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- (window.webpackJsonp=window.webpackJsonp||[]).push([[169],{499: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:"泛型"}},[t("a",{staticClass:"header-anchor",attrs:{href:"#泛型"}},[s._v("#")]),s._v(" 泛型")]),s._v(" "),t("p",[s._v("软件工程中，我们不仅要创建定义良好且一致的 API，同时也要考虑可重用性。 组件不仅能够支持当前的数据类型，同时也能支持未来的数据类型，这在创建大型系统时为你提供了十分灵活的功能。")]),s._v(" "),t("p",[s._v("在像 C# 和 Java 这样的语言中，可以使用泛型来创建可重用的组件，一个组件可以支持多种类型的数据。 这样用户就可以以自己的数据类型来使用组件。")]),s._v(" "),t("h2",{attrs:{id:"基础示例"}},[t("a",{staticClass:"header-anchor",attrs:{href:"#基础示例"}},[s._v("#")]),s._v(" 基础示例")]),s._v(" "),t("p",[s._v("下面来创建第一个使用泛型的例子："),t("code",[s._v("identity")]),s._v(" 函数。 这个函数会返回任何传入它的值。 你可以把这个函数当成是 "),t("code",[s._v("echo")]),s._v(" 命令。")]),s._v(" "),t("p",[s._v("不用泛型的话，这个函数可能是下面这样：")]),s._v(" "),t("div",{staticClass:"language-typescript 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"),t("code",[s._v("()")]),s._v("。")]),s._v(" "),t("p",[s._v("第二种方法更普遍。利用了"),t("em",[s._v("类型推论")]),s._v(" -- 即编译器会根据传入的参数自动地帮助我们确定 "),t("code",[s._v("T")]),s._v(" 的类型：")]),s._v(" "),t("div",{staticClass:"language-typescript line-numbers-mode"},[t("pre",{pre:!0,attrs:{class:"language-typescript"}},[t("code",[t("span",{pre:!0,attrs:{class:"token keyword"}},[s._v("let")]),s._v(" output "),t("span",{pre:!0,attrs:{class:"token operator"}},[s._v("=")]),s._v(" "),t("span",{pre:!0,attrs:{class:"token function"}},[s._v("identity")]),t("span",{pre:!0,attrs:{class:"token punctuation"}},[s._v("(")]),t("span",{pre:!0,attrs:{class:"token string"}},[s._v("'myString'")]),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("p",[s._v("注意我们没必要使用尖括号（"),t("code",[s._v("<>")]),s._v("）来明确地传入类型；编译器可以查看 "),t("code",[s._v("myString")]),s._v(" 的值，然后把 "),t("code",[s._v("T")]),s._v(" 设置为它的类型。 类型推论帮助我们保持代码精简和高可读性。如果编译器不能够自动地推断出类型的话，只能像上面那样明确的传入 "),t("code",[s._v("T")]),s._v(" 的类型，在一些复杂的情况下，这是可能出现的。")]),s._v(" "),t("h2",{attrs:{id:"使用泛型变量"}},[t("a",{staticClass:"header-anchor",attrs:{href:"#使用泛型变量"}},[s._v("#")]),s._v(" 使用泛型变量")]),s._v(" "),t("p",[s._v("使用泛型创建像 "),t("code",[s._v("identity")]),s._v(" 这样的泛型函数时，编译器要求你在函数体必须正确的使用这个通用的类型。 换句话说，你必须把这些参数当做是任意或所有类型。")]),s._v(" "),t("p",[s._v("看下之前 "),t("code",[s._v("identity")]),s._v(" 例子：")]),s._v(" "),t("div",{staticClass:"language-typescript line-numbers-mode"},[t("pre",{pre:!0,attrs:{class:"language-typescript"}},[t("code",[t("span",{pre:!0,attrs:{class:"token keyword"}},[s._v("function")]),s._v(" "),t("span",{pre:!0,attrs:{class:"token generic-function"}},[t("span",{pre:!0,attrs:{class:"token function"}},[s._v("identity")]),t("span",{pre:!0,attrs:{class:"token generic class-name"}},[t("span",{pre:!0,attrs:{class:"token operator"}},[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("p",[s._v("如果我们想打印出 "),t("code",[s._v("arg")]),s._v(" 的长度。 我们很可能会这样做：")]),s._v(" "),t("div",{staticClass:"language-typescript line-numbers-mode"},[t("pre",{pre:!0,attrs:{class:"language-typescript"}},[t("code",[t("span",{pre:!0,attrs:{class:"token keyword"}},[s._v("function")]),s._v(" "),t("span",{pre:!0,attrs:{class:"token generic-function"}},[t("span",{pre:!0,attrs:{class:"token function"}},[s._v("loggingIdentity")]),t("span",{pre:!0,attrs:{class:"token generic class-name"}},[t("span",{pre:!0,attrs:{class:"token operator"}},[s._v("<")]),t("span",{pre:!0,attrs:{class:"token constant"}},[s._v("T")]),t("span",{pre:!0,attrs:{class:"token operator"}},[s._v(">")])])]),t("span",{pre:!0,attrs:{class:"token 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arg\n"),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("span",{staticClass:"line-number"},[s._v("3")]),t("br"),t("span",{staticClass:"line-number"},[s._v("4")]),t("br")])]),t("p",[s._v("如果这么做，编译器会报错说我们使用了 "),t("code",[s._v("arg")]),s._v(" 的 "),t("code",[s._v(".length")]),s._v(" 属性，但是没有地方指明 "),t("code",[s._v("arg")]),s._v(" 具有这个属性。