text: (window.webpackJsonp=window.webpackJsonp||[]).push([[102],{557:function(t,s,a){"use strict";a.r(s);var n=a(21),e=Object(n.a)({},(function(){var t=this,s=t.$createElement,a=t._self._c||s;return a("ContentSlotsDistributor",{attrs:{"slot-key":t.$parent.slotKey}},[a("h1",{attrs:{id:"generator-函数的异步应用"}},[a("a",{staticClass:"header-anchor",attrs:{href:"#generator-函数的异步应用"}},[t._v("#")]),t._v(" Generator 函数的异步应用")]),t._v(" "),a("p",[t._v("异步编程对 JavaScript 语言太重要。JavaScript 语言的执行环境是“单线程”的，如果没有异步编程，根本没法用，非卡死不可。本章主要介绍 Generator 函数如何完成异步操作。\n")]),t._v(" "),a("h2",{attrs:{id:"传统方法"}},[a("a",{staticClass:"header-anchor",attrs:{href:"#传统方法"}},[t._v("#")]),t._v(" 传统方法")]),t._v(" "),a("p",[t._v("ES6 诞生以前，异步编程的方法，大概有下面四种。")]),t._v(" "),a("ul",[a("li",[t._v("回调函数")]),t._v(" "),a("li",[t._v("事件监听")]),t._v(" "),a("li",[t._v("发布/订阅")]),t._v(" "),a("li",[t._v("Promise 对象")])]),t._v(" "),a("p",[t._v("Generator 函数将 JavaScript 异步编程带入了一个全新的阶段。")]),t._v(" "),a("h2",{attrs:{id:"基本概念"}},[a("a",{staticClass:"header-anchor",attrs:{href:"#基本概念"}},[t._v("#")]),t._v(" 基本概念")]),t._v(" "),a("h3",{attrs:{id:"异步"}},[a("a",{staticClass:"header-anchor",attrs:{href:"#异步"}},[t._v("#")]),t._v(" 异步")]),t._v(" "),a("p",[t._v('所谓"异步"，简单说就是一个任务不是连续完成的，可以理解成该任务被人为分成两段，先执行第一段，然后转而执行其他任务，等做好了准备，再回过头执行第二段。')]),t._v(" "),a("p",[t._v("比如，有一个任务是读取文件进行处理，任务的第一段是向操作系统发出请求，要求读取文件。然后，程序执行其他任务，等到操作系统返回文件，再接着执行任务的第二段（处理文件）。这种不连续的执行，就叫做异步。")]),t._v(" "),a("p",[t._v("相应地，连续的执行就叫做同步。由于是连续执行，不能插入其他任务，所以操作系统从硬盘读取文件的这段时间，程序只能干等着。")]),t._v(" "),a("h3",{attrs:{id:"回调函数"}},[a("a",{staticClass:"header-anchor",attrs:{href:"#回调函数"}},[t._v("#")]),t._v(" 回调函数")]),t._v(" "),a("p",[t._v("JavaScript 语言对异步编程的实现，就是回调函数。所谓回调函数，就是把任务的第二段单独写在一个函数里面，等到重新执行这个任务的时候，就直接调用这个函数。回调函数的英语名字"),a("code",[t._v("callback")]),t._v('，直译过来就是"重新调用"。')]),t._v(" "),a("p",[t._v("读取文件进行处理，是这样写的。")]),t._v(" "),a("div",{staticClass:"language-javascript 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"),a("h3",{attrs:{id:"协程"}},[a("a",{staticClass:"header-anchor",attrs:{href:"#协程"}},[t._v("#")]),t._v(" 协程")]),t._v(" "),a("p",[t._v('传统的编程语言，早有异步编程的解决方案（其实是多任务的解决方案）。其中有一种叫做"协程"（coroutine），意思是'),a("strong",[t._v("多个线程互相协作，完成异步任务")]),t._v("。")]),t._v(" "),a("p",[t._v("协程有点像函数，又有点像线程。它的运行流程大致如下。")]),t._v(" "),a("ul",[a("li",[t._v("第一步，协程"),a("code",[t._v("A")]),t._v("开始执行。")]),t._v(" "),a("li",[t._v("第二步，协程"),a("code",[t._v("A")]),t._v("执行到一半，进入暂停，执行权转移到协程"),a("code",[t._v("B")]),t._v("。")]),t._v(" "),a("li",[t._v("第三步，（一段时间后）协程"),a("code",[t._v("B")]),t._v("交还执行权。")]),t._v(" "),a("li",[t._v("第四步，协程"),a("code",[t._v("A")]),t._v("恢复执行。")])]),t._v(" "),a("p",[t._v("上面流程的协程"),a("code",[t._v("A")]),t._v("，就是异步任务，因为它分成两段（或多段）执行。")]),t._v(" "),a("p",[t._v("举例来说，读取文件的协程写法如下。")]),t._v(" "),a("div",{staticClass:"language-javascript extra-class"},[a("pre",{pre:!0,attrs:{class:"language-javascript"}},[a("code",[a("span",{pre:!0,attrs:{class:"token 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punctuation"}},[t._v("(")]),t._v("data"),a("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v(")")]),a("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v(";")]),t._v("\n"),a("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v("}")]),a("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v(")")]),a("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v(";")]),t._v("\n")])])]),a("p",[t._v("上面代码中，首先执行 Generator 函数，获取遍历器对象，然后使用"),a("code",[t._v("next")]),t._v("方法（第二行），执行异步任务的第一阶段。