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(window.webpackJsonp=window.webpackJsonp||[]).push([[139],{466:function(s,t,a){"use strict";a.r(t);var n=a(3),e=Object(n.a)({},(function(){var s=this,t=s._self._c;return t("ContentSlotsDistributor",{attrs:{"slot-key":s.$parent.slotKey}},[t("h1",{attrs:{id:"异步操作"}},[t("a",{staticClass:"header-anchor",attrs:{href:"#异步操作"}},[s._v("#")]),s._v(" 异步操作")]),s._v(" "),t("h2",{attrs:{id:"一、异步操作概述"}},[t("a",{staticClass:"header-anchor",attrs:{href:"#一、异步操作概述"}},[s._v("#")]),s._v(" 一、异步操作概述")]),s._v(" "),t("h3",{attrs:{id:"_1、单线程模型"}},[t("a",{staticClass:"header-anchor",attrs:{href:"#_1、单线程模型"}},[s._v("#")]),s._v(" 1、单线程模型")]),s._v(" "),t("p",[s._v("单线程模型指的是，"),t("strong",[s._v("JavaScript 只在一个线程上运行")]),s._v("。也就是说，"),t("strong",[s._v("JavaScript 同时只能执行一个任务")]),s._v("，其他任务都必须在后面排队等待。")]),s._v(" "),t("p",[s._v("注意，"),t("strong",[s._v("JavaScript 只在一个线程上运行，不代表 JavaScript 引擎只有一个线程")]),s._v("。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。")]),s._v(" "),t("p",[s._v("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，"),t("strong",[s._v("为了避免复杂性")]),s._v("，"),t("strong",[s._v("JavaScript 一开始就是单线程")]),s._v("，这已经成了这门语言的核心特征，将来也不会改变。")]),s._v(" "),t("p",[s._v("这种模式的"),t("strong",[s._v("好处")]),s._v("是"),t("strong",[s._v("实现起来比较简单，执行环境相对单纯")]),s._v("；"),t("strong",[s._v("坏处")]),s._v("是"),t("strong",[s._v("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")]),s._v("。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。")]),s._v(" "),t("p",[s._v("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 "),t("strong",[s._v("IO 操作（输入输出）"),t("strong",[s._v("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，"),t("strong",[s._v("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")]),s._v("。这种机制就是 JavaScript 内部采用的")]),s._v("“事件循环”机制（Event Loop）")]),s._v("。")]),s._v(" "),t("p",[s._v("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 "),t("strong",[s._v("Node 可以用很少的资源，应付大流量访问")]),s._v("的原因。")]),s._v(" "),t("p",[s._v("为了利用多核 CPU 的计算能力，"),t("strong",[s._v("HTML5 提出 Web Worker 标准")]),s._v("，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。")]),s._v(" "),t("h3",{attrs:{id:"_2、同步任务和异步任务"}},[t("a",{staticClass:"header-anchor",attrs:{href:"#_2、同步任务和异步任务"}},[s._v("#")]),s._v(" 2、同步任务和异步任务")]),s._v(" "),t("p",[s._v("程序里面所有的任务，可以"),t("strong",[s._v("分成两类：同步任务（synchronous）和异步任务（asynchronous）")]),s._v("。")]),s._v(" "),t("p",[t("strong",[s._v("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")]),s._v("。只有前一个任务执行完毕，才能执行后一个任务。")]),s._v(" "),t("p",[t("strong",[s._v("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")]),s._v("。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），"),t("strong",[s._v("该任务（采用回调函数的形式）才会进入主线程执行")]),s._v("。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。")]),s._v(" "),t("p",[s._v("举例来说，"),t("strong",[s._v("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")]),s._v("。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。")]),s._v(" "),t("h3",{attrs:{id:"_3、任务队列和事件循环"}},[t("a",{staticClass:"header-anchor",attrs:{href:"#_3、任务队列和事件循环"}},[s._v("#")]),s._v(" 3、任务队列和事件循环")]),s._v(" "),t("p",[s._v("JavaScript 运行时，除了一个正在运行的主线程，引擎还提供一个"),t("strong",[s._v("任务队列")]),s._v("（task queue），"),t("strong",[s._v("里面是各种需要当前程序处理的异步任务")]),s._v("。（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。）")]),s._v(" "),t("p",[s._v("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。")]),s._v(" "),t("p",[t("strong",[s._v("异步任务的写法通常是回调函数")]),s._v("。