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docs: React Note update
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@ -341,4 +341,695 @@ root.render(<App />)
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- All in JS
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- 不同于Vue的模板语法 不需要专门学习模板语法中的指令(v-for/v-if/v-bind)
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### JSX的使用
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#### 书写JSX的规范与注意事项
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- JSX的顶层只能有一个根元素 元素必须包裹在单独的闭合标签中
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- 后续会接触到Fragment标签 Vue3也是将元素包裹在了Fragments标签中
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- 为了方便阅读 通常在JSX外层包裹一个小括号`()`方便阅读
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#### JSX的注释
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在JSX中编写注释，需要以`{/* ... */}`的形式，在`.jsx/.tsx`文件中，通过快捷键就可以快捷的生成注释内容
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本质上是通过花括号语法`{}`嵌入了一段JavaScript表达式，在表达式中书写注释
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```tsx{4}
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...
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return (
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<div>
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{/* Some Comment... */}
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<h1>Count: {count}</h1>
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<button onClick={this.addCount}>Add</button>
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<button onClick={this.subCount}>Sub</button>
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</div>
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)
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...
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```
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#### JSX嵌入变量作为子元素
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可以通过花括号语法将变量内容嵌入到JSX语法中：
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```tsx
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const message = 'Hello, React!'
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const arr = ['abc', 'cba', 'nba']
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return (
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<div>
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<h1>{ message }</h1>
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<div>{ arr }</div>
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</div>
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)
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```
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- 变量类型为number string array类型时，可以直接展示
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- 变量类型为null undefined boolean类型时，内容为空
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- 如果希望可以展示null/undefined/boolean类型，需要通过`.toString()`方法将其转为字符串
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- 空字符串拼接、String构造函数等方式
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- Object对象类型不能作为子元素 (Objects are not valid as a React child)
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下例中，只有number类型会被正常展示，而其余变量则不会展示在视图中
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```tsx
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render() {
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const number = 123
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const n = null
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const u = undefined
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const b = true
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return (
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<div>
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<div>
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Number: {number}
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</div>
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<div>
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Null: {n}
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</div>
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<div>
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Undefined: {u}
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</div>
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<div>
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Boolean: {b}
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</div>
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</div>
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)
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}
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```
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将对象类型变量嵌入到JSX语法中，React会抛出错误：
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```tsx {6}
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...
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render() {
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const obj = { name: 'Ziu' }
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return (
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<div>
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{ obj }
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</div>
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)
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}
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...
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```
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#### JSX的属性绑定
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- 在Vue中我们通过`v-bind`绑定属性
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- 在React中如何绑定元素属性？
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- `title` `src` `href` `class` 内联`style`等
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下例中，我们通过花括号语法对元素的属性进行了动态绑定，点击按钮可以切换className状态
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同时，动态绑定的内联样式也会发生改变，通过花括号语法动态绑定style属性
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```tsx
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class App extends React.Component {
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constructor() {
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super()
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this.state = {
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isActive: false,
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title: 'Description'
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}
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this.changeActive = this.changeActive.bind(this)
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}
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changeActive() {
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this.setState({
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isActive: !this.state.isActive
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})
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}
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render() {
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const { isActive, title } = this.state
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const classList = ['title', isActive ? 'active' : '']
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return (
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<div>
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<div
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className={classList.join(' ')}
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title={title}
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style={{ color: isActive ? 'red' : 'blue' }}
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>
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Hello, React!
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</div>
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<button onClick={this.changeActive}>Change Active</button>
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</div>
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)
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}
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}
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```
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当我们通过脚手架创建项目时，可以使用第三方库来帮我们完成className的绑定
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- `classnames`库 `pnpm add classnames`
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- 提供了多种创建className的语法
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### JSX事件绑定
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先前的例子中，我们已经通过`onClick`给按钮绑定过事件处理函数了，其中涉及了this绑定
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回顾一下this的四种绑定规则：
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1. 默认绑定 独立执行 foo() this => undefined
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2. 隐式绑定 被一个对象执行 obj.foo() this => obj
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3. 显式绑定 call/bind/apply foo.call('aaa') this => String('aaa')
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4. new绑定 new Foo() 创建一个新对象，并且赋值给this
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除了之前通过`function + bind`绑定事件处理函数的方式，还可以通过箭头函数来帮我们完成处理
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箭头函数的内部使用this时会自动向上层作用域查找this 实际开发中这种方式并不常用
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```tsx {2}
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...
