| 1 |
- (window.webpackJsonp=window.webpackJsonp||[]).push([[115],{445: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:"arraybuffer"}},[t("a",{staticClass:"header-anchor",attrs:{href:"#arraybuffer"}},[s._v("#")]),s._v(" ArrayBuffer")]),s._v(" "),t("p",[t("code",[s._v("ArrayBuffer")]),s._v("对象、"),t("code",[s._v("TypedArray")]),s._v("视图和"),t("code",[s._v("DataView")]),s._v("视图是 JavaScript 操作二进制数据的一个接口。这些对象早就存在，属于独立的规格（2011 年 2 月发布），ES6 将它们纳入了 ECMAScript 规格，并且增加了新的方法。它们都是以数组的语法处理二进制数据，所以统称为二进制数组。\n"),s._v("\n这个接口的原始设计目的，与 WebGL 项目有关。所谓 WebGL，就是指浏览器与显卡之间的通信接口，为了满足 JavaScript 与显卡之间大量的、实时的数据交换，它们之间的数据通信必须是二进制的，而不能是传统的文本格式。文本格式传递一个 32 位整数，两端的 JavaScript 脚本与显卡都要进行格式转化，将非常耗时。这时要是存在一种机制，可以像 C 语言那样，直接操作字节，将 4 个字节的 32 位整数，以二进制形式原封不动地送入显卡，脚本的性能就会大幅提升。")]),s._v(" "),t("p",[s._v("二进制数组就是在这种背景下诞生的。它很像 C 语言的数组，允许开发者以数组下标的形式，直接操作内存，大大增强了 JavaScript 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字节的"),t("code",[s._v("ArrayBuffer")]),s._v("对象，然后在它的基础上，建立了一个 32 位整数的视图。由于每个 32 位整数占据 4 个字节，所以一共可以写入 4 个整数，依次为 0，2，4，6。")]),s._v(" "),t("p",[s._v("如果在这段数据上接着建立一个 16 位整数的视图，则可以读出完全不一样的结果。")]),s._v(" "),t("div",{staticClass:"language-javascript line-numbers-mode"},[t("pre",{pre:!0,attrs:{class:"language-javascript"}},[t("code",[t("span",{pre:!0,attrs:{class:"token keyword"}},[s._v("const")]),s._v(" int16View "),t("span",{pre:!0,attrs:{class:"token operator"}},[s._v("=")]),s._v(" "),t("span",{pre:!0,attrs:{class:"token keyword"}},[s._v("new")]),s._v(" "),t("span",{pre:!0,attrs:{class:"token class-name"}},[s._v("Int16Array")]),t("span",{pre:!0,attrs:{class:"token punctuation"}},[s._v("(")]),s._v("buffer"),t("span",{pre:!0,attrs:{class:"token punctuation"}},[s._v(")")]),t("span",{pre:!0,attrs:{class:"token punctuation"}},[s._v(";")]),s._v("\n\n"),t("span",{pre:!0,attrs:{class:"token keyword"}},[s._v("for")]),s._v(" "),t("span",{pre:!0,attrs:{class:"token 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"),t("h3",{attrs:{id:"typedarray-of"}},[t("a",{staticClass:"header-anchor",attrs:{href:"#typedarray-of"}},[s._v("#")]),s._v(" TypedArray.of()")]),s._v(" "),t("p",[s._v("TypedArray 数组的所有构造函数，都有一个静态方法"),t("code",[s._v("of")]),s._v("，用于将参数转为一个"),t("code",[s._v("TypedArray")]),s._v("实例。")]),s._v(" "),t("div",{staticClass:"language-javascript line-numbers-mode"},[t("pre",{pre:!0,attrs:{class:"language-javascript"}},[t("code",[s._v("Float32Array"),t("span",{pre:!0,attrs:{class:"token punctuation"}},[s._v(".")]),t("span",{pre:!0,attrs:{class:"token function"}},[s._v("of")]),t("span",{pre:!0,attrs:{class:"token punctuation"}},[s._v("(")]),t("span",{pre:!0,attrs:{class:"token number"}},[s._v("0.151")]),t("span",{pre:!0,attrs:{class:"token punctuation"}},[s._v(",")]),s._v(" "),t("span",{pre:!0,attrs:{class:"token operator"}},[s._v("-")]),t("span",{pre:!0,attrs:{class:"token