)
React 组件状态(State)
组件可以拥有状态（state），它是组件数据的私有部分，可以用来管理动态数据。
状态仅适用于类组件，或者使用 React 的 Hook 时可以在函数组件中使用。
React 把组件看成是一个状态机（State Machines）。通过与用户的交互，实现不同状态，然后渲染 UI，让用户界面和数据保持一致。
React 里，只需更新组件的 state，然后根据新的 state 重新渲染用户界面（不要操作 DOM）。
以下实例创建一个名称扩展为 React.Component 的 ES6 类，在 render() 方法中使用 this.state 来修改当前的时间。
添加一个类构造函数来初始化状态 this.state，类组件应始终使用 props 调用基础构造函数。
类组件中的状态管理
创建一个有状态的类组件：
Counter.js 文件
import React, { Component } from 'react';
class Counter extends Component {
constructor(props) {
super(props);
this.state = { count: 0 };
}
increment = () => {
this.setState({ count: this.state.count + 1 });
}
render() {
return (
<div>
<p>Count: {this.state.count}</p>
<button onClick={this.increment}>Increment</button>
</div>
);
}
}
export default Counter;
在 src/index.js 中渲染该组件：
实例
import React from 'react';
import ReactDOM from 'react-dom';
import './index.css';
import Counter from './Counter';
const root = ReactDOM.createRoot(document.getElementById("root"));
// 渲染 Counter 组件
root.render(<Counter />);
函数组件中的状态管理（使用 Hook）
使用 React Hook 可以在函数组件中使用状态。最常用的 Hook 是 useState。
创建一个有状态的函数组件：
Counter.js 文件
import React, { useState } from 'react';
function Counter() {
const [count, setCount] = useState(0);
return (
<div>
<p>Count: {count}</p>
<button onClick={() => setCount(count + 1)}>Increment</button>
</div>
);
}
export default Counter;
在 src/index.js 中渲染该组件：
实例
import React from 'react';
import ReactDOM from 'react-dom';
import './index.css';
import Counter from './Counter';
const root = ReactDOM.createRoot(document.getElementById("root"));
// 渲染 Counter 组件
root.render(<Counter />);
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实例
创建一个时间点实例来理解组件状态：
React 实例
class Clock extends React.Component { constructor(props) { super(props); this.state = {date: new Date()}; } render() { return ( <div> <h1>Hello, world!</h1> <h2>现在是 {this.state.date.toLocaleTimeString()}.</h2> </div> ); } } const root = ReactDOM.createRoot(document.getElementById("root")); root.render( <Clock /> );
尝试一下 »
接下来，我们将使Clock设置自己的计时器并每秒更新一次。
将生命周期方法添加到类中
在具有许多组件的应用程序中，在销毁时释放组件所占用的资源非常重要。
每当 Clock 组件第一次加载到 DOM 中的时候，我们都想生成定时器，这在 React 中被称为挂载。
同样，每当 Clock 生成的这个 DOM 被移除的时候，我们也会想要清除定时器，这在 React 中被称为卸载。
我们可以在组件类上声明特殊的方法，当组件挂载或卸载时，来运行一些代码：
React 实例
class Clock extends React.Component { constructor(props) { super(props); this.state = {date: new Date()}; } componentDidMount() { this.timerID = setInterval( () => this.tick(), 1000 ); } componentWillUnmount() { clearInterval(this.timerID); } tick() { this.setState({ date: new Date() }); } render() { return ( <div> <h1>Hello, world!</h1> <h2>现在是 {this.state.date.toLocaleTimeString()}.</h2> </div> ); } } const root = ReactDOM.createRoot(document.getElementById("root")); root.render( <Clock /> );