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changeActive = () => {
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this.setState({
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isActive: !this.state.isActive
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})
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}
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...
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```
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相比之下更推荐使用的，是下面这种方式：
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```tsx {2}
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...
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<button onClick={() => this.changeActive()}>Change Active</button>
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...
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```
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这样书写有几种好处：
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- 给事件处理函数传递参数更方便
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- 书写更方便 不必主动考虑this绑定问题
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它的原理是，我们对外暴露的本质上是一个箭头函数，当调用箭头函数时，本质上是执行`this.changeActive`，这是 一种隐式绑定，找到的this为当前组件实例
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### 事件绑定参数传递
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- Event参数传递
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- 额外参数传递
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事件回调函数的第一个默认参数就是Event对象，这个Event对象是经过React包装后的，但是原生的属性都包含在内，React对其进行了一些扩展
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```tsx {13}
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...
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changeActive(ev) {
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console.log('Event', ev)
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}
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render() {
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return (
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<div>
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{/* event将作为默认入参传递给changeActive */}
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<button onClick={this.changeActive}>Change Active</button>
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{/* 通过箭头函数绑定事件监听回调函数时 需要手动透传一下event */}
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<button onClick={(ev) => this.changeActive(ev)}>Change Active</button>
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</div>
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)
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}
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...
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```
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当我们需要传递额外的参数时，通过箭头函数传递也更容易：
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```tsx {13}
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changeActive(ev, name, age) {
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console.log('Event', ev)
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console.log('Name', name)
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console.log('Age', age)
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}
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render() {
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return (
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<div>
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{/* NOT Recommand */}
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<button onClick={this.changeActive.bind(this, 'Ziu', 18)}>Change Active</button>
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{/* Recommand */}
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<button onClick={(ev) => this.changeActive(ev, 'Ziu', 18)}>Change Active</button>
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</div>
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)
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}
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```
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需要注意，当通过`.bind`传递额外参数时，最后一个参数才是默认传递的Event对象，这会导致非预期行为
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```sh
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> Event 'Ziu'
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> Name 18
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> Age {Event}
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```
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### JSX事件绑定案例
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创建一个Tab栏，选中哪个选项，哪个选项被激活切换为红色，同一时间仅有一个激活项目
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结合之前学习的内容，很容易就可以写出下述 代码：
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```tsx
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class App extends React.Component {
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constructor() {
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super()
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this.state = {
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activeIndex: 0,
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tabList: ['Home', 'Recommend', 'Hot', 'User']
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}
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}
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changeActive(index) {
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this.setState({
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activeIndex: index
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})
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}
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render() {
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const { activeIndex, tabList } = this.state
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return (
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<div>
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<div className="tabs">
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{tabList.map((item, index) => (
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<button
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className={'tab ' + index === activeIndex ? 'active' : ''}