number"}},[s._v("8")]),t("span",{pre:!0,attrs:{class:"token punctuation"}},[s._v(",")]),s._v(" 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class-name"}},[s._v("Uint8Array")]),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 number"}},[s._v("1")]),t("span",{pre:!0,attrs:{class:"token punctuation"}},[s._v(",")]),t("span",{pre:!0,attrs:{class:"token number"}},[s._v("2")]),t("span",{pre:!0,attrs:{class:"token punctuation"}},[s._v(",")]),t("span",{pre:!0,attrs:{class:"token number"}},[s._v("3")]),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 punctuation"}},[s._v(";")]),s._v("\n\n"),t("span",{pre:!0,attrs:{class:"token comment"}},[s._v("// 方法二")]),s._v("\n"),t("span",{pre:!0,attrs:{class:"token keyword"}},[s._v("let")]),s._v(" tarr "),t("span",{pre:!0,attrs:{class:"token operator"}},[s._v("=")]),s._v(" Uint8Array"),t("span",{pre:!0,attrs:{class:"token punctuation"}},[s._v(".")]),t("span",{pre:!0,attrs:{class:"token function"}},[s._v("of")]),t("span",{pre:!0,attrs:{class:"token punctuation"}},[s._v("(")]),t("span",{pre:!0,attrs:{class:"token number"}},[s._v("1")]),t("span",{pre:!0,attrs:{class:"token punctuation"}},[s._v(",")]),t("span",{pre:!0,attrs:{class:"token number"}},[s._v("2")]),t("span",{pre:!0,attrs:{class:"token punctuation"}},[s._v(",")]),t("span",{pre:!0,attrs:{class:"token number"}},[s._v("3")]),t("span",{pre:!0,attrs:{class:"token punctuation"}},[s._v(")")]),t("span",{pre:!0,attrs:{class:"token punctuation"}},[s._v(";")]),s._v("\n\n"),t("span",{pre:!0,attrs:{class:"token comment"}},[s._v("// 方法三")]),s._v("\n"),t("span",{pre:!0,attrs:{class:"token keyword"}},[s._v("let")]),s._v(" tarr "),t("span",{pre:!0,attrs:{class:"token operator"}},[s._v("=")]),s._v(" "),t("span",{pre:!0,attrs:{class:"token keyword"}},[s._v("new")]),s._v(" "),t("span",{pre:!0,attrs:{class:"token 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"),t("div",{staticClass:"line-numbers-wrapper"},[t("span",{staticClass:"line-number"},[s._v("1")]),t("br"),t("span",{staticClass:"line-number"},[s._v("2")]),t("br"),t("span",{staticClass:"line-number"},[s._v("3")]),t("br"),t("span",{staticClass:"line-number"},[s._v("4")]),t("br"),t("span",{staticClass:"line-number"},[s._v("5")]),t("br"),t("span",{staticClass:"line-number"},[s._v("6")]),t("br"),t("span",{staticClass:"line-number"},[s._v("7")]),t("br"),t("span",{staticClass:"line-number"},[s._v("8")]),t("br"),t("span",{staticClass:"line-number"},[s._v("9")]),t("br"),t("span",{staticClass:"line-number"},[s._v("10")]),t("br"),t("span",{staticClass:"line-number"},[s._v("11")]),t("br")])]),t("h3",{attrs:{id:"typedarray-from"}},[t("a",{staticClass:"header-anchor",attrs:{href:"#typedarray-from"}},[s._v("#")]),s._v(" TypedArray.from()")]),s._v(" "),t("p",[s._v("静态方法"),t("code",[s._v("from")]),s._v("接受一个可遍历的数据结构（比如数组）作为参数，返回一个基于这个结构的"),t("code",[s._v("TypedArray")]),s._v("实例。")]),s._v(" "),t("div",{staticClass:"language-javascript