Kezdőlap Felfedezés Súgó
Regisztráció Bejelentkezés
root
/
blog_web
1
0
Másolás 0
Fájlok Problémák 0 Beolvasztási kérések 0 Wiki
Fa: 000ff68bb8
Branch-ok Tag-ek
blog_web
/
assets
/
js
/
155.5ee5b308.js

155.5ee5b308.js 62 KB
Előzmények Nyers

1 
  1. (window.webpackJsonp=window.webpackJsonp||[]).push([[155],{485: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:"_01-hook概述"}},[s("a",{staticClass:"header-anchor",attrs:{href:"#_01-hook概述"}},[t._v("#")]),t._v(" 01. Hook概述")]),t._v(" "),s("p",[t._v("Hook可以让你在不编写 class 的情况下使用 state 以及其他的 React 特性。")]),t._v(" "),s("h2",{attrs:{id:"什么是-hook"}},[s("a",{staticClass:"header-anchor",attrs:{href:"#什么是-hook"}},[t._v("#")]),t._v(" 什么是 Hook?")]),t._v(" "),s("p",[t._v("Hook 是一个特殊的函数，可以让你在函数组件里“钩入” React "),s("strong",[t._v("state")]),t._v(" 及 "),s("strong",[t._v("生命周期")]),t._v("  等特性的函数。")]),t._v(" "),s("blockquote",[s("ol",[s("li",[s("p",[t._v("Hook 不能在 class 组件中使用 —— 这使得你不使用 class 也能使用 React。")])]),t._v(" "),s("li",[s("p",[t._v("React 内置了一些像 "),s("code",[t._v("useState")]),t._v(" 这样的 Hook。你也可以创建你自己的 Hook 来复用不同组件之间的状态逻辑。我们会先介绍这些内置的 Hook。")])])])]),t._v(" "),s("h2",{attrs:{id:"usestate-状态钩子"}},[s("a",{staticClass:"header-anchor",attrs:{href:"#usestate-状态钩子"}},[t._v("#")]),t._v(" useState（状态钩子）")]),t._v(" "),s("p",[s("strong",[t._v("模式：")])]),t._v(" "),s("div",{staticClass:"language-js line-numbers-mode"},[s("pre",{pre:!0,attrs:{class:"language-js"}},[s("code",[s("span",{pre:!0,attrs:{class:"token keyword"}},[t._v("const")]),t._v(" "),s("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v("[")]),s("span",{pre:!0,attrs:{class:"token operator"}},[t._v("<")]),t._v("取值"),s("span",{pre:!0,attrs:{class:"token operator"}},[t._v(">")]),s("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v(",")]),t._v(" "),s("span",{pre:!0,attrs:{class:"token operator"}},[t._v("<")]),t._v("设值"),s("span",{pre:!0,attrs:{class:"token operator"}},[t._v(">")]),s("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v("]")]),t._v(" "),s("span",{pre:!0,attrs:{class:"token operator"}},[t._v("=")]),t._v(" "),s("span",{pre:!0,attrs:{class:"token function"}},[t._v("useState")]),s("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v("(")]),s("span",{pre:!0,attrs:{class:"token operator"}},[t._v("<")]),t._v("初始值"),s("span",{pre:!0,attrs:{class:"token operator"}},[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("p",[t._v("上面的三个值均可自定义名称，分别是：")]),t._v(" "),s("ul",[s("li",[t._v("state取值变量名")]),t._v(" "),s("li",[t._v("state设值方法名")]),t._v(" "),s("li",[t._v("初始值\n"),s("ul",[s("li",[t._v("初始值可以是数字、字符串、对象、数组等")])])])]),t._v(" "),s("p",[s("strong",[t._v("例子：")])]),t._v(" "),s("div",{staticClass:"language-jsx line-numbers-mode"},[s("pre",{pre:!0,attrs:{class:"language-jsx"}},[s("code",[s("span",{pre:!0,attrs:{class:"token keyword"}},[t._v("import")]),t._v(" React"),s("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v(",")]),t._v(" "),s("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v("{")]),t._v(" useState "),s("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v("}")]),t._v(" "),s("span",{pre:!0,attrs:{class:"token keyword"}},[t._v("from")]),t._v(" "),s("span",{pre:!0,attrs:{class:"token string"}},[t._v("'react'")]),s("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v(";")]),t._v("\n\n"),s("span",{pre:!0,attrs:{class:"token keyword"}},[t._v("function")]),t._v(" "),s("span",{pre:!0,attrs:{class:"token function"}},[t._v("Example")]),s("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v("(")]),s("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v(")")]),t._v(" "),s("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v("{")]),t._v("\n  "),s("span",{pre:!0,attrs:{class:"token comment"}},[t._v("// 声明一个新的叫做 “count” 的 state 变量")]),t._v("\n  "),s("span",{pre:!0,attrs:{class:"token keyword"}},[t._v("const")]),t._v(" "),s("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v("[")]),t._v("count"),s("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v(",")]),t._v(" setCount"),s("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v("]")]),t._v(" "),s("span",{pre:!0,attrs:{class:"token operator"}},[t._v("=")]),t._v(" "),s("span",{pre:!0,attrs:{class:"token function"}},[t._v("useState")]),s("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v("(")]),s("span",{pre:!0,attrs:{class:"token number"}},[t._v("0")]),s("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v(")")]),s("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v(";")]),t._v("\n\n  "),s("span",{pre:!0,attrs:{class:"token keyword"}},[t._v("return")]),t._v(" "),s("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v("(")]),t._v("\n    "),s("span",{pre:!0,attrs:{class:"token tag"}},[s("span",{pre:!0,attrs:{class:"token tag"}},[s("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v("<")]),t._v("div")]),s("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v(">")])]),s("span",{pre:!0,attrs:{class:"token plain-text"}},[t._v("\n      ")]),s("span",{pre:!0,attrs:{class:"token tag"}},[s("span",{pre:!0,attrs:{class:"token tag"}},[s("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v("<")]),t._v("p")]),s("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v(">")])]),s("span",{pre:!0,attrs:{class:"token plain-text"}},[t._v("You clicked ")]),s("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v("{")]),t._v("count"),s("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v("}")]),s("span",{pre:!0,attrs:{class:"token plain-text"}},[t._v(" times")]),s("span",{pre:!0,attrs:{class:"token tag"}},[s("span",{pre:!0,attrs:{class:"token tag"}},[s("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v("</")]),t._v("p")]),s("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v(">")])]),s("span",{pre:!0,attrs:{class:"token plain-text"}},[t._v("\n      ")]),s("span",{pre:!0,attrs:{class:"token tag"}},[s("span",{pre:!0,attrs:{class:"token tag"}},[s("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v("<")]),t._v("button")]),t._v(" "),s("span",{pre:!0,attrs:{class:"token attr-name"}},[t._v("onClick")]),s("span",{pre:!0,attrs:{class:"token script language-javascript"}},[s("span",{pre:!0,attrs:{class:"token script-punctuation