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- (window.webpackJsonp=window.webpackJsonp||[]).push([[105],{432:function(t,s,a){"use strict";a.r(s);var n=a(3),e=Object(n.a)({},(function(){var t=this,s=t._self._c;return s("ContentSlotsDistributor",{attrs:{"slot-key":t.$parent.slotKey}},[s("h1",{attrs:{id:"generator-函数的异步应用"}},[s("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(" "),s("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(" "),s("h2",{attrs:{id:"传统方法"}},[s("a",{staticClass:"header-anchor",attrs:{href:"#传统方法"}},[t._v("#")]),t._v(" 传统方法")]),t._v(" "),s("p",[t._v("ES6 诞生以前，异步编程的方法，大概有下面四种。")]),t._v(" "),s("ul",[s("li",[t._v("回调函数")]),t._v(" "),s("li",[t._v("事件监听")]),t._v(" "),s("li",[t._v("发布/订阅")]),t._v(" "),s("li",[t._v("Promise 对象")])]),t._v(" "),s("p",[t._v("Generator 函数将 JavaScript 异步编程带入了一个全新的阶段。")]),t._v(" "),s("h2",{attrs:{id:"基本概念"}},[s("a",{staticClass:"header-anchor",attrs:{href:"#基本概念"}},[t._v("#")]),t._v(" 基本概念")]),t._v(" "),s("h3",{attrs:{id:"异步"}},[s("a",{staticClass:"header-anchor",attrs:{href:"#异步"}},[t._v("#")]),t._v(" 异步")]),t._v(" "),s("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(" "),s("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(" "),s("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(" "),s("h3",{attrs:{id:"回调函数"}},[s("a",{staticClass:"header-anchor",attrs:{href:"#回调函数"}},[t._v("#")]),t._v(" 回调函数")]),t._v(" "),s("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"),s("code",[t._v("callback")]),t._v('，直译过来就是"重新调用"。')]),t._v(" "),s("p",[t._v("读取文件进行处理，是这样写的。")]),t._v(" "),s("div",{staticClass:"language-javascript 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"),s("div",{staticClass:"line-numbers-wrapper"},[s("span",{staticClass:"line-number"},[t._v("1")]),s("br"),s("span",{staticClass:"line-number"},[t._v("2")]),s("br"),s("span",{staticClass:"line-number"},[t._v("3")]),s("br"),s("span",{staticClass:"line-number"},[t._v("4")]),s("br"),s("span",{staticClass:"line-number"},[t._v("5")]),s("br"),s("span",{staticClass:"line-number"},[t._v("6")]),s("br"),s("span",{staticClass:"line-number"},[t._v("7")]),s("br"),s("span",{staticClass:"line-number"},[t._v("8")]),s("br"),s("span",{staticClass:"line-number"},[t._v("9")]),s("br"),s("span",{staticClass:"line-number"},[t._v("10")]),s("br"),s("span",{staticClass:"line-number"},[t._v("11")]),s("br"),s("span",{staticClass:"line-number"},[t._v("12")]),s("br"),s("span",{staticClass:"line-number"},[t._v("13")]),s("br"),s("span",{staticClass:"line-number"},[t._v("14")]),s("br"),s("span",{staticClass:"line-number"},[t._v("15")]),s("br")])]),s("p",[t._v("上面代码中，我使用了"),s("code",[t._v("fs-readfile-promise")]),t._v("模块，它的作用就是返回一个 Promise 版本的"),s("code",[t._v("readFile")]),t._v("函数。Promise 提供"),s("code",[t._v("then")]),t._v("方法加载回调函数，"),s("code",[t._v("catch")]),t._v("方法捕捉执行过程中抛出的错误。")]),t._v(" "),s("p",[t._v("可以看到，Promise 的写法只是回调函数的改进，使用"),s("code",[t._v("then")]),t._v("方法以后，异步任务的两段执行看得更清楚了，除此以外，并无新意。")]),t._v(" "),s("p",[t._v("Promise 的最大问题是代码冗余，原来的任务被 Promise 包装了一下，不管什么操作，一眼看去都是一堆"),s("code",[t._v("then")]),t._v("，原来的语义变得很不清楚。")]),t._v(" "),s("p",[t._v("那么，有没有更好的写法呢？")]),t._v(" "),s("h2",{attrs:{id:"generator-函数"}},[s("a",{staticClass:"header-anchor",attrs:{href:"#generator-函数"}},[t._v("#")]),t._v(" Generator 函数")]),t._v(" "),s("h3",{attrs:{id:"协程"}},[s("a",{staticClass:"header-anchor",attrs:{href:"#协程"}},[t._v("#")]),t._v(" 协程")]),t._v(" "),s("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'),s("strong",[t._v("多个线程互相协作，完成异步任务")]),t._v("。")]),t._v(" "),s("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(" "),s("ul",[s("li",[t._v("第一步，协程"),s("code",[t._v("A")]),t._v("开始执行。")]),t._v(" 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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(" "),s("h2",{attrs:{id:"thunk-函数"}},[s("a",{staticClass:"header-anchor",attrs:{href:"#thunk-函数"}},[t._v("#")]),t._v(" Thunk 函数")]),t._v(" "),s("p",[t._v("Thunk 函数是自动执行 Generator 函数的一种方法。")]),t._v(" "),s("h3",{attrs:{id:"参数的求值策略"}},[s("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(" "),s("p",[t._v("Thunk 函数早在上个世纪 60 年代就诞生了。")]),t._v(" "),s("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(" "),s("div",{staticClass:"language-javascript line-numbers-mode"},[s("pre",{pre:!0,attrs:{class:"language-javascript"}},[s("code",[s("span",{pre:!0,attrs:{class:"token keyword"}},[t._v("var")]),t._v(" x "),s("span",{pre:!0,attrs:{class:"token operator"}},[t._v("=")]),t._v(" "),s("span",{pre:!0,attrs:{class:"token number"}},[t._v("1")]),s("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v(";")]),t._v("\n\n"),s("span",{pre:!0,attrs:{class:"token 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"),s("div",{staticClass:"line-numbers-wrapper"},[s("span",{staticClass:"line-number"},[t._v("1")]),s("br"),s("span",{staticClass:"line-number"},[t._v("2")]),s("br"),s("span",{staticClass:"line-number"},[t._v("3")]),s("br")])]),s("p",[t._v("另一种意见是“传名调用”（call by name），即直接将表达式"),s("code",[t._v("x + 5")]),t._v("传入函数体，只在用到它的时候求值。