line-numbers-mode"},[t("pre",{pre:!0,attrs:{class:"language-javascript"}},[t("code",[s._v("Uint16Array"),t("span",{pre:!0,attrs:{class:"token punctuation"}},[s._v(".")]),t("span",{pre:!0,attrs:{class:"token function"}},[s._v("from")]),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 number"}},[s._v("0")]),t("span",{pre:!0,attrs:{class:"token punctuation"}},[s._v(",")]),s._v(" "),t("span",{pre:!0,attrs:{class:"token number"}},[s._v("1")]),t("span",{pre:!0,attrs:{class:"token punctuation"}},[s._v(",")]),s._v(" "),t("span",{pre:!0,attrs:{class:"token number"}},[s._v("2")]),t("span",{pre:!0,attrs:{class:"token punctuation"}},[s._v("]")]),t("span",{pre:!0,attrs:{class:"token punctuation"}},[s._v(")")]),s._v("\n"),t("span",{pre:!0,attrs:{class:"token comment"}},[s._v("// Uint16Array [ 0, 1, 2 ]")]),s._v("\n")])]),s._v(" "),t("div",{staticClass:"line-numbers-wrapper"},[t("span",{staticClass:"line-number"},[s._v("1")]),t("br"),t("span",{staticClass:"line-number"},[s._v("2")]),t("br")])]),t("p",[s._v("这个方法还可以将一种"),t("code",[s._v("TypedArray")]),s._v("实例，转为另一种。")]),s._v(" "),t("div",{staticClass:"language-javascript line-numbers-mode"},[t("pre",{pre:!0,attrs:{class:"language-javascript"}},[t("code",[t("span",{pre:!0,attrs:{class:"token keyword"}},[s._v("const")]),s._v(" ui16 "),t("span",{pre:!0,attrs:{class:"token operator"}},[s._v("=")]),s._v(" Uint16Array"),t("span",{pre:!0,attrs:{class:"token punctuation"}},[s._v(".")]),t("span",{pre:!0,attrs:{class:"token function"}},[s._v("from")]),t("span",{pre:!0,attrs:{class:"token punctuation"}},[s._v("(")]),s._v("Uint8Array"),t("span",{pre:!0,attrs:{class:"token punctuation"}},[s._v(".")]),t("span",{pre:!0,attrs:{class:"token function"}},[s._v("of")]),t("span",{pre:!0,attrs:{class:"token punctuation"}},[s._v("(")]),t("span",{pre:!0,attrs:{class:"token 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"),t("div",{staticClass:"line-numbers-wrapper"},[t("span",{staticClass:"line-number"},[s._v("1")]),t("br"),t("span",{staticClass:"line-number"},[s._v("2")]),t("br")])]),t("p",[t("code",[s._v("from")]),s._v("方法还可以接受一个函数，作为第二个参数，用来对每个元素进行遍历，功能类似"),t("code",[s._v("map")]),s._v("方法。")]),s._v(" "),t("div",{staticClass:"language-javascript line-numbers-mode"},[t("pre",{pre:!0,attrs:{class:"language-javascript"}},[t("code",[s._v("Int8Array"),t("span",{pre:!0,attrs:{class:"token punctuation"}},[s._v(".")]),t("span",{pre:!0,attrs:{class:"token function"}},[s._v("of")]),t("span",{pre:!0,attrs:{class:"token punctuation"}},[s._v("(")]),t("span",{pre:!0,attrs:{class:"token number"}},[s._v("127")]),t("span",{pre:!0,attrs:{class:"token punctuation"}},[s._v(",")]),s._v(" "),t("span",{pre:!0,attrs:{class:"token number"}},[s._v("126")]),t("span",{pre:!0,attrs:{class:"token