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155.f16ed794.js

155.f16ed794.js 27 KB
Cronologia Originale

1 
  1. 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初始值为 "),s("code",[t._v("{ count: 0 }")]),t._v(" ，当用户点击按钮后，我们通过调用 "),s("code",[t._v("this.setState()")]),t._v(" 来增加 "),s("code",[t._v("state.count")]),t._v("。整个章节中都将使用该 class 的代码片段做示例。")]),t._v(" "),s("h3",{attrs:{id:"hook-在-class-内部是不起作用"}},[s("a",{staticClass:"header-anchor",attrs:{href:"#hook-在-class-内部是不起作用"}},[t._v("#")]),t._v(" Hook 在 class 内部是不起作用")]),t._v(" "),s("p",[s("strong",[t._v("Hook 在 class 内部是不起作用的")]),t._v("。但你可以使用它们来取代 class 。")]),t._v(" "),s("h2",{attrs:{id:"调用-usestate-方法的时候做了什么"}},[s("a",{staticClass:"header-anchor",attrs:{href:"#调用-usestate-方法的时候做了什么"}},[t._v("#")]),t._v(" 调用 "),s("code",[t._v("useState")]),t._v(" 方法的时候做了什么?")]),t._v(" "),s("p",[t._v("它定义一个 “state 变量”。我们的变量叫 "),s("code",[t._v("count")]),t._v("， 但是我们可以叫他任何名字，比如 "),s("code",[t._v("banana")]),t._v("。这是一种在函数调用时保存变量的方式 —— "),s("code",[t._v("useState")]),t._v(" 是一种新方法，它与 class 里面的 "),s("code",[t._v("this.state")]),t._v(" 提供的功能完全相同。"),s("strong",[t._v("一般来说，在函数退出后变量就会”消失”，而 state 中的变量会被 React 保留。")])]),t._v(" "),s("h2",{attrs:{id:"usestate-需要哪些参数"}},[s("a",{staticClass:"header-anchor",attrs:{href:"#usestate-需要哪些参数"}},[t._v("#")]),t._v(" "),s("code",[t._v("useState")]),t._v(" 需要哪些参数？")]),t._v(" "),s("p",[s("code",[t._v("useState()")]),t._v(" 方法里面"),s("strong",[t._v("唯一的参数就是初始 state")]),t._v("。不同于 class 的是，我们可以按照需要使用数字或字符串对其进行赋值，而不一定是对象。在示例中，只需使用数字来记录用户点击次数，所以我们传了 "),s("code",[t._v("0")]),t._v(" 作为变量的初始 state。（如果我们想要在 state 中存储两个不同的变量，只需调用 "),s("code",[t._v("useState()")]),t._v(" 两次即可。）")]),t._v(" "),s("h2",{attrs:{id:"usestate-方法的返回值是什么"}},[s("a",{staticClass:"header-anchor",attrs:{href:"#usestate-方法的返回值是什么"}},[t._v("#")]),t._v(" "),s("code",[t._v("useState")]),t._v(" 方法的返回值是什么？")]),t._v(" "),s("p",[t._v("返回值为：当前 state 以及更新 state 的函数。这就是我们写 "),s("code",[t._v("const [count, setCount] = useState()")]),t._v(" 的原因。这与 class 里面 "),s("code",[t._v("this.state.count")]),t._v(" 和 "),s("code",[t._v("this.setState")]),t._v(" 类似，唯一区别就是你需要成对的获取它们。")]),t._v(" "),s("h2",{attrs:{id:"理解usestate"}},[s("a",{staticClass:"header-anchor",attrs:{href:"#理解usestate"}},[t._v("#")]),t._v(" 理解useState")]),t._v(" "),s("p",[t._v("既然我们知道了 "),s("code",[t._v("useState")]),t._v(" 的作用，我们的示例应该更容易理解了：")]),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 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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(" 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punctuation"}},[t._v(")")]),s("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v(";")]),t._v("\n")])]),t._v(" "),s("div",{staticClass:"line-numbers-wrapper"},[s("span",{staticClass:"line-number"},[t._v("1")]),s("br"),s("span",{staticClass:"line-number"},[t._v("2")]),s("br"),s("span",{staticClass:"line-number"},[t._v("3")]),s("br"),s("span",{staticClass:"line-number"},[t._v("4")]),s("br"),s("span",{staticClass:"line-number"},[t._v("5")]),s("br")])]),s("p",[t._v("在以上组件中，我们有局部变量 "),s("code",[t._v("age")]),t._v("，"),s("code",[t._v("fruit")]),t._v(" 和 "),s("code",[t._v("todos")]),t._v("，并且我们可以单独更新它们：")]),t._v(" "),s("div",{staticClass:"language-jsx line-numbers-mode"},[s("pre",{pre:!0,attrs:{class:"language-jsx"}},[s("code",[t._v("  "),s("span",{pre:!0,attrs:{class:"token keyword"}},[t._v("function")]),t._v(" "),s("span",{pre:!0,attrs:{class:"token function"}},[t._v("handleOrangeClick")]),s("span",{pre:!0,attrs:{class:"token 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    2.