punctuation"}},[t._v("=")]),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(" "),s("span",{pre:!0,attrs:{class:"token operator"}},[t._v("=>")]),t._v(" "),s("span",{pre:!0,attrs:{class:"token function"}},[t._v("setCount")]),s("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v("(")]),t._v("count "),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(")")]),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 plain-text"}},[t._v("\n        Click me\n      ")]),s("span",{pre:!0,attrs:{class:"token tag"}},[s("span",{pre:!0,attrs:{class:"token tag"}},[s("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v("</")]),t._v("button")]),s("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v(">")])]),s("span",{pre:!0,attrs:{class:"token plain-text"}},[t._v("\n    ")]),s("span",{pre:!0,attrs:{class:"token tag"}},[s("span",{pre:!0,attrs:{class:"token tag"}},[s("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v("</")]),t._v("div")]),s("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v(">")])]),t._v("\n  "),s("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v(")")]),s("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v(";")]),t._v("\n"),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("h3",{attrs:{id:"声明多个-state-变量"}},[s("a",{staticClass:"header-anchor",attrs:{href:"#声明多个-state-变量"}},[t._v("#")]),t._v(" 声明多个 state 变量")]),t._v(" "),s("div",{staticClass:"language-jsx line-numbers-mode"},[s("pre",{pre:!0,attrs:{class:"language-jsx"}},[s("code",[s("span",{pre:!0,attrs:{class:"token keyword"}},[t._v("function")]),t._v(" "),s("span",{pre:!0,attrs:{class:"token function"}},[t._v("ExampleWithManyStates")]),s("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v("(")]),s("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v(")")]),t._v(" "),s("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v("{")]),t._v("\n  "),s("span",{pre:!0,attrs:{class:"token comment"}},[t._v("// 声明多个 state 变量！")]),t._v("\n  "),s("span",{pre:!0,attrs:{class:"token keyword"}},[t._v("const")]),t._v(" "),s("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v("[")]),t._v("age"),s("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v(",")]),t._v(" setAge"),s("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v("]")]),t._v(" "),s("span",{pre:!0,attrs:{class:"token operator"}},[t._v("=")]),t._v(" "),s("span",{pre:!0,attrs:{class:"token function"}},[t._v("useState")]),s("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v("(")]),s("span",{pre:!0,attrs:{class:"token number"}},[t._v("42")]),s("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v(")")]),s("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v(";")]),t._v("\n  "),s("span",{pre:!0,attrs:{class:"token keyword"}},[t._v("const")]),t._v(" "),s("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v("[")]),t._v("fruit"),s("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v(",")]),t._v(" setFruit"),s("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v("]")]),t._v(" "),s("span",{pre:!0,attrs:{class:"token operator"}},[t._v("=")]),t._v(" "),s("span",{pre:!0,attrs:{class:"token function"}},[t._v("useState")]),s("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v("(")]),s("span",{pre:!0,attrs:{class:"token string"}},[t._v("'banana'")]),s("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v(")")]),s("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v(";")]),t._v("\n  "),s("span",{pre:!0,attrs:{class:"token keyword"}},[t._v("const")]),t._v(" "),s("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v("[")]),t._v("todos"),s("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v(",")]),t._v(" setTodos"),s("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v("]")]),t._v(" "),s("span",{pre:!0,attrs:{class:"token operator"}},[t._v("=")]),t._v(" "),s("span",{pre:!0,attrs:{class:"token function"}},[t._v("useState")]),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(" "),s("span",{pre:!0,attrs:{class:"token literal-property property"}},[t._v("text")]),s("span",{pre:!0,attrs:{class:"token operator"}},[t._v(":")]),t._v(" "),s("span",{pre:!0,attrs:{class:"token string"}},[t._v("'Learn Hooks'")]),t._v(" "),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(")")]),s("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v(";")]),t._v("\n  "),s("span",{pre:!0,attrs:{class:"token comment"}},[t._v("// ...")]),t._v("\n"),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("h2",{attrs:{id:"effect-hook-副作用钩子"}},[s("a",{staticClass:"header-anchor",attrs:{href:"#effect-hook-副作用钩子"}},[t._v("#")]),t._v(" Effect Hook (副作用钩子)")]),t._v(" "),s("p",[t._v("你之前可能已经在 React 组件中执行过"),s("strong",[t._v("数据获取、订阅 或 者手动修改 DOM")]),t._v("。我们统一把这些操作称为“"),s("strong",[t._v("副作用")]),t._v("”，或者简称为“作用”。")]),t._v(" "),s("h3",{attrs:{id:"useeffect-生命周期钩子"}},[s("a",{staticClass:"header-anchor",attrs:{href:"#useeffect-生命周期钩子"}},[t._v("#")]),t._v(" useEffect（生命周期钩子）")]),t._v(" "),s("p",[s("code",[t._v("useEffect")]),t._v(" 就是一个 Effect Hook，"),s("strong",[t._v("给函数组件增加了操作副作用的能力")]),t._v("。它跟 class 组件中的 "),s("code",[t._v("componentDidMount")]),t._v("、"),s("code",[t._v("componentDidUpdate")]),t._v(" 和 "),s("code",[t._v("componentWillUnmount")]),t._v(" 具有相同的用途，只不过被合并成了一个 API。（我们会在"),s("a",{attrs:{href:"https://zh-hans.reactjs.org/docs/hooks-effect.html",target:"_blank",rel:"noopener noreferrer"}},[t._v("使用 Effect Hook"),s("OutboundLink")],1),t._v(" 里展示对比 "),s("code",[t._v("useEffect")]),t._v(" 和这些方法的例子。）")]),t._v(" "),s("p",[t._v("例如，下面这个组件在 React 更新 DOM 后会设置一个页面标题：")]),t._v(" "),s("div",{staticClass:"language-jsx line-numbers-mode"},[s("pre",{pre:!0,attrs:{class:"language-jsx"}},[s("code",[s("span",{pre:!0,attrs:{class:"token keyword"}},[t._v("import")]),t._v(" React"),s("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v(",")]),t._v(" "),s("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v("{")]),t._v(" useState"),s("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v(",")]),t._v(" useEffect "),s("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v("}")]),t._v(" "),s("span",{pre:!0,attrs:{class:"token keyword"}},[t._v("from")]),t._v(" "),s("span",{pre:!0,attrs:{class:"token