punctuation"}},[s._v(",")]),s._v(" "),t("span",{pre:!0,attrs:{class:"token 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x"),t("span",{pre:!0,attrs:{class:"token punctuation"}},[s._v(")")]),s._v("\n"),t("span",{pre:!0,attrs:{class:"token comment"}},[s._v("// Int16Array [ 254, 252, 250 ]")]),s._v("\n")])]),s._v(" "),t("div",{staticClass:"line-numbers-wrapper"},[t("span",{staticClass:"line-number"},[s._v("1")]),t("br"),t("span",{staticClass:"line-number"},[s._v("2")]),t("br"),t("span",{staticClass:"line-number"},[s._v("3")]),t("br"),t("span",{staticClass:"line-number"},[s._v("4")]),t("br"),t("span",{staticClass:"line-number"},[s._v("5")]),t("br")])]),t("p",[s._v("上面的例子中，"),t("code",[s._v("from")]),s._v("方法没有发生溢出，这说明遍历不是针对原来的 8 位整数数组。也就是说，"),t("code",[s._v("from")]),s._v("会将第一个参数指定的 TypedArray 数组，拷贝到另一段内存之中，处理之后再将结果转成指定的数组格式。")]),s._v(" "),t("h2",{attrs:{id:"复合视图"}},[t("a",{staticClass:"header-anchor",attrs:{href:"#复合视图"}},[s._v("#")]),s._v(" 复合视图")]),s._v(" "),t("p",[s._v("由于视图的构造函数可以指定起始位置和长度，所以在同一段内存之中，可以依次存放不同类型的数据，这叫做“复合视图”。")]),s._v(" "),t("div",{staticClass:"language-javascript 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"),t("span",{pre:!0,attrs:{class:"token keyword"}},[s._v("new")]),s._v(" "),t("span",{pre:!0,attrs:{class:"token class-name"}},[s._v("DataView")]),t("span",{pre:!0,attrs:{class:"token punctuation"}},[s._v("(")]),s._v("buffer"),t("span",{pre:!0,attrs:{class:"token punctuation"}},[s._v(")")]),t("span",{pre:!0,attrs:{class:"token punctuation"}},[s._v(";")]),s._v("\n")])]),s._v(" "),t("div",{staticClass:"line-numbers-wrapper"},[t("span",{staticClass:"line-number"},[s._v("1")]),t("br"),t("span",{staticClass:"line-number"},[s._v("2")]),t("br")])]),t("p",[t("code",[s._v("DataView")]),s._v("实例有以下属性，含义与"),t("code",[s._v("TypedArray")]),s._v("实例的同名方法相同。")]),s._v(" "),t("ul",[t("li",[t("code",[s._v("DataView.prototype.buffer")]),s._v("：返回对应的 ArrayBuffer 对象")]),s._v(" "),t("li",[t("code",[s._v("DataView.prototype.byteLength")]),s._v("：返回占据的内存字节长度")]),s._v(" "),t("li",[t("code",[s._v("DataView.prototype.byteOffset")]),s._v("：返回当前视图从对应的 ArrayBuffer 对象的哪个字节开始")])]),s._v(" 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"),t("p",[s._v("如果一次读取两个或两个以上字节，就必须明确数据的存储方式，到底是小端字节序还是大端字节序。默认情况下，"),t("code",[s._v("DataView")]),s._v("的"),t("code",[s._v("get")]),s._v("方法使用大端字节序解读数据，如果需要使用小端字节序解读，必须在"),t("code",[s._v("get")]),s._v("方法的第二个参数指定"),t("code",[s._v("true")]),s._v("。")]),s._v(" "),t("div",{staticClass:"language-javascript line-numbers-mode"},[t("pre",{pre:!0,attrs:{class:"language-javascript"}},[t("code",[t("span",{pre:!0,attrs:{class:"token comment"}},[s._v("// 小端字节序")]),s._v("\n"),t("span",{pre:!0,attrs:{class:"token keyword"}},[s._v("const")]),s._v(" v1 "),t("span",{pre:!0,attrs:{class:"token operator"}},[s._v("=")]),s._v(" dv"),t("span",{pre:!0,attrs:{class:"token