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style={{
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color: index === activeIndex ? 'red' : 'black'
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}}
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key={index}
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onClick={() => this.changeActive(index)}
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>
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{item}
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</button>
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))}
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</div>
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</div>
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)
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}
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}
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const root = ReactDOM.createRoot(document.querySelector('#root'))
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root.render(<App />)
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```
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### 条件渲染
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控制元素按照某种条件渲染，以加载状态为例
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列表未加载出来时，展示`加载中`，加载完毕则渲染完整内容：
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```tsx
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class App extends React.Component {
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constructor() {
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super()
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this.state = {
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isLoading: true
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}
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}
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changeLoading() {
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this.setState({
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isLoading: !this.state.isLoading
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})
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}
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render() {
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const { isLoading } = this.state
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return (
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<div>
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{isLoading ? (
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<div className="loading"> Loading ... </div>
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) : (
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<div className="list">Some Content</div>
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)}
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<button onClick={() => this.changeLoading()}>Toggle Loading</button>
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</div>
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)
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}
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}
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const root = ReactDOM.createRoot(document.querySelector('#root'))
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root.render(<App />)
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```
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常用的条件渲染方式
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- `if/else/else-if`
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- 适合判断逻辑较复杂的情况 将条件渲染抽离出来
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- 三元运算符 `?:`
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- 适合判断逻辑简单的情况
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- 逻辑与运算符 `&&`
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- 如果条件成立则渲染某个组件，否则什么内容都不渲染
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- 可选链 `user?.info?.name`
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下例中通过逻辑与运算符`&&`决定`VIP`标签是否展示在视图中
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```tsx
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class App extends React.Component {
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constructor() {
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super()
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this.state = {
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isVip: false
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}
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}
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changeVip() {
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this.setState({
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isVip: !this.state.isVip
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})
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}
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render() {
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const { isVip } = this.state
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return (
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<div>
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<div class="user">
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<span>username: Ziu</span>
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{isVip && <span className="vip-banner"> VIP </span>}
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</div>
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<button onClick={() => this.changeVip()}>Toggle Vip</button>
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</div>
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)
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}
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}
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const root = ReactDOM.createRoot(document.querySelector('#root'))
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root.render(<App />)
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```
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#### 在React中简单写一个"v-show"
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`v-show`是Vue提供的语法糖，不同于`v-if`，它只切换元素的`display`属性。