string"}},[t._v("'react'")]),s("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v(";")]),t._v("\n"),s("span",{pre:!0,attrs:{class:"token keyword"}},[t._v("function")]),t._v(" "),s("span",{pre:!0,attrs:{class:"token function"}},[t._v("Example")]),s("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v("(")]),s("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v(")")]),t._v(" "),s("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v("{")]),t._v("\n  "),s("span",{pre:!0,attrs:{class:"token keyword"}},[t._v("const")]),t._v(" "),s("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v("[")]),t._v("count"),s("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v(",")]),t._v(" setCount"),s("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v("]")]),t._v(" "),s("span",{pre:!0,attrs:{class:"token operator"}},[t._v("=")]),t._v(" "),s("span",{pre:!0,attrs:{class:"token function"}},[t._v("useState")]),s("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v("(")]),s("span",{pre:!0,attrs:{class:"token number"}},[t._v("0")]),s("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v(")")]),s("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v(";")]),t._v("\n\n  "),s("span",{pre:!0,attrs:{class:"token comment"}},[t._v("// 相当于 componentDidMount 和 componentDidUpdate:")]),t._v("\n  "),s("span",{pre:!0,attrs:{class:"token function"}},[t._v("useEffect")]),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(" "),s("span",{pre:!0,attrs:{class:"token operator"}},[t._v("=>")]),t._v(" "),s("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v("{")]),t._v("\n    "),s("span",{pre:!0,attrs:{class:"token comment"}},[t._v("// 第一次渲染DOM以及更新DOM后执行此函数")]),t._v("\n    "),s("span",{pre:!0,attrs:{class:"token comment"}},[t._v("// 使用浏览器的 API 更新页面标题")]),t._v("\n    document"),s("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v(".")]),t._v("title "),s("span",{pre:!0,attrs:{class:"token operator"}},[t._v("=")]),t._v(" "),s("span",{pre:!0,attrs:{class:"token template-string"}},[s("span",{pre:!0,attrs:{class:"token template-punctuation string"}},[t._v("`")]),s("span",{pre:!0,attrs:{class:"token string"}},[t._v("You clicked ")]),s("span",{pre:!0,attrs:{class:"token interpolation"}},[s("span",{pre:!0,attrs:{class:"token interpolation-punctuation punctuation"}},[t._v("${")]),t._v("count"),s("span",{pre:!0,attrs:{class:"token interpolation-punctuation punctuation"}},[t._v("}")])]),s("span",{pre:!0,attrs:{class:"token string"}},[t._v(" times")]),s("span",{pre:!0,attrs:{class:"token template-punctuation string"}},[t._v("`")])]),s("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v(";")]),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  "),s("span",{pre:!0,attrs:{class:"token keyword"}},[t._v("return")]),t._v(" "),s("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v("(")]),t._v("\n    "),s("span",{pre:!0,attrs:{class:"token tag"}},[s("span",{pre:!0,attrs:{class:"token tag"}},[s("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v("<")]),t._v("div")]),s("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v(">")])]),s("span",{pre:!0,attrs:{class:"token plain-text"}},[t._v("\n      ")]),s("span",{pre:!0,attrs:{class:"token tag"}},[s("span",{pre:!0,attrs:{class:"token tag"}},[s("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v("<")]),t._v("p")]),s("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v(">")])]),s("span",{pre:!0,attrs:{class:"token plain-text"}},[t._v("You clicked ")]),s("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v("{")]),t._v("count"),s("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v("}")]),s("span",{pre:!0,attrs:{class:"token plain-text"}},[t._v(" times")]),s("span",{pre:!0,attrs:{class:"token tag"}},[s("span",{pre:!0,attrs:{class:"token tag"}},[s("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v("</")]),t._v("p")]),s("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v(">")])]),s("span",{pre:!0,attrs:{class:"token plain-text"}},[t._v("\n      ")]),s("span",{pre:!0,attrs:{class:"token tag"}},[s("span",{pre:!0,attrs:{class:"token tag"}},[s("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v("<")]),t._v("button")]),t._v(" "),s("span",{pre:!0,attrs:{class:"token attr-name"}},[t._v("onClick")]),s("span",{pre:!0,attrs:{class:"token script language-javascript"}},[s("span",{pre:!0,attrs:{class:"token script-punctuation punctuation"}},[t._v("=")]),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(" "),s("span",{pre:!0,attrs:{class:"token operator"}},[t._v("=>")]),t._v(" "),s("span",{pre:!0,attrs:{class:"token function"}},[t._v("setCount")]),s("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v("(")]),t._v("count "),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(")")]),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 plain-text"}},[t._v("\n        Click me\n      ")]),s("span",{pre:!0,attrs:{class:"token tag"}},[s("span",{pre:!0,attrs:{class:"token tag"}},[s("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v("</")]),t._v("button")]),s("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v(">")])]),s("span",{pre:!0,attrs:{class:"token plain-text"}},[t._v("\n    ")]),s("span",{pre:!0,attrs:{class:"token tag"}},[s("span",{pre:!0,attrs:{class:"token tag"}},[s("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v("</")]),t._v("div")]),s("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v(">")])]),t._v("\n  "),s("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v(")")]),s("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v(";")]),t._v("\n"),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("p",[t._v("当你调用 "),s("code",[t._v("useEffect")]),t._v(" 时，就是在告诉 React "),s("strong",[t._v("在完成对 DOM 的更改后运行你的“副作用”函数")]),t._v("。由于副作用函数是在组件内声明的，所以它们可以访问到组件的 props 和 state。默认情况下，React 会在每次渲染后调用副作用函数 —— "),s("strong",[t._v("包括")]),t._v("第一次渲染的时候。")]),t._v(" "),s("h3",{attrs:{id:"清除-副作用"}},[s("a",{staticClass:"header-anchor",attrs:{href:"#清除-副作用"}},[t._v("#")]),t._v(" “清除”副作用")]),t._v(" "),s("p",[t._v("副作用函数还可以通过返回一个函数来指定如何“清除”副作用。