punctuation"}},[s._v(".")]),t("span",{pre:!0,attrs:{class:"token function"}},[s._v("getUint16")]),t("span",{pre:!0,attrs:{class:"token punctuation"}},[s._v("(")]),t("span",{pre:!0,attrs:{class:"token number"}},[s._v("1")]),t("span",{pre:!0,attrs:{class:"token punctuation"}},[s._v(",")]),s._v(" "),t("span",{pre:!0,attrs:{class:"token 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"),t("li",[t("strong",[t("code",[s._v("setInt32")])]),s._v("：写入 4 个字节的 32 位整数。")]),s._v(" "),t("li",[t("strong",[t("code",[s._v("setUint32")])]),s._v("：写入 4 个字节的 32 位无符号整数。")]),s._v(" "),t("li",[t("strong",[t("code",[s._v("setFloat32")])]),s._v("：写入 4 个字节的 32 位浮点数。")]),s._v(" "),t("li",[t("strong",[t("code",[s._v("setFloat64")])]),s._v("：写入 8 个字节的 64 位浮点数。")])]),s._v(" "),t("p",[s._v("这一系列"),t("code",[s._v("set")]),s._v("方法，接受两个参数，第一个参数是字节序号，表示从哪个字节开始写入，第二个参数为写入的数据。对于那些写入两个或两个以上字节的方法，需要指定第三个参数，"),t("code",[s._v("false")]),s._v("或者"),t("code",[s._v("undefined")]),s._v("表示使用大端字节序写入，"),t("code",[s._v("true")]),s._v("表示使用小端字节序写入。")]),s._v(" "),t("div",{staticClass:"language-javascript line-numbers-mode"},[t("pre",{pre:!0,attrs:{class:"language-javascript"}},[t("code",[t("span",{pre:!0,attrs:{class:"token comment"}},[s._v("// 在第1个字节，以大端字节序写入值为25的32位整数")]),s._v("\ndv"),t("span",{pre:!0,attrs:{class:"token punctuation"}},[s._v(".")]),t("span",{pre:!0,attrs:{class:"token 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ev"),t("span",{pre:!0,attrs:{class:"token punctuation"}},[s._v(".")]),s._v("data"),t("span",{pre:!0,attrs:{class:"token punctuation"}},[s._v(";")]),s._v("\n\n "),t("span",{pre:!0,attrs:{class:"token comment"}},[s._v("// 在共享内存上建立视图，方便读写")]),s._v("\n "),t("span",{pre:!0,attrs:{class:"token keyword"}},[s._v("const")]),s._v(" sharedArray "),t("span",{pre:!0,attrs:{class:"token operator"}},[s._v("=")]),s._v(" "),t("span",{pre:!0,attrs:{class:"token keyword"}},[s._v("new")]),s._v(" "),t("span",{pre:!0,attrs:{class:"token class-name"}},[s._v("Int32Array")]),t("span",{pre:!0,attrs:{class:"token punctuation"}},[s._v("(")]),s._v("sharedBuffer"),t("span",{pre:!0,attrs:{class:"token punctuation"}},[s._v(")")]),t("span",{pre:!0,attrs:{class:"token punctuation"}},[s._v(";")]),s._v("\n\n "),t("span",{pre:!0,attrs:{class:"token comment"}},[s._v("// ...")]),s._v("\n"),t("span",{pre:!0,attrs:{class:"token punctuation"}},[s._v("}")]),t("span",{pre:!0,attrs:{class:"token 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"),t("span",{pre:!0,attrs:{class:"token keyword"}},[s._v("new")]),s._v(" "),t("span",{pre:!0,attrs:{class:"token class-name"}},[s._v("PrimeGenerator")]),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 punctuation"}},[s._v(";")]),s._v("\n\n"),t("span",{pre:!0,attrs:{class:"token comment"}},[s._v("// 将 10 万个质数，写入这段内存空间")]),s._v("\n"),t("span",{pre:!0,attrs:{class:"token