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下面我们在React中简单复现一个`v-show`的效果：
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```tsx
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class App extends React.Component {
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constructor() {
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super()
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this.state = {
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isShow: true
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}
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}
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changeShow() {
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this.setState({
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isShow: !this.state.isShow
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})
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}
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render() {
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const { isShow } = this.state
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return (
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<div>
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<div style={{ display: isShow ? '' : 'none' }}>Target Element</div>
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<button onClick={() => this.changeShow()}>Toggle Show</button>
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</div>
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)
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}
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}
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```
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实际使用中，将其封装为hooks来调用更具通用性，也更方便管理
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### 列表渲染中的高阶函数
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- `filter`函数 过滤器
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- `slice`函数 分页
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- `sorc`函数 排序
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- ...
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```tsx
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class App extends React.Component {
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constructor() {
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super()
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this.state = {
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stuList: [
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{ name: 'Ziu', age: 18, score: 88 },
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{ name: 'Kobe', age: 19, score: 59 },
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{ name: 'Why', age: 20, score: 61 },
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{ name: 'James', age: 21, score: 99 }
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]
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}
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}
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render() {
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const { stuList } = this.state
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// 及格的学生
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const passStuList = stuList.filter((item) => item.score >= 60)
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// 分数最高的两个学生
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const top2StuList = stuList.sort((a, b) => b.score - a.score).slice(0, 2)
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return (
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<div>
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<div className="list">
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{stuList.map(({ name, age, score }) => (
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<div className="item" key={name}>
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<span className="name">{name}</span>
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<span className="age">{age}</span>
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<span className="score">{score}</span>
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</div>
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))}
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</div>
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</div>
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)
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}
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}
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const root = ReactDOM.createRoot(document.querySelector('#root'))
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root.render(<App />)
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```
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### JSX的本质
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#### JSX的转换过程
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假设我们有下面的JSX代码：
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```tsx
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class App extends React.Component {
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constructor() {
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super()
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}
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render() {
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const page = (
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<div className="page">
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<div className="header">Header</div>
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<div className="content">
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Content
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<div className="banner">Banner</div>
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<ul>
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<li>Item 1</li>
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<li>Item 2</li>
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<li>Item 3</li>
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<li>Item 4</li>