例如，在下面的组件中使用副作用函数来订阅好友的在线状态，并通过取消订阅来进行清除操作：")]),t._v(" "),s("div",{staticClass:"language-jsx line-numbers-mode"},[s("pre",{pre:!0,attrs:{class:"language-jsx"}},[s("code",[s("span",{pre:!0,attrs:{class:"token keyword"}},[t._v("import")]),t._v(" React"),s("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v(",")]),t._v(" "),s("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v("{")]),t._v(" useState"),s("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v(",")]),t._v(" useEffect "),s("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v("}")]),t._v(" "),s("span",{pre:!0,attrs:{class:"token keyword"}},[t._v("from")]),t._v(" "),s("span",{pre:!0,attrs:{class:"token string"}},[t._v("'react'")]),s("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v(";")]),t._v("\n\n"),s("span",{pre:!0,attrs:{class:"token keyword"}},[t._v("function")]),t._v(" "),s("span",{pre:!0,attrs:{class:"token function"}},[t._v("FriendStatus")]),s("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v("(")]),s("span",{pre:!0,attrs:{class:"token parameter"}},[t._v("props")]),s("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v(")")]),t._v(" "),s("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v("{")]),t._v("\n  "),s("span",{pre:!0,attrs:{class:"token keyword"}},[t._v("const")]),t._v(" "),s("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v("[")]),t._v("isOnline"),s("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v(",")]),t._v(" setIsOnline"),s("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v("]")]),t._v(" "),s("span",{pre:!0,attrs:{class:"token operator"}},[t._v("=")]),t._v(" "),s("span",{pre:!0,attrs:{class:"token function"}},[t._v("useState")]),s("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v("(")]),s("span",{pre:!0,attrs:{class:"token keyword"}},[t._v("null")]),s("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v(")")]),s("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v(";")]),t._v("\n\n  "),s("span",{pre:!0,attrs:{class:"token keyword"}},[t._v("function")]),t._v(" "),s("span",{pre:!0,attrs:{class:"token function"}},[t._v("handleStatusChange")]),s("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v("(")]),s("span",{pre:!0,attrs:{class:"token parameter"}},[t._v("status")]),s("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v(")")]),t._v(" "),s("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v("{")]),t._v("\n    "),s("span",{pre:!0,attrs:{class:"token function"}},[t._v("setIsOnline")]),s("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v("(")]),t._v("status"),s("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v(".")]),t._v("isOnline"),s("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v(")")]),s("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v(";")]),t._v("\n  "),s("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v("}")]),t._v("\n\n  "),s("span",{pre:!0,attrs:{class:"token function"}},[t._v("useEffect")]),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(" "),s("span",{pre:!0,attrs:{class:"token operator"}},[t._v("=>")]),t._v(" "),s("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v("{")]),t._v(" "),s("span",{pre:!0,attrs:{class:"token comment"}},[t._v("// 组件挂载完成和更新完成后执行")]),t._v("\n    "),s("span",{pre:!0,attrs:{class:"token comment"}},[t._v("// 创建订阅")]),t._v("\n    ChatAPI"),s("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v(".")]),s("span",{pre:!0,attrs:{class:"token function"}},[t._v("subscribeToFriendStatus")]),s("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v("(")]),t._v("props"),s("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v(".")]),t._v("friend"),s("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v(".")]),t._v("id"),s("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v(",")]),t._v(" handleStatusChange"),s("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v(")")]),s("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v(";")]),t._v("\n    "),s("span",{pre:!0,attrs:{class:"token keyword"}},[t._v("return")]),t._v(" "),s("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v("(")]),s("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v(")")]),t._v(" "),s("span",{pre:!0,attrs:{class:"token operator"}},[t._v("=>")]),t._v(" "),s("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v("{")]),t._v(" "),s("span",{pre:!0,attrs:{class:"token comment"}},[t._v("// 组件销毁时执行")]),t._v("\n      "),s("span",{pre:!0,attrs:{class:"token comment"}},[t._v("// 取消订阅")]),t._v("\n      ChatAPI"),s("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v(".")]),s("span",{pre:!0,attrs:{class:"token function"}},[t._v("unsubscribeFromFriendStatus")]),s("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v("(")]),t._v("props"),s("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v(".")]),t._v("friend"),s("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v(".")]),t._v("id"),s("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v(",")]),t._v(" handleStatusChange"),s("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v(")")]),s("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v(";")]),t._v("\n    "),s("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v("}")]),s("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v(";")]),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  "),s("span",{pre:!0,attrs:{class:"token keyword"}},[t._v("if")]),t._v(" "),s("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v("(")]),t._v("isOnline "),s("span",{pre:!0,attrs:{class:"token operator"}},[t._v("===")]),t._v(" "),s("span",{pre:!0,attrs:{class:"token keyword"}},[t._v("null")]),s("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v(")")]),t._v(" "),s("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v("{")]),t._v("\n    "),s("span",{pre:!0,attrs:{class:"token keyword"}},[t._v("return")]),t._v(" "),s("span",{pre:!0,attrs:{class:"token string"}},[t._v("'Loading...'")]),s("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v(";")]),t._v("\n  "),s("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v("}")]),t._v("\n  "),s("span",{pre:!0,attrs:{class:"token keyword"}},[t._v("return")]),t._v(" isOnline "),s("span",{pre:!0,attrs:{class:"token operator"}},[t._v("?")]),t._v(" "),s("span",{pre:!0,attrs:{class:"token string"}},[t._v("'Online'")]),t._v(" "),s("span",{pre:!0,attrs:{class:"token operator"}},[t._v(":")]),t._v(" "),s("span",{pre:!0,attrs:{class:"token string"}},[t._v("'Offline'")]),s("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v(";")]),t._v("\n"),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("p",[t._v("在这个示例中，React 会在组件销毁时取消对 "),s("code",[t._v("ChatAPI")]),t._v(" 的订阅，然后在后续渲染时重新执行副作用函数。