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  1. (window.webpackJsonp=window.webpackJsonp||[]).push([[157],{487: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:"_03-使用effect-hook-副作用钩子"}},[s("a",{staticClass:"header-anchor",attrs:{href:"#_03-使用effect-hook-副作用钩子"}},[t._v("#")]),t._v(" 03. 使用Effect Hook（副作用钩子）")]),t._v(" "),s("p",[t._v("如果你熟悉 React class 的生命周期函数，你可以把 "),s("code",[t._v("useEffect")]),t._v(" Hook 看做 "),s("code",[t._v("componentDidMount")]),t._v("(挂载完成)，"),s("code",[t._v("componentDidUpdate")]),t._v("(更新完成) 和 "),s("code",[t._v("componentWillUnmount")]),t._v("（即将销毁前） 这三个函数的组合。")]),t._v(" "),s("blockquote",[s("p",[t._v("Did : 做了...     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class 中，我们需要在两个生命周期函数中编写重复的代码。")])]),t._v(" "),s("p",[t._v("这是因为很多情况下，我们希望在组件加载和更新时执行同样的操作。"),s("strong",[t._v("从概念上说，我们希望它在每次渲染之后执行")]),t._v(" —— 但 React 的 class 组件没有提供这样的方法。即使我们提取出一个方法，我们还是要在两个地方调用它。")]),t._v(" "),s("p",[t._v("现在让我们来看看如何使用 "),s("code",[t._v("useEffect")]),t._v(" 执行相同的操作。")]),t._v(" "),s("h3",{attrs:{id:"使用-hook-的示例"}},[s("a",{staticClass:"header-anchor",attrs:{href:"#使用-hook-的示例"}},[t._v("#")]),t._v(" 使用 Hook 的示例")]),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 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做了什么？")]),t._v(" 通过使用这个 Hook，你可以告诉 React 组件需要在渲染后执行某些操作。React 会保存你传递的函数（我们将它称之为 “effect”），并且在执行 DOM 更新之后调用它。在这个 effect 中，我们设置了 document 的 title 属性，不过我们也可以执行数据获取或调用其他命令式的 API。")]),t._v(" "),s("p",[s("strong",[t._v("为什么在组件内部调用 "),s("code",[t._v("useEffect")]),t._v("？")]),t._v(" 将 "),s("code",[t._v("useEffect")]),t._v(" 放在组件内部让我们可以在 effect 中直接访问 "),s("code",[t._v("count")]),t._v(" state 变量（或其他 props）。我们不需要特殊的 API 来读取它 —— 它已经保存在函数作用域中。Hook 使用了 JavaScript 的闭包机制，而不用在 JavaScript 已经提供了解决方案的情况下，还引入特定的 React API。")]),t._v(" "),s("p",[s("strong",[s("code",[t._v("useEffect")]),t._v(" 会在每次渲染后都执行吗？")]),t._v(" 是的，默认情况下，它在第一次渲染之后"),s("em",[t._v("和")]),t._v("每次更新之后都会执行。（我们稍后会谈到"),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("如何控制它"),s("OutboundLink")],1),t._v("。）你可能会更容易接受 effect 发生在“渲染之后”这种概念，不用再去考虑“挂载”还是“更新”。React 保证了每次运行 effect 的同时，DOM 都已经更新完毕。")]),t._v(" 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使用 Effect 的提示")]),t._v(" "),s("h3",{attrs:{id:"使用hook的目的"}},[s("a",{staticClass:"header-anchor",attrs:{href:"#使用hook的目的"}},[t._v("#")]),t._v(" 使用Hook的目的")]),t._v(" "),s("p",[t._v("使用 Hook 其中一个"),s("a",{attrs:{href:"https://zh-hans.reactjs.org/docs/hooks-intro.html#complex-components-become-hard-to-understand",target:"_blank",rel:"noopener noreferrer"}},[t._v("目的"),s("OutboundLink")],1),t._v("就是要解决 class 中生命周期函数经常包含不相关的逻辑，但又把相关逻辑分离到了几个不同方法中的问题。")]),t._v(" "),s("h3",{attrs:{id:"提示-使用多个-effect-实现关注点分离"}},[s("a",{staticClass:"header-anchor",attrs:{href:"#提示-使用多个-effect-实现关注点分离"}},[t._v("#")]),t._v(" 提示: 使用多个 Effect 实现关注点分离")]),t._v(" "),s("p",[t._v("可以使用多个 effect。这会将不相关逻辑分离到不同的 effect 中：")]),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 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"),s("div",{staticClass:"line-numbers-wrapper"},[s("span",{staticClass:"line-number"},[t._v("1")]),s("br"),s("span",{staticClass:"line-number"},[t._v("2")]),s("br"),s("span",{staticClass:"line-number"},[t._v("3")]),s("br"),s("span",{staticClass:"line-number"},[t._v("4")]),s("br"),s("span",{staticClass:"line-number"},[t._v("5")]),s("br"),s("span",{staticClass:"line-number"},[t._v("6")]),s("br"),s("span",{staticClass:"line-number"},[t._v("7")]),s("br"),s("span",{staticClass:"line-number"},[t._v("8")]),s("br"),s("span",{staticClass:"line-number"},[t._v("9")]),s("br"),s("span",{staticClass:"line-number"},[t._v("10")]),s("br"),s("span",{staticClass:"line-number"},[t._v("11")]),s("br"),s("span",{staticClass:"line-number"},[t._v("12")]),s("br"),s("span",{staticClass:"line-number"},[t._v("13")]),s("br"),s("span",{staticClass:"line-number"},[t._v("14")]),s("br"),s("span",{staticClass:"line-number"},[t._v("15")]),s("br"),s("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("h3",{attrs:{id:"解释-为什么每次更新的时候都要运行-effect"}},[s("a",{staticClass:"header-anchor",attrs:{href:"#解释-为什么每次更新的时候都要运行-effect"}},[t._v("#")]),t._v(" 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    2.