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<li>Item 5</li>
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</ul>
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</div>
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<div className="footer">Footer</div>
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</div>
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)
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console.log(page)
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return <div>{page}</div>
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}
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}
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```
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通过JSX语法描述出来的template会经过Babel转化，转化为JavaScript树的数据结构
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|
||||
在控制台中我们可以看到，子节点都存放进了父节点的`props.children`中
|
||||
|
||||
#### 虚拟DOM树
|
||||
|
||||
JSX仅仅是`React.createElement(component, props, ...children)`的语法糖
|
||||
|
||||
所有的JSX语法都会被Babel转化为这样的命令式语法
|
||||
|
||||
.createElement函数的参数
|
||||
|
||||
- type
|
||||
- 当前ReactElement的类型
|
||||
- 如果是标签元素，值为字符串如：`"div"`
|
||||
- 如果是组件元素，那么值为组件的名称
|
||||
- config
|
||||
- 所有JSX中绑定的属性都在config中以键值对的形式存储
|
||||
- 例如`className` => `class`
|
||||
|
||||
我们借助Babel官网的Playground来检查一下JSX语法的转化
|
||||
|
||||
```js
|
||||
import { jsx as _jsx } from "react/jsx-runtime";
|
||||
import { jsxs as _jsxs } from "react/jsx-runtime";
|
||||
const page = /*#__PURE__*/_jsxs("div", {
|
||||
className: "page",
|
||||
children: [/*#__PURE__*/_jsx("div", {
|
||||
className: "header",
|
||||
children: "Header"
|
||||
}), /*#__PURE__*/_jsxs("div", {
|
||||
className: "content",
|
||||
children: ["Content", /*#__PURE__*/_jsx("div", {
|
||||
className: "banner",
|
||||
children: "Banner"
|
||||
}), /*#__PURE__*/_jsxs("ul", {
|
||||
children: [/*#__PURE__*/_jsx("li", {
|
||||
children: "Item 1"
|
||||
}), /*#__PURE__*/_jsx("li", {
|
||||
children: "Item 2"
|
||||
}), /*#__PURE__*/_jsx("li", {
|
||||
children: "Item 3"
|
||||
}), /*#__PURE__*/_jsx("li", {
|
||||
children: "Item 4"
|
||||
}), /*#__PURE__*/_jsx("li", {
|
||||
children: "Item 5"
|
||||
})]
|
||||
})]
|
||||
}), /*#__PURE__*/_jsx("div", {
|
||||
className: "footer",
|
||||
children: "Footer"
|
||||
})]
|
||||
});
|
||||
console.log(page);
|
||||
```
|
||||
|
||||
这时经过Babel转义后的纯JS函数，这段函数可以在浏览器中直接运行
|
||||
|
||||
如果移除了相关JSX代码，并将他们都替换为`React.createElement`函数调用，那么得到的代码也可以直接在浏览器中运行。Babel帮助我们完成了转化，提高了开发效率，相比于通过调用`React.createElement`来描述视图，通过JSX编写的代码更加容易维护
|
||||
|
||||
这些代码最终形成的就是虚拟DOM树，React可以将虚拟DOM渲染到页面上，形成真实DOM
|
||||
|
||||
虚拟DOM允许React可以通过diff算法，高效地对真实DOM树进行更新
|
||||
|
||||
### 声明式编程
|
||||
|
||||
- 虚拟DOM帮我们从命令式编程转到了声明式编程的模式
|
||||
- 对虚拟DOM作何处理，如何渲染是由React决定的，由于做了一层抽象，那么同样可以将虚拟DOM渲染成原生组件（React Native）
|
||||
|
||||
### 购物车案例
|
||||
|
||||
下面写一个经典的购物车案例
|
||||
|
||||
```tsx
|
||||
function formatPrice(price) {
|
||||
return `$ ${price.toFixed(2)}`
|
||||
}
|
||||
|
||||
class App extends React.Component {
|
||||
constructor() {
|
||||
super()
|
||||
this.state = {
|
||||
books: [
|
||||
{ name: 'book1', author: 'author1', price: 100, count: 0 },
|
||||
{ name: 'book2', author: 'author2', price: 200, count: 0 },
|
||||
{ name: 'book3', author: 'author3', price: 300, count: 0 },
|
||||
{ name: 'book4', author: 'author4', price: 400, count: 0 }
|
||||
]
|
||||
}
|
||||
}
|
||||
|
||||
changeCount(index, count) {
|
||||
this.setState((state) => {
|
||||
const books = [...state.books]
|
||||
books[index].count += count
|
||||
return { books }
|
||||
})
|
||||
}
|
||||
|
||||
removeItem(index) {
|
||||
this.setState((state) => {
|
||||
const books = [...state.books]
|
||||
books.splice(index, 1)
|
||||
return { books }
|
||||
})
|
||||
}
|
||||
|
||||
getTotal() {
|
||||
const { books } = this.state
|
||||
return books.reduce((acc, { price, count }) => acc + price * count, 0)
|
||||
}
|
||||
|
||||
renderBookCart() {
|
||||
const { books } = this.state
|
||||
const total = this.getTotal()
|
||||
return (
|
||||
<div className="shopping-cart">
|
||||
<h1>Shopping Cart</h1>
|
||||
<div className="books">
|
||||
{books.map(({ name, author, price, count }, index) => (
|
||||
<div className="book" key={name}>
|
||||
<span className="idx">{index + 1}</span>
|
||||
<span className="name">{name}</span>
|
||||
<span className="author">{author}</span>
|
||||
<span className="price">{formatPrice(price)}</span>
|
||||
<span className="counter">
|
||||
<button onClick={() => this.changeCount(index, -1)} disabled={count <= 0}>
|
||||
-
|
||||
</button>
|
||||
<span className="counter-number">{count}</span>
|
||||
<button onClick={() => this.changeCount(index, 1)}>+</button>
|
||||
</span>
|
||||
<button onClick={() => this.removeItem(index)}>Delete</button>
|
||||
</div>
|
||||
))}
|
||||
</div>
|
||||
<div className="total">
|
||||
<span>Total: {formatPrice(total)}</span>
|
||||
</div>
|
||||
</div>
|
||||
)
|
||||
}
|
||||
|
||||
renderEmptyTip() {
|
||||
return <div className="empty">Shopping Cart is Empty</div>
|
||||
}
|
||||
|
||||
render() {
|
||||
const isEmpty = this.state.books.length === 0
|
||||
|
||||
return !isEmpty ? this.renderBookCart() : this.renderEmptyTip()
|
||||
}
|
||||
}
|
||||
|
||||
const root = ReactDOM.createRoot(document.querySelector('#root'))
|
||||
root.render(<App />)
|
||||
```
|
||||
|
||||
## React脚手架
|
||||
|
||||
- 认识脚手架工具
|
||||
- create-react-app
|
||||
- 创建React项目
|
||||
- Webpack的配置
|
||||
|
||||
类似于Vue提供的 `pnpm create vite` 创建一个模板，React也可以通过 `create-react-app` 来初始化一个空的React模板
|
||||
|
||||
```sh
|
||||
pnpm add create-react-app -g # 全局安装create-react-app
|
||||
create-react-app react-app # 创建一个名为react-app的React项目
|
||||
# 删除node_modules package-lock.json
|
||||
cd react-app
|
||||
pnpm i # 使用pnpm重新安装依赖
|
||||
```
|
||||
|
||||
```tsx
|
||||
// index.js
|
||||
import ReactDOM from 'react-dom/client'
|
||||
import App from './App'
|
||||
|
||||
const root = ReactDOM.createRoot(document.querySelector('#root'))
|
||||
root.render(<App />)
|
||||
|
||||
// App.js
|
||||
import React from 'react'
|
||||
|
||||
export default class App extends React.Component {
|
||||
render() {
|
||||
return <div>Hello, React!</div>
|
||||
}
|
||||
}
|
||||
```
|
||||
Loading…
x
Reference in New Issue
Block a user