（如果传给 "),s("code",[t._v("ChatAPI")]),t._v(" 的 "),s("code",[t._v("props.friend.id")]),t._v(" 没有变化，你也可以"),s("a",{attrs:{href:"https://zh-hans.reactjs.org/docs/hooks-effect.html#tip-optimizing-performance-by-skipping-effects",target:"_blank",rel:"noopener noreferrer"}},[t._v("告诉 React 跳过重新订阅"),s("OutboundLink")],1),t._v("。）")]),t._v(" "),s("p",[t._v("跟 "),s("code",[t._v("useState")]),t._v(" 一样，你可以在组件中多次使用 "),s("code",[t._v("useEffect")]),t._v(" ：")]),t._v(" "),s("div",{staticClass:"language-jsx line-numbers-mode"},[s("pre",{pre:!0,attrs:{class:"language-jsx"}},[s("code",[s("span",{pre:!0,attrs:{class:"token keyword"}},[t._v("function")]),t._v(" "),s("span",{pre:!0,attrs:{class:"token function"}},[t._v("FriendStatusWithCounter")]),s("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v("(")]),s("span",{pre:!0,attrs:{class:"token parameter"}},[t._v("props")]),s("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v(")")]),t._v(" "),s("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v("{")]),t._v("\n  "),s("span",{pre:!0,attrs:{class:"token keyword"}},[t._v("const")]),t._v(" "),s("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v("[")]),t._v("count"),s("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v(",")]),t._v(" setCount"),s("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v("]")]),t._v(" "),s("span",{pre:!0,attrs:{class:"token operator"}},[t._v("=")]),t._v(" "),s("span",{pre:!0,attrs:{class:"token function"}},[t._v("useState")]),s("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v("(")]),s("span",{pre:!0,attrs:{class:"token number"}},[t._v("0")]),s("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v(")")]),s("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v(";")]),t._v("\n  "),s("span",{pre:!0,attrs:{class:"token function"}},[t._v("useEffect")]),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(" "),s("span",{pre:!0,attrs:{class:"token operator"}},[t._v("=>")]),t._v(" "),s("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v("{")]),t._v("\n    document"),s("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v(".")]),t._v("title "),s("span",{pre:!0,attrs:{class:"token operator"}},[t._v("=")]),t._v(" "),s("span",{pre:!0,attrs:{class:"token template-string"}},[s("span",{pre:!0,attrs:{class:"token template-punctuation string"}},[t._v("`")]),s("span",{pre:!0,attrs:{class:"token string"}},[t._v("You clicked ")]),s("span",{pre:!0,attrs:{class:"token interpolation"}},[s("span",{pre:!0,attrs:{class:"token interpolation-punctuation punctuation"}},[t._v("${")]),t._v("count"),s("span",{pre:!0,attrs:{class:"token interpolation-punctuation punctuation"}},[t._v("}")])]),s("span",{pre:!0,attrs:{class:"token string"}},[t._v(" times")]),s("span",{pre:!0,attrs:{class:"token template-punctuation string"}},[t._v("`")])]),s("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v(";")]),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\n  "),s("span",{pre:!0,attrs:{class:"token keyword"}},[t._v("const")]),t._v(" "),s("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v("[")]),t._v("isOnline"),s("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v(",")]),t._v(" setIsOnline"),s("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v("]")]),t._v(" "),s("span",{pre:!0,attrs:{class:"token operator"}},[t._v("=")]),t._v(" "),s("span",{pre:!0,attrs:{class:"token function"}},[t._v("useState")]),s("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v("(")]),s("span",{pre:!0,attrs:{class:"token keyword"}},[t._v("null")]),s("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v(")")]),s("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v(";")]),t._v("\n  "),s("span",{pre:!0,attrs:{class:"token function"}},[t._v("useEffect")]),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(" "),s("span",{pre:!0,attrs:{class:"token operator"}},[t._v("=>")]),t._v(" "),s("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v("{")]),t._v("\n    ChatAPI"),s("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v(".")]),s("span",{pre:!0,attrs:{class:"token function"}},[t._v("subscribeToFriendStatus")]),s("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v("(")]),t._v("props"),s("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v(".")]),t._v("friend"),s("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v(".")]),t._v("id"),s("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v(",")]),t._v(" handleStatusChange"),s("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v(")")]),s("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v(";")]),t._v("\n    "),s("span",{pre:!0,attrs:{class:"token keyword"}},[t._v("return")]),t._v(" "),s("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v("(")]),s("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v(")")]),t._v(" "),s("span",{pre:!0,attrs:{class:"token operator"}},[t._v("=>")]),t._v(" "),s("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v("{")]),t._v("\n      ChatAPI"),s("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v(".")]),s("span",{pre:!0,attrs:{class:"token function"}},[t._v("unsubscribeFromFriendStatus")]),s("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v("(")]),t._v("props"),s("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v(".")]),t._v("friend"),s("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v(".")]),t._v("id"),s("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v(",")]),t._v(" handleStatusChange"),s("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v(")")]),s("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v(";")]),t._v("\n    "),s("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v("}")]),s("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v(";")]),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\n  "),s("span",{pre:!0,attrs:{class:"token keyword"}},[t._v("function")]),t._v(" "),s("span",{pre:!0,attrs:{class:"token