line-number">14</span><br><span class="line-number">15</span><br><span class="line-number">16</span><br><span class="line-number">17</span><br><span class="line-number">18</span><br><span class="line-number">19</span><br><span class="line-number">20</span><br><span class="line-number">21</span><br><span class="line-number">22</span><br><span class="line-number">23</span><br><span class="line-number">24</span><br><span class="line-number">25</span><br><span class="line-number">26</span><br><span class="line-number">27</span><br><span class="line-number">28</span><br><span class="line-number">29</span><br><span class="line-number">30</span><br><span class="line-number">31</span><br></div></div><p>上面代码中，最多只能同时运行两个异步任务。变量<code>running</code>记录当前正在运行的任务数，只要低于门槛值，就再启动一个新的任务，如果等于<code>0</code>，就表示所有任务都执行完了，这时就执行<code>final</code>函数。</p> <p>这段代码需要三秒完成整个脚本，处在串行执行和并行执行之间。<strong>通过调节<code>limit</code>变量，达到效率和资源的最佳平衡</strong>。</p> <h2 id="文档"><a href="#文档" class="header-anchor">#</a> 文档</h2> <p>学习文档：<a href="https://wangdoc.com/javascript/" target="_blank" rel="noopener noreferrer">https://wangdoc.com/javascript/<span><svg xmlns="http://www.w3.org/2000/svg" aria-hidden="true" focusable="false" x="0px" y="0px" viewBox="0 0 100 100" width="15" height="15" class="icon outbound"><path fill="currentColor" d="M18.8,85.1h56l0,0c2.2,0,4-1.8,4-4v-32h-8v28h-48v-48h28v-8h-32l0,0c-2.2,0-4,1.8-4,4v56C14.8,83.3,16.6,85.1,18.8,85.1z"></path> <polygon fill="currentColor" points="45.7,48.7 51.3,54.3 77.2,28.5 77.2,37.2 85.2,37.2 85.2,14.9 62.8,14.9 62.8,22.9 71.5,22.9"></polygon></svg> <span class="sr-only">(opens new window)</span></span></a></p></div></div>  <div class="page-edit"><div class="edit-link"><a href="https://github.com/heBody/blog/edit/master/docs/《JavaScript教程》笔记/04.异步操作.md" target="_blank" rel="noopener noreferrer">编辑</a> <span><svg xmlns="http://www.w3.org/2000/svg" aria-hidden="true" focusable="false" x="0px" y="0px" viewBox="0 0 100 100" width="15" height="15" class="icon outbound"><path fill="currentColor" d="M18.8,85.1h56l0,0c2.2,0,4-1.8,4-4v-32h-8v28h-48v-48h28v-8h-32l0,0c-2.2,0-4,1.8-4,4v56C14.8,83.3,16.6,85.1,18.8,85.1z"></path> <polygon fill="currentColor" points="45.7,48.7 51.3,54.3 77.2,28.5 77.2,37.2 85.2,37.2 85.2,14.9 62.8,14.9 62.8,22.9 71.5,22.9"></polygon></svg> <span class="sr-only">(opens new window)</span></span></div> <div class="tags"><a href="/blog/tags/?tag=JavaScript" title="标签">#JavaScript</a></div> <div class="last-updated"><span class="prefix">上次更新:</span> <span class="time">2022/12/14, 19:36:42</span></div></div> <div class="page-nav-wapper"><div class="page-nav-centre-wrap"><a href="/blog/pages/659b5af5e2e704e0/" class="page-nav-centre page-nav-centre-prev"><div class="tooltip">面向对象</div></a> <a href="/blog/pages/7d961b8030c6099e/" class="page-nav-centre page-nav-centre-next"><div class="tooltip">DOM</div></a></div> <div class="page-nav"><p class="inner"><span class="prev">
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  174.             CSS给table的tbody添加滚动条
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