Haskell 语言采用这种策略。")]),t._v(" "),s("div",{staticClass:"language-javascript line-numbers-mode"},[s("pre",{pre:!0,attrs:{class:"language-javascript"}},[s("code",[s("span",{pre:!0,attrs:{class:"token function"}},[t._v("f")]),s("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v("(")]),t._v("x "),s("span",{pre:!0,attrs:{class:"token operator"}},[t._v("+")]),t._v(" "),s("span",{pre:!0,attrs:{class:"token number"}},[t._v("5")]),s("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v(")")]),t._v("\n"),s("span",{pre:!0,attrs:{class:"token comment"}},[t._v("// 传名调用时，等同于")]),t._v("\n"),s("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v("(")]),t._v("x 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"),s("div",{staticClass:"line-numbers-wrapper"},[s("span",{staticClass:"line-number"},[t._v("1")]),s("br"),s("span",{staticClass:"line-number"},[t._v("2")]),s("br"),s("span",{staticClass:"line-number"},[t._v("3")]),s("br"),s("span",{staticClass:"line-number"},[t._v("4")]),s("br"),s("span",{staticClass:"line-number"},[t._v("5")]),s("br"),s("span",{staticClass:"line-number"},[t._v("6")]),s("br"),s("span",{staticClass:"line-number"},[t._v("7")]),s("br"),s("span",{staticClass:"line-number"},[t._v("8")]),s("br")])]),s("p",[t._v("上面代码中，函数"),s("code",[t._v("g")]),t._v("封装了"),s("code",[t._v("n")]),t._v("个异步的读取文件操作，只要执行"),s("code",[t._v("run")]),t._v("函数，这些操作就会自动完成。这样一来，异步操作不仅可以写得像同步操作，而且一行代码就可以执行。")]),t._v(" "),s("p",[t._v("Thunk 函数并不是 Generator 函数自动执行的唯一方案。因为自动执行的关键是，必须有一种机制，自动控制 Generator 函数的流程，接收和交还程序的执行权。回调函数可以做到这一点，Promise 对象也可以做到这一点。")]),t._v(" "),s("h2",{attrs:{id:"co-模块"}},[s("a",{staticClass:"header-anchor",attrs:{href:"#co-模块"}},[t._v("#")]),t._v(" co 模块")]),t._v(" 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1120")]),t._v("\n"),s("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v("}")]),s("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v(")")]),s("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v(";")]),t._v("\n")])]),t._v(" "),s("div",{staticClass:"line-numbers-wrapper"},[s("span",{staticClass:"line-number"},[t._v("1")]),s("br"),s("span",{staticClass:"line-number"},[t._v("2")]),s("br"),s("span",{staticClass:"line-number"},[t._v("3")]),s("br"),s("span",{staticClass:"line-number"},[t._v("4")]),s("br"),s("span",{staticClass:"line-number"},[t._v("5")]),s("br"),s("span",{staticClass:"line-number"},[t._v("6")]),s("br"),s("span",{staticClass:"line-number"},[t._v("7")]),s("br"),s("span",{staticClass:"line-number"},[t._v("8")]),s("br"),s("span",{staticClass:"line-number"},[t._v("9")]),s("br"),s("span",{staticClass:"line-number"},[t._v("10")]),s("br"),s("span",{staticClass:"line-number"},[t._v("11")]),s("br"),s("span",{staticClass:"line-number"},[t._v("12")]),s("br"),s("span",{staticClass:"line-number"},[t._v("13")]),s("br"),s("span",{staticClass:"line-number"},[t._v("14")]),s("br"),s("span",{staticClass:"line-number"},[t._v("15")]),s("br"),s("span",{staticClass:"line-number"},[t._v("16")]),s("br"),s("span",{staticClass:"line-number"},[t._v("17")]),s("br"),s("span",{staticClass:"line-number"},[t._v("18")]),s("br"),s("span",{staticClass:"line-number"},[t._v("19")]),s("br"),s("span",{staticClass:"line-number"},[t._v("20")]),s("br"),s("span",{staticClass:"line-number"},[t._v("21")]),s("br"),s("span",{staticClass:"line-number"},[t._v("22")]),s("br"),s("span",{staticClass:"line-number"},[t._v("23")]),s("br")])]),s("p",[t._v("上面代码采用 Stream 模式读取《悲惨世界》的文本文件，对于每个数据块都使用"),s("code",[t._v("stream.once")]),t._v("方法，在"),s("code",[t._v("data")]),t._v("、"),s("code",[t._v("end")]),t._v("、"),s("code",[t._v("error")]),t._v("三个事件上添加一次性回调函数。变量"),s("code",[t._v("res")]),t._v("只有在"),s("code",[t._v("data")]),t._v("事件发生时才有值，然后累加每个数据块之中"),s("code",[t._v("valjean")]),t._v("这个词出现的次数。")])])}),[],!1,null,null,null);s.default=e.exports}}]);
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