function"}},[t._v("handleStatusChange")]),s("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v("(")]),s("span",{pre:!0,attrs:{class:"token parameter"}},[t._v("status")]),s("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v(")")]),t._v(" "),s("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v("{")]),t._v("\n    "),s("span",{pre:!0,attrs:{class:"token function"}},[t._v("setIsOnline")]),s("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v("(")]),t._v("status"),s("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v(".")]),t._v("isOnline"),s("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v(")")]),s("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v(";")]),t._v("\n  "),s("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v("}")]),t._v("\n  "),s("span",{pre:!0,attrs:{class:"token comment"}},[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("p",[t._v("通过使用 Hook，你可以把组件内相关的副作用组织在一起（例如创建订阅及取消订阅），而不要把它们拆分到不同的生命周期函数里。")]),t._v(" "),s("blockquote",[s("p",[t._v("详细说明")]),t._v(" "),s("p",[t._v("你可以在这一章节了解更多关于 "),s("code",[t._v("useEffect")]),t._v(" 的内容："),s("a",{attrs:{href:"https://zh-hans.reactjs.org/docs/hooks-effect.html",target:"_blank",rel:"noopener noreferrer"}},[t._v("使用 Effect Hook"),s("OutboundLink")],1)])]),t._v(" "),s("h2",{attrs:{id:"hook-使用规则"}},[s("a",{staticClass:"header-anchor",attrs:{href:"#hook-使用规则"}},[t._v("#")]),t._v(" Hook 使用规则")]),t._v(" "),s("p",[t._v("Hook 就是 JavaScript 函数，但是使用它们会有两个额外的规则：")]),t._v(" "),s("ul",[s("li",[t._v("只能在"),s("strong",[t._v("函数最外层")]),t._v("调用 Hook。不要在循环、条件判断或者子函数中调用。")]),t._v(" "),s("li",[t._v("只能在 "),s("strong",[t._v("React 的函数组件")]),t._v("中调用 Hook。不要在其他 JavaScript 函数中调用。（还有一个地方可以调用 Hook —— 就是自定义的 Hook 中，我们稍后会学习到。）")])]),t._v(" "),s("p",[t._v("同时，我们提供了 "),s("a",{attrs:{href:"https://www.npmjs.com/package/eslint-plugin-react-hooks",target:"_blank",rel:"noopener noreferrer"}},[t._v("linter 插件"),s("OutboundLink")],1),t._v("来自动执行这些规则。这些规则乍看起来会有一些限制和令人困惑，但是要让 Hook 正常工作，它们至关重要。")]),t._v(" "),s("blockquote",[s("p",[t._v("详细说明")]),t._v(" "),s("p",[t._v("你可以在这章节了解更多关于这些规则的内容："),s("a",{attrs:{href:"https://zh-hans.reactjs.org/docs/hooks-rules.html",target:"_blank",rel:"noopener noreferrer"}},[t._v("Hook 使用规则"),s("OutboundLink")],1),t._v("。")])]),t._v(" "),s("h2",{attrs:{id:"自定义-hook-hook的使用逻辑封装"}},[s("a",{staticClass:"header-anchor",attrs:{href:"#自定义-hook-hook的使用逻辑封装"}},[t._v("#")]),t._v(" 自定义 Hook （Hook的使用逻辑封装）")]),t._v(" "),s("p",[t._v("有时候我们会想要在组件之间重用一些状态逻辑。目前为止，有两种主流方案来解决这个问题："),s("a",{attrs:{href:"https://zh-hans.reactjs.org/docs/higher-order-components.html",target:"_blank",rel:"noopener noreferrer"}},[t._v("高阶组件"),s("OutboundLink")],1),t._v("和 "),s("a",{attrs:{href:"https://zh-hans.reactjs.org/docs/render-props.html",target:"_blank",rel:"noopener noreferrer"}},[t._v("render props"),s("OutboundLink")],1),t._v("。自定义 Hook 可以让你在不增加组件的情况下达到同样的目的。")]),t._v(" "),s("p",[t._v("前面，我们介绍了一个叫 "),s("code",[t._v("FriendStatus")]),t._v(" 的组件，它通过调用 "),s("code",[t._v("useState")]),t._v(" 和 "),s("code",[t._v("useEffect")]),t._v(" 的 Hook 来订阅一个好友的在线状态。假设我们想在另一个组件里重用这个订阅逻辑。")]),t._v(" "),s("p",[t._v("首先，我们把这个逻辑抽取到一个叫做 "),s("code",[t._v("useFriendStatus")]),t._v(" 的自定义 Hook 里：")]),t._v(" "),s("div",{staticClass:"language-jsx line-numbers-mode"},[s("pre",{pre:!0,attrs:{class:"language-jsx"}},[s("code",[s("span",{pre:!0,attrs:{class:"token keyword"}},[t._v("import")]),t._v(" React"),s("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v(",")]),t._v(" "),s("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v("{")]),t._v(" useState"),s("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v(",")]),t._v(" useEffect "),s("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v("}")]),t._v(" "),s("span",{pre:!0,attrs:{class:"token keyword"}},[t._v("from")]),t._v(" "),s("span",{pre:!0,attrs:{class:"token string"}},[t._v("'react'")]),s("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v(";")]),t._v("\n\n"),s("span",{pre:!0,attrs:{class:"token comment"}},[t._v("// 逻辑抽取，方便复用")]),t._v("\n"),s("span",{pre:!0,attrs:{class:"token keyword"}},[t._v("function")]),t._v(" "),s("span",{pre:!0,attrs:{class:"token function"}},[t._v("useFriendStatus")]),s("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v("(")]),s("span",{pre:!0,attrs:{class:"token parameter"}},[t._v("friendID")]),s("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v(")")]),t._v(" "),s("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v("{")]),t._v("\n  "),s("span",{pre:!0,attrs:{class:"token comment"}},[t._v("// 每次调用时的state都是独立的")]),t._v("\n  "),s("span",{pre:!0,attrs:{class:"token keyword"}},[t._v("const")]),t._v(" "),s("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v("[")]),t._v("isOnline"),s("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v(",")]),t._v(" setIsOnline"),s("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v("]")]),t._v(" "),s("span",{pre:!0,attrs:{class:"token operator"}},[t._v("=")]),t._v(" "),s("span",{pre:!0,attrs:{class:"token function"}},[t._v("useState")]),s("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v("(")]),s("span",{pre:!0,attrs:{class:"token keyword"}},[t._v("null")]),s("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v(")")]),s("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v(";")]),t._v("\n\n  "),s("span",{pre:!0,attrs:{class:"token keyword"}},[t._v("function")]),t._v(" "),s("span",{pre:!0,attrs:{class:"token function"}},[t._v("handleStatusChange")]),s("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v("(")]),s("span",{pre:!0,attrs:{class:"token parameter"}},[t._v("status")]),s("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v(")")]),t._v(" "),s("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v("{")]),t._v("\n    "),s("span",{pre:!0,attrs:{class:"token function"}},[t._v("setIsOnline")]),s("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v("(")]),t._v("status"),s("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v(".")]),t._v("isOnline"),s("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v(")")]),s("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v(";")]),t._v("\n  "),s("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v("}")]),t._v("\n\n  "),s("span",{pre:!0,attrs:{class:"token function"}},[t._v("useEffect")]),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(" "),s("span",{pre:!0,attrs:{class:"token operator"}},[t._v("=>")]),t._v(" "),s("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v("{")]),t._v("\n    ChatAPI"),s("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v(".")]),s("span",{pre:!0,attrs:{class:"token function"}},[t._v("subscribeToFriendStatus")]),s("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v("(")]),t._v("friendID"),s("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v(",")]),t._v(" handleStatusChange"),s("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v(")")]),s("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v(";")]),t._v("\n    "),s("span",{pre:!0,attrs:{class:"token keyword"}},[t._v("return")]),t._v(" "),s("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v("(")]),s("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v(")")]),t._v(" "),s("span",{pre:!0,attrs:{class:"token operator"}},[t._v("=>")]),t._v(" "),s("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v("{")]),t._v("\n      ChatAPI"),s("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v(".")]),s("span",{pre:!0,attrs:{class:"token function"}},[t._v("unsubscribeFromFriendStatus")]),s("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v("(")]),t._v("friendID"),s("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v(",")]),t._v(" handleStatusChange"),s("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v(")")]),s("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v(";")]),t._v("\n    "),s("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v("}")]),s("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v(";")]),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\n  "),s("span",{pre:!0,attrs:{class:"token keyword"}},[t._v("return")]),t._v(" isOnline"),s("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v(";")]),t._v("\n"),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("p",[t._v("它将 "),s("code",[t._v("friendID")]),t._v(" 作为参数，并返回该好友是否在线：")]),t._v(" "),s("p",[t._v("现在我们可以在两个组件中使用它：")]),t._v(" "),s("div",{staticClass:"language-jsx line-numbers-mode"},[s("pre",{pre:!0,attrs:{class:"language-jsx"}},[s("code",[s("span",{pre:!0,attrs:{class:"token keyword"}},[t._v("function")]),t._v(" "),s("span",{pre:!0,attrs:{class:"token function"}},[t._v("FriendStatus")]),s("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v("(")]),s("span",{pre:!0,attrs:{class:"token parameter"}},[t._v("props")]),s("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v(")")]),t._v(" "),s("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v("{")]),t._v("\n  "),s("span",{pre:!0,attrs:{class:"token keyword"}},[t._v("const")]),t._v(" isOnline "),s("span",{pre:!0,attrs:{class:"token operator"}},[t._v("=")]),t._v(" "),s("span",{pre:!0,attrs:{class:"token function"}},[t._v("useFriendStatus")]),s("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v("(")]),t._v("props"),s("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v(".")]),t._v("friend"),s("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v(".")]),t._v("id"),s("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v(")")]),s("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v(";")]),t._v("\n  "),s("span",{pre:!0,attrs:{class:"token keyword"}},[t._v("if")]),t._v(" "),s("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v("(")]),t._v("isOnline "),s("span",{pre:!0,attrs:{class:"token operator"}},[t._v("===")]),t._v(" "),s("span",{pre:!0,attrs:{class:"token keyword"}},[t._v("null")]),s("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v(")")]),t._v(" "),s("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v("{")]),t._v("\n    "),s("span",{pre:!0,attrs:{class:"token keyword"}},[t._v("return")]),t._v(" "),s("span",{pre:!0,attrs:{class:"token string"}},[t._v("'Loading...'")]),s("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v(";")]),t._v("\n  "),s("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v("}")]),t._v("\n  "),s("span",{pre:!0,attrs:{class:"token keyword"}},[t._v("return")]),t._v(" isOnline "),s("span",{pre:!0,attrs:{class:"token operator"}},[t._v("?")]),t._v(" "),s("span",{pre:!0,attrs:{class:"token string"}},[t._v("'Online'")]),t._v(" "),s("span",{pre:!0,attrs:{class:"token operator"}},[t._v(":")]),t._v(" "),s("span",{pre:!0,attrs:{class:"token string"}},[t._v("'Offline'")]),s("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v(";")]),t._v("\n"),s("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v("}")]),t._v("\n"),s("span",{pre:!0,attrs:{class:"token keyword"}},[t._v("function")]),t._v(" "),s("span",{pre:!0,attrs:{class:"token function"}},[t._v("FriendListItem")]),s("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v("(")]),s("span",{pre:!0,attrs:{class:"token parameter"}},[t._v("props")]),s("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v(")")]),t._v(" "),s("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v("{")]),t._v("\n  "),s("span",{pre:!0,attrs:{class:"token keyword"}},[t._v("const")]),t._v(" isOnline "),s("span",{pre:!0,attrs:{class:"token operator"}},[t._v("=")]),t._v(" "),s("span",{pre:!0,attrs:{class:"token function"}},[t._v("useFriendStatus")]),s("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v("(")]),t._v("props"),s("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v(".")]),t._v("friend"),s("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v(".")]),t._v("id"),s("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v(")")]),s("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v(";")]),t._v("\n  "),s("span",{pre:!0,attrs:{class:"token keyword"}},[t._v("return")]),t._v(" "),s("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v("(")]),t._v("\n    "),s("span",{pre:!0,attrs:{class:"token tag"}},[s("span",{pre:!0,attrs:{class:"token tag"}},[s("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v("<")]),t._v("li")]),t._v(" "),s("span",{pre:!0,attrs:{class:"token attr-name"}},[t._v("style")]),s("span",{pre:!0,attrs:{class:"token script