keyword"}},[s._v("for")]),s._v(" "),t("span",{pre:!0,attrs:{class:"token punctuation"}},[s._v("(")]),s._v(" "),t("span",{pre:!0,attrs:{class:"token keyword"}},[s._v("let")]),s._v(" i"),t("span",{pre:!0,attrs:{class:"token operator"}},[s._v("=")]),t("span",{pre:!0,attrs:{class:"token number"}},[s._v("0")]),s._v(" "),t("span",{pre:!0,attrs:{class:"token punctuation"}},[s._v(";")]),s._v(" i "),t("span",{pre:!0,attrs:{class:"token operator"}},[s._v("<")]),s._v(" ia"),t("span",{pre:!0,attrs:{class:"token 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"),t("div",{staticClass:"language-javascript line-numbers-mode"},[t("pre",{pre:!0,attrs:{class:"language-javascript"}},[t("code",[t("span",{pre:!0,attrs:{class:"token comment"}},[s._v("// Worker 线程")]),s._v("\n"),t("span",{pre:!0,attrs:{class:"token keyword"}},[s._v("let")]),s._v(" ia"),t("span",{pre:!0,attrs:{class:"token punctuation"}},[s._v(";")]),s._v("\n"),t("span",{pre:!0,attrs:{class:"token function-variable function"}},[s._v("onmessage")]),s._v(" "),t("span",{pre:!0,attrs:{class:"token operator"}},[s._v("=")]),s._v(" "),t("span",{pre:!0,attrs:{class:"token keyword"}},[s._v("function")]),s._v(" "),t("span",{pre:!0,attrs:{class:"token punctuation"}},[s._v("(")]),t("span",{pre:!0,attrs:{class:"token parameter"}},[s._v("ev")]),t("span",{pre:!0,attrs:{class:"token punctuation"}},[s._v(")")]),s._v(" "),t("span",{pre:!0,attrs:{class:"token punctuation"}},[s._v("{")]),s._v("\n ia "),t("span",{pre:!0,attrs:{class:"token operator"}},[s._v("=")]),s._v(" ev"),t("span",{pre:!0,attrs:{class:"token punctuation"}},[s._v(".")]),s._v("data"),t("span",{pre:!0,attrs:{class:"token punctuation"}},[s._v(";")]),s._v("\n console"),t("span",{pre:!0,attrs:{class:"token punctuation"}},[s._v(".")]),t("span",{pre:!0,attrs:{class:"token function"}},[s._v("log")]),t("span",{pre:!0,attrs:{class:"token punctuation"}},[s._v("(")]),s._v("ia"),t("span",{pre:!0,attrs:{class:"token punctuation"}},[s._v(".")]),s._v("length"),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 comment"}},[s._v("// 100000")]),s._v("\n console"),t("span",{pre:!0,attrs:{class:"token punctuation"}},[s._v(".")]),t("span",{pre:!0,attrs:{class:"token function"}},[s._v("log")]),t("span",{pre:!0,attrs:{class:"token punctuation"}},[s._v("(")]),s._v("ia"),t("span",{pre:!0,attrs:{class:"token punctuation"}},[s._v("[")]),t("span",{pre:!0,attrs:{class:"token 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"),t("div",{staticClass:"line-numbers-wrapper"},[t("span",{staticClass:"line-number"},[s._v("1")]),t("br"),t("span",{staticClass:"line-number"},[s._v("2")]),t("br"),t("span",{staticClass:"line-number"},[s._v("3")]),t("br"),t("span",{staticClass:"line-number"},[s._v("4")]),t("br"),t("span",{staticClass:"line-number"},[s._v("5")]),t("br"),t("span",{staticClass:"line-number"},[s._v("6")]),t("br"),t("span",{staticClass:"line-number"},[s._v("7")]),t("br")])]),t("h2",{attrs:{id:"atomics-对象"}},[t("a",{staticClass:"header-anchor",attrs:{href:"#atomics-对象"}},[s._v("#")]),s._v(" Atomics 对象")]),s._v(" "),t("p",[s._v("多线程共享内存，最大的问题就是如何防止两个线程同时修改某个地址，或者说，当一个线程修改共享内存以后，必须有一个机制让其他线程同步。