记住，这些类型变量代表的是任意类型，所以使用这个函数的人可能传入的是个数字，而数字是没有 "),t("code",[s._v(".length")]),s._v(" 属性的。")]),s._v(" "),t("p",[s._v("现在假设我们想操作 "),t("code",[s._v("T")]),s._v(" 类型的数组而不直接是 "),t("code",[s._v("T")]),s._v("。由于我们操作的是数组，所以 "),t("code",[s._v(".length")]),s._v(" 属性是应该存在的。我们可以像创建其它数组一样创建这个数组：")]),s._v(" "),t("div",{staticClass:"language-typescript line-numbers-mode"},[t("pre",{pre:!0,attrs:{class:"language-typescript"}},[t("code",[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("p",[s._v("你可以这样理解 "),t("code",[s._v("loggingIdentity")]),s._v(" 的类型：泛型函数 "),t("code",[s._v("loggingIdentity")]),s._v("，接收类型参数 "),t("code",[s._v("T")]),s._v(" 和参数 "),t("code",[s._v("arg")]),s._v("，它是个元素类型是 "),t("code",[s._v("T")]),s._v(" 的数组，并返回元素类型是"),t("code",[s._v("T")]),s._v(" 的数组。 如果我们传入数字数组，将返回一个数字数组，因为此时 "),t("code",[s._v("T")]),s._v(" 的的类型为 "),t("code",[s._v("number")]),s._v("。 这可以让我们把泛型变量 "),t("code",[s._v("T")]),s._v(" 当做类型的一部分使用，而不是整个类型，增加了灵活性。")]),s._v(" "),t("h2",{attrs:{id:"泛型类型"}},[t("a",{staticClass:"header-anchor",attrs:{href:"#泛型类型"}},[s._v("#")]),s._v(" 泛型类型")]),s._v(" "),t("p",[s._v("上一节，我们创建了 "),t("code",[s._v("identity")]),s._v(" 通用函数，可以适用于不同的类型。 在这节，我们研究一下函数本身的类型，以及如何创建泛型接口。")]),s._v(" "),t("p",[s._v("泛型函数的类型与非泛型函数的类型没什么不同，只是有一个类型参数在最前面，像函数声明一样：")]),s._v(" "),t("div",{staticClass:"language-typescript line-numbers-mode"},[t("pre",{pre:!0,attrs:{class:"language-typescript"}},[t("code",[t("span",{pre:!0,attrs:{class:"token keyword"}},[s._v("function")]),s._v(" "),t("span",{pre:!0,attrs:{class:"token generic-function"}},[t("span",{pre:!0,attrs:{class:"token function"}},[s._v("identity")]),t("span",{pre:!0,attrs:{class:"token generic class-name"}},[t("span",{pre:!0,attrs:{class:"token operator"}},[s._v("<")]),t("span",{pre:!0,attrs:{class:"token constant"}},[s._v("T")]),t("span",{pre:!0,attrs:{class:"token operator"}},[s._v(">")])])]),t("span",{pre:!0,attrs:{class:"token punctuation"}},[s._v("(")]),s._v("arg"),t("span",{pre:!0,attrs:{class:"token operator"}},[s._v(":")]),s._v(" "),t("span",{pre:!0,attrs:{class:"token constant"}},[s._v("T")]),t("span",{pre:!0,attrs:{class:"token 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punctuation"}},[s._v(")")]),s._v(" "),t("span",{pre:!0,attrs:{class:"token operator"}},[s._v("=>")]),s._v(" "),t("span",{pre:!0,attrs:{class:"token constant"}},[s._v("T")]),s._v(" "),t("span",{pre:!0,attrs:{class:"token operator"}},[s._v("=")]),s._v(" identity\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("我们也可以使用不同的泛型参数名，只要在数量上和使用方式上能对应上就可以。")]),s._v(" "),t("div",{staticClass:"language-typescript line-numbers-mode"},[t("pre",{pre:!0,attrs:{class:"language-typescript"}},[t("code",[t("span",{pre:!0,attrs:{class:"token keyword"}},[s._v("function")]),s._v(" "),t("span",{pre:!0,attrs:{class:"token generic-function"}},[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("p",[s._v("我们还可以使用带有调用签名的对象字面量来定义泛型函数：")]),s._v(" "),t("div",{staticClass:"language-typescript line-numbers-mode"},[t("pre",{pre:!0,attrs:{class:"language-typescript"}},[t("code",[t("span",{pre:!0,attrs:{class:"token keyword"}},[s._v("function")]),s._v(" "),t("span",{pre:!0,attrs:{class:"token generic-function"}},[t("span",{pre:!0,attrs:{class:"token function"}},[s._v("identity")]),t("span",{pre:!0,attrs:{class:"token generic class-name"}},[t("span",{pre:!0,attrs:{class:"token operator"}},[s._v("<")]),t("span",{pre:!0,attrs:{class:"token constant"}},[s._v("T")]),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("p",[s._v("这引导我们去写第一个泛型接口了。