language-javascript"}},[s("span",{pre:!0,attrs:{class:"token script-punctuation 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(" "),s("span",{pre:!0,attrs:{class:"token literal-property property"}},[t._v("color")]),s("span",{pre:!0,attrs:{class:"token operator"}},[t._v(":")]),t._v(" isOnline "),s("span",{pre:!0,attrs:{class:"token operator"}},[t._v("?")]),t._v(" "),s("span",{pre:!0,attrs:{class:"token string"}},[t._v("'green'")]),t._v(" "),s("span",{pre:!0,attrs:{class:"token operator"}},[t._v(":")]),t._v(" "),s("span",{pre:!0,attrs:{class:"token string"}},[t._v("'black'")]),t._v(" "),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(">")])]),s("span",{pre:!0,attrs:{class:"token plain-text"}},[t._v("\n      ")]),s("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v("{")]),t._v("props"),s("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v(".")]),t._v("friend"),s("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v(".")]),t._v("name"),s("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v("}")]),s("span",{pre:!0,attrs:{class:"token plain-text"}},[t._v("\n    ")]),s("span",{pre:!0,attrs:{class:"token tag"}},[s("span",{pre:!0,attrs:{class:"token tag"}},[s("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v("</")]),t._v("li")]),s("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v(">")])]),t._v("\n  "),s("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v(")")]),s("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v(";")]),t._v("\n"),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("p",[s("strong",[t._v("每个组件间的 state 是完全独立的。Hook 是一种复用"),s("em",[t._v("状态逻辑")]),t._v("的方式，它不复用 state 本身")]),t._v("。事实上 Hook 的每次"),s("em",[t._v("调用")]),t._v("都有一个完全独立的 state —— 因此你可以在单个组件中多次调用同一个自定义 Hook。")]),t._v(" "),s("p",[t._v("自定义 Hook 更像是一种约定而不是功能。"),s("strong",[t._v("如果函数的名字以 “"),s("code",[t._v("use")]),t._v("” 开头并调用其他 Hook，我们就说这是一个自定义 Hook")]),t._v("。 "),s("code",[t._v("useSomething")]),t._v(" 的命名约定可以让我们的 linter 插件在使用 Hook 的代码中找到 bug。")]),t._v(" "),s("p",[t._v("你可以创建涵盖各种场景的自定义 Hook，如表单处理、动画、订阅声明、计时器，甚至可能还有更多我们没想到的场景。我们很期待看到 React 社区会出现什么样的自定义 Hook。")]),t._v(" "),s("blockquote",[s("p",[t._v("详细说明")]),t._v(" "),s("p",[t._v("我们会在这一章节介绍更多关于自定义 Hook 的内容： "),s("a",{attrs:{href:"https://zh-hans.reactjs.org/docs/hooks-custom.html",target:"_blank",rel:"noopener noreferrer"}},[t._v("创建你自己的 Hook"),s("OutboundLink")],1),t._v("。")])]),t._v(" "),s("h2",{attrs:{id:"其他-hook"}},[s("a",{staticClass:"header-anchor",attrs:{href:"#其他-hook"}},[t._v("#")]),t._v(" 其他 Hook")]),t._v(" "),s("p",[t._v("除此之外，还有一些使用频率较低的但是很有用的 Hook。比如，"),s("a",{attrs:{href:"https://zh-hans.reactjs.org/docs/hooks-reference.html#usecontext",target:"_blank",rel:"noopener noreferrer"}},[s("code",[t._v("useContext")]),s("OutboundLink")],1),t._v(" 让你不使用组件嵌套就可以订阅 React 的 Context。")]),t._v(" "),s("div",{staticClass:"language-jsx line-numbers-mode"},[s("pre",{pre:!0,attrs:{class:"language-jsx"}},[s("code",[s("span",{pre:!0,attrs:{class:"token keyword"}},[t._v("function")]),t._v(" "),s("span",{pre:!0,attrs:{class:"token function"}},[t._v("Example")]),s("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v("(")]),s("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v(")")]),t._v(" "),s("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v("{")]),t._v("\n  "),s("span",{pre:!0,attrs:{class:"token keyword"}},[t._v("const")]),t._v(" locale "),s("span",{pre:!0,attrs:{class:"token operator"}},[t._v("=")]),t._v(" "),s("span",{pre:!0,attrs:{class:"token function"}},[t._v("useContext")]),s("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v("(")]),t._v("LocaleContext"),s("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v(")")]),s("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v(";")]),t._v("\n  "),s("span",{pre:!0,attrs:{class:"token keyword"}},[t._v("const")]),t._v(" theme "),s("span",{pre:!0,attrs:{class:"token operator"}},[t._v("=")]),t._v(" "),s("span",{pre:!0,attrs:{class:"token function"}},[t._v("useContext")]),s("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v("(")]),t._v("ThemeContext"),s("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v(")")]),s("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v(";")]),t._v("\n  "),s("span",{pre:!0,attrs:{class:"token comment"}},[t._v("// ...")]),t._v("\n"),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("p",[t._v("另外 "),s("a",{attrs:{href:"https://zh-hans.reactjs.org/docs/hooks-reference.html#usereducer",target:"_blank",rel:"noopener noreferrer"}},[s("code",[t._v("useReducer")]),s("OutboundLink")],1),t._v(" 可以让你通过 reducer 来管理组件本地的复杂 state。")]),t._v(" "),s("div",{staticClass:"language-jsx line-numbers-mode"},[s("pre",{pre:!0,attrs:{class:"language-jsx"}},[s("code",[s("span",{pre:!0,attrs:{class:"token keyword"}},[t._v("function")]),t._v(" "),s("span",{pre:!0,attrs:{class:"token function"}},[t._v("Todos")]),s("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v("(")]),s("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v(")")]),t._v(" "),s("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v("{")]),t._v("\n  "),s("span",{pre:!0,attrs:{class:"token keyword"}},[t._v("const")]),t._v(" "),s("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v("[")]),t._v("todos"),s("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v(",")]),t._v(" dispatch"),s("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v("]")]),t._v(" "),s("span",{pre:!0,attrs:{class:"token operator"}},[t._v("=")]),t._v(" "),s("span",{pre:!0,attrs:{class:"token function"}},[t._v("useReducer")]),s("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v("(")]),t._v("todosReducer"),s("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v(")")]),s("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v(";")]),t._v("  "),s("span",{pre:!0,attrs:{class:"token comment"}},[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("blockquote",[s("p",[t._v("详细说明")]),t._v(" "),s("p",[t._v("你可以在这一章节了解更多关于所有内置 Hook 的内容："),s("a",{attrs:{href:"https://zh-hans.reactjs.org/docs/hooks-reference.html",target:"_blank",rel:"noopener noreferrer"}},[t._v("Hook API 索引"),s("OutboundLink")],1),t._v("。")])])])}),[],!1,null,null,null);s.default=e.exports}}]);
    2.