SharedArrayBuffer API 提供"),t("code",[s._v("Atomics")]),s._v("对象，保证所有共享内存的操作都是“原子性”的，并且可以在所有线程内同步。")]),s._v(" "),t("p",[s._v("什么叫“原子性操作”呢？现代编程语言中，一条普通的命令被编译器处理以后，会变成多条机器指令。如果是单线程运行，这是没有问题的；多线程环境并且共享内存时，就会出问题，因为这一组机器指令的运行期间，可能会插入其他线程的指令，从而导致运行结果出错。请看下面的例子。")]),s._v(" "),t("div",{staticClass:"language-javascript line-numbers-mode"},[t("pre",{pre:!0,attrs:{class:"language-javascript"}},[t("code",[t("span",{pre:!0,attrs:{class:"token comment"}},[s._v("// 主线程")]),s._v("\nia"),t("span",{pre:!0,attrs:{class:"token punctuation"}},[s._v("[")]),t("span",{pre:!0,attrs:{class:"token number"}},[s._v("42")]),t("span",{pre:!0,attrs:{class:"token punctuation"}},[s._v("]")]),s._v(" "),t("span",{pre:!0,attrs:{class:"token operator"}},[s._v("=")]),s._v(" "),t("span",{pre:!0,attrs:{class:"token number"}},[s._v("314159")]),t("span",{pre:!0,attrs:{class:"token punctuation"}},[s._v(";")]),s._v(" "),t("span",{pre:!0,attrs:{class:"token comment"}},[s._v("// 原先的值 191")]),s._v("\nia"),t("span",{pre:!0,attrs:{class:"token punctuation"}},[s._v("[")]),t("span",{pre:!0,attrs:{class:"token number"}},[s._v("37")]),t("span",{pre:!0,attrs:{class:"token punctuation"}},[s._v("]")]),s._v(" "),t("span",{pre:!0,attrs:{class:"token operator"}},[s._v("=")]),s._v(" "),t("span",{pre:!0,attrs:{class:"token number"}},[s._v("123456")]),t("span",{pre:!0,attrs:{class:"token punctuation"}},[s._v(";")]),s._v(" "),t("span",{pre:!0,attrs:{class:"token comment"}},[s._v("// 原先的值 163")]),s._v("\n\n"),t("span",{pre:!0,attrs:{class:"token comment"}},[s._v("// Worker 线程")]),s._v("\nconsole"),t("span",{pre:!0,attrs:{class:"token punctuation"}},[s._v(".")]),t("span",{pre:!0,attrs:{class:"token function"}},[s._v("log")]),t("span",{pre:!0,attrs:{class:"token punctuation"}},[s._v("(")]),s._v("ia"),t("span",{pre:!0,attrs:{class:"token punctuation"}},[s._v("[")]),t("span",{pre:!0,attrs:{class:"token number"}},[s._v("37")]),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 punctuation"}},[s._v(";")]),s._v("\nconsole"),t("span",{pre:!0,attrs:{class:"token punctuation"}},[s._v(".")]),t("span",{pre:!0,attrs:{class:"token function"}},[s._v("log")]),t("span",{pre:!0,attrs:{class:"token punctuation"}},[s._v("(")]),s._v("ia"),t("span",{pre:!0,attrs:{class:"token punctuation"}},[s._v("[")]),t("span",{pre:!0,attrs:{class:"token number"}},[s._v("42")]),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 punctuation"}},[s._v(";")]),s._v("\n"),t("span",{pre:!0,attrs:{class:"token comment"}},[s._v("// 可能的结果")]),s._v("\n"),t("span",{pre:!0,attrs:{class:"token comment"}},[s._v("// 