我们把上面例子里的对象字面量拿出来做为一个接口：")]),s._v(" "),t("div",{staticClass:"language-typescript line-numbers-mode"},[t("pre",{pre:!0,attrs:{class:"language-typescript"}},[t("code",[t("span",{pre:!0,attrs:{class:"token keyword"}},[s._v("interface")]),s._v(" "),t("span",{pre:!0,attrs:{class:"token class-name"}},[s._v("GenericIdentityFn")]),s._v(" "),t("span",{pre:!0,attrs:{class:"token punctuation"}},[s._v("{")]),s._v("\n "),t("span",{pre:!0,attrs:{class:"token operator"}},[s._v("<")]),t("span",{pre:!0,attrs:{class:"token constant"}},[s._v("T")]),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("p",[s._v("我们甚至可以把泛型参数当作整个接口的一个参数。 这样我们就能清楚的知道使用的具体是哪个泛型类型（比如： "),t("code",[s._v("Dictionary<string>")]),s._v(" 而不只是"),t("code",[s._v("Dictionary")]),s._v("）。这样接口里的其它成员也能知道这个参数的类型了。")]),s._v(" "),t("div",{staticClass:"language-typescript line-numbers-mode"},[t("pre",{pre:!0,attrs:{class:"language-typescript"}},[t("code",[t("span",{pre:!0,attrs:{class:"token keyword"}},[s._v("interface")]),s._v(" "),t("span",{pre:!0,attrs:{class:"token class-name"}},[s._v("GenericIdentityFn"),t("span",{pre:!0,attrs:{class:"token operator"}},[s._v("<")]),t("span",{pre:!0,attrs:{class:"token constant"}},[s._v("T")]),t("span",{pre:!0,attrs:{class:"token operator"}},[s._v(">")])]),s._v(" "),t("span",{pre:!0,attrs:{class:"token punctuation"}},[s._v("{")]),s._v("\n "),t("span",{pre:!0,attrs:{class:"token punctuation"}},[s._v("(")]),s._v("arg"),t("span",{pre:!0,attrs:{class:"token operator"}},[s._v(":")]),s._v(" "),t("span",{pre:!0,attrs:{class:"token constant"}},[s._v("T")]),t("span",{pre:!0,attrs:{class:"token punctuation"}},[s._v(")")]),t("span",{pre:!0,attrs:{class:"token operator"}},[s._v(":")]),s._v(" "),t("span",{pre:!0,attrs:{class:"token constant"}},[s._v("T")]),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 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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("p",[s._v("注意，我们的示例做了少许改动。 不再描述泛型函数，而是把非泛型函数签名作为泛型类型一部分。 当我们使用 "),t("code",[s._v("GenericIdentityFn")]),s._v(" 的时候，还得传入一个类型参数来指定泛型类型（这里是："),t("code",[s._v("number")]),s._v("），锁定了之后代码里使用的类型。对于描述哪部分类型属于泛型部分来说，理解何时把参数放在调用签名里和何时放在接口上是很有帮助的。")]),s._v(" "),t("p",[s._v("除了泛型接口，我们还可以创建泛型类。 注意，无法创建泛型枚举和泛型命名空间。")]),s._v(" "),t("h2",{attrs:{id:"泛型类"}},[t("a",{staticClass:"header-anchor",attrs:{href:"#泛型类"}},[s._v("#")]),s._v(" 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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("p",[s._v("与接口一样，直接把泛型类型放在类后面，可以帮助我们确认类的所有属性都在使用相同的类型。")]),s._v(" "),t("p",[s._v("我们在"),t("a",{attrs:{href:"/chapter2/class"}},[s._v("类")]),s._v("那节说过，类有两部分：静态部分和实例部分。 泛型类指的是实例部分的类型，所以类的静态属性不能使用这个泛型类型。")]),s._v(" "),t("h2",{attrs:{id:"泛型约束"}},[t("a",{staticClass:"header-anchor",attrs:{href:"#泛型约束"}},[s._v("#")]),s._v(" 泛型约束")]),s._v(" "),t("p",[s._v("我们有时候想操作某类型的一组值，并且我们知道这组值具有什么样的属性。在 "),t("code",[s._v("loggingIdentity")]),s._v(" 例子中，我们想访问 "),t("code",[s._v("arg")]),s._v(" 的 "),t("code",[s._v("length")]),s._v(" 属性，但是编译器并不能证明每种类型都有 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