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。")]),s._v(" "),t("p",[s._v("JavaScript 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"),t("div",{staticClass:"language-js line-numbers-mode"},[t("pre",{pre:!0,attrs:{class:"language-js"}},[t("code",[t("span",{pre:!0,attrs:{class:"token keyword"}},[s._v("function")]),s._v(" "),t("span",{pre:!0,attrs:{class:"token function"}},[s._v("f1")]),t("span",{pre:!0,attrs:{class:"token punctuation"}},[s._v("(")]),t("span",{pre:!0,attrs:{class:"token punctuation"}},[s._v(")")]),s._v(" "),t("span",{pre:!0,attrs:{class:"token punctuation"}},[s._v("{")]),s._v(" "),t("span",{pre:!0,attrs:{class:"token comment"}},[s._v("// 此函数是异步操作")]),s._v("\n  "),t("span",{pre:!0,attrs:{class:"token comment"}},[s._v("// ...")]),s._v("\n"),t("span",{pre:!0,attrs:{class:"token punctuation"}},[s._v("}")]),s._v("\n\n"),t("span",{pre:!0,attrs:{class:"token keyword"}},[s._v("function")]),s._v(" "),t("span",{pre:!0,attrs:{class:"token function"}},[s._v("f2")]),t("span",{pre:!0,attrs:{class:"token punctuation"}},[s._v("(")]),t("span",{pre:!0,attrs:{class:"token punctuation"}},[s._v(")")]),s._v(" 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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。")]),s._v("\n")])]),s._v(" 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noreferrer"}},[s._v("耦合"),t("OutboundLink")],1),s._v("（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。")]),s._v(" "),t("h4",{attrs:{id:"_2-事件监听"}},[t("a",{staticClass:"header-anchor",attrs:{href:"#_2-事件监听"}},[s._v("#")]),s._v(" （2）事件监听")]),s._v(" "),t("p",[s._v("另一种思路是采用事件驱动模式。"),t("strong",[s._v("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")]),s._v("。")]),s._v(" "),t("p",[s._v("还是以"),t("code",[s._v("f1")]),s._v("和"),t("code",[s._v("f2")]),s._v("为例。首先，为"),t("code",[s._v("f1")]),s._v("绑定一个事件（这里采用的 jQuery 的"),t("a",{attrs:{href:"https://api.jquery.com/on/",target:"_blank",rel:"noopener noreferrer"}},[s._v("写法"),t("OutboundLink")],1),s._v("）。")]),s._v(" "),t("div",{staticClass:"language-js line-numbers-mode"},[t("pre",{pre:!0,attrs:{class:"language-js"}},[t("code",[s._v("f1"),t("span",{pre:!0,attrs:{class:"token punctuation"}},[s._v(".")]),t("span",{pre:!0,attrs:{class:"token function"}},[s._v("on")]),t("span",{pre:!0,attrs:{class:"token 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"),t("a",{attrs:{href:"https://gist.github.com/661855",target:"_blank",rel:"noopener noreferrer"}},[s._v("Tiny Pub/Sub"),t("OutboundLink")],1),s._v("，这是 jQuery 的一个插件。")]),s._v(" "),t("p",[s._v("首先，"),t("code",[s._v("f2")]),s._v("向信号中心"),t("code",[s._v("jQuery")]),s._v("订阅"),t("code",[s._v("done")]),s._v("信号。")]),s._v(" "),t("div",{staticClass:"language-js line-numbers-mode"},[t("pre",{pre:!0,attrs:{class:"language-js"}},[t("code",[s._v("jQuery"),t("span",{pre:!0,attrs:{class:"token punctuation"}},[s._v(".")]),t("span",{pre:!0,attrs:{class:"token function"}},[s._v("subscribe")]),t("span",{pre:!0,attrs:{class:"token punctuation"}},[s._v("(")]),t("span",{pre:!0,attrs:{class:"token string"}},[s._v("'done'")]),t("span",{pre:!0,attrs:{class:"token punctuation"}},[s._v(",")]),s._v(" f2"),t("span",{pre:!0,attrs:{class:"token punctuation"}},[s._v(")")]),t("span",{pre:!0,attrs:{class:"token punctuation"}},[s._v(";")]),t("span",{pre:!0,attrs:{class:"token comment"}},[s._v("// 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    2.