123456")]),s._v("\n"),t("span",{pre:!0,attrs:{class:"token comment"}},[s._v("// 191")]),s._v("\n")])]),s._v(" "),t("div",{staticClass:"line-numbers-wrapper"},[t("span",{staticClass:"line-number"},[s._v("1")]),t("br"),t("span",{staticClass:"line-number"},[s._v("2")]),t("br"),t("span",{staticClass:"line-number"},[s._v("3")]),t("br"),t("span",{staticClass:"line-number"},[s._v("4")]),t("br"),t("span",{staticClass:"line-number"},[s._v("5")]),t("br"),t("span",{staticClass:"line-number"},[s._v("6")]),t("br"),t("span",{staticClass:"line-number"},[s._v("7")]),t("br"),t("span",{staticClass:"line-number"},[s._v("8")]),t("br"),t("span",{staticClass:"line-number"},[s._v("9")]),t("br"),t("span",{staticClass:"line-number"},[s._v("10")]),t("br")])]),t("p",[s._v("上面代码中，主线程的原始顺序是先对 42 号位置赋值，再对 37 号位置赋值。但是，编译器和 CPU 为了优化，可能会改变这两个操作的执行顺序（因为它们之间互不依赖），先对 37 号位置赋值，再对 42 号位置赋值。而执行到一半的时候，Worker 线程可能就会来读取数据，导致打印出"),t("code",[s._v("123456")]),s._v("和"),t("code",[s._v("191")]),s._v("。")]),s._v(" "),t("p",[s._v("下面是另一个例子。")]),s._v(" "),t("div",{staticClass:"language-javascript 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"),t("span",{pre:!0,attrs:{class:"token operator"}},[s._v("<")]),s._v(" ia"),t("span",{pre:!0,attrs:{class:"token punctuation"}},[s._v(".")]),s._v("length"),t("span",{pre:!0,attrs:{class:"token punctuation"}},[s._v(";")]),s._v(" i"),t("span",{pre:!0,attrs:{class:"token operator"}},[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("\n ia"),t("span",{pre:!0,attrs:{class:"token punctuation"}},[s._v("[")]),s._v("i"),t("span",{pre:!0,attrs:{class:"token punctuation"}},[s._v("]")]),s._v(" "),t("span",{pre:!0,attrs:{class:"token operator"}},[s._v("=")]),s._v(" primes"),t("span",{pre:!0,attrs:{class:"token punctuation"}},[s._v(".")]),t("span",{pre:!0,attrs:{class:"token function"}},[s._v("next")]),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 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"),t("span",{pre:!0,attrs:{class:"token number"}},[s._v("112")]),t("span",{pre:!0,attrs:{class:"token punctuation"}},[s._v(",")]),s._v(" "),t("span",{pre:!0,attrs:{class:"token number"}},[s._v("1")]),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 comment"}},[s._v("// 正确")]),s._v("\n")])]),s._v(" 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"),t("li",[t("code",[s._v("Atomics.isLockFree(size)")]),s._v("：返回一个布尔值，表示"),t("code",[s._v("Atomics")]),s._v("对象是否可以处理某个"),t("code",[s._v("size")]),s._v("的内存锁定。如果返回"),t("code",[s._v("false")]),s._v("，应用程序就需要自己来实现锁定。")])]),s._v(" "),t("p",[t("code",[s._v("Atomics.compareExchange")]),s._v("的一个用途是，从 SharedArrayBuffer 读取一个值，然后对该值进行某个操作，操作结束以后，检查一下 SharedArrayBuffer 里面原来那个值是否发生变化（即被其他线程改写过）。如果没有改写过，就将它写回原来的位置，否则读取新的值，再重头进行一次操作。")])])}),[],!1,null,null,null);t.default=e.exports}}]);
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