由于"),a("code",[t._v("Fetch")]),t._v("模块返回的是一个 Promise 对象，因此要用"),a("code",[t._v("then")]),t._v("方法调用下一个"),a("code",[t._v("next")]),t._v("方法。")]),t._v(" "),a("p",[t._v("可以看到，"),a("strong",[t._v("虽然 Generator 函数将异步操作表示得很简洁，但是流程管理却不方便")]),t._v("（即何时执行第一阶段、何时执行第二阶段）。")]),t._v(" "),a("h2",{attrs:{id:"thunk-函数"}},[a("a",{staticClass:"header-anchor",attrs:{href:"#thunk-函数"}},[t._v("#")]),t._v(" Thunk 函数")]),t._v(" "),a("p",[t._v("Thunk 函数是自动执行 Generator 函数的一种方法。")]),t._v(" "),a("h3",{attrs:{id:"参数的求值策略"}},[a("a",{staticClass:"header-anchor",attrs:{href:"#参数的求值策略"}},[t._v("#")]),t._v(" 参数的求值策略")]),t._v(" "),a("p",[t._v("Thunk 函数早在上个世纪 60 年代就诞生了。")]),t._v(" "),a("p",[t._v('那时，编程语言刚刚起步，计算机学家还在研究，编译器怎么写比较好。一个争论的焦点是"求值策略"，即函数的参数到底应该何时求值。')]),t._v(" "),a("div",{staticClass:"language-javascript extra-class"},[a("pre",{pre:!0,attrs:{class:"language-javascript"}},[a("code",[a("span",{pre:!0,attrs:{class:"token keyword"}},[t._v("var")]),t._v(" x "),a("span",{pre:!0,attrs:{class:"token operator"}},[t._v("=")]),t._v(" "),a("span",{pre:!0,attrs:{class:"token number"}},[t._v("1")]),a("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v(";")]),t._v("\n\n"),a("span",{pre:!0,attrs:{class:"token keyword"}},[t._v("function")]),t._v(" "),a("span",{pre:!0,attrs:{class:"token function"}},[t._v("f")]),a("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v("(")]),a("span",{pre:!0,attrs:{class:"token parameter"}},[t._v("m")]),a("span",{pre:!0,attrs:{class:"token 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value），即在进入函数体之前，就计算'),a("code",[t._v("x + 5")]),t._v("的值（等于 6），再将这个值传入函数"),a("code",[t._v("f")]),t._v("。C 语言就采用这种策略。")]),t._v(" "),a("div",{staticClass:"language-javascript extra-class"},[a("pre",{pre:!0,attrs:{class:"language-javascript"}},[a("code",[a("span",{pre:!0,attrs:{class:"token function"}},[t._v("f")]),a("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v("(")]),t._v("x "),a("span",{pre:!0,attrs:{class:"token operator"}},[t._v("+")]),t._v(" "),a("span",{pre:!0,attrs:{class:"token number"}},[t._v("5")]),a("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v(")")]),t._v("\n"),a("span",{pre:!0,attrs:{class:"token comment"}},[t._v("// 传值调用时，等同于")]),t._v("\n"),a("span",{pre:!0,attrs:{class:"token function"}},[t._v("f")]),a("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v("(")]),a("span",{pre:!0,attrs:{class:"token number"}},[t._v("6")]),a("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v(")")]),t._v("\n")])])]),a("p",[t._v("另一种意见是“传名调用”（call by 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