React 原理详解

import React from 'react'
import ReactDOM from 'react-dom'
class Opp extends React.Component {
	state = {
		count: 1,
	}
	handleClick = () => {
		// 异步更新数据
		this.setState({
			count: this.state.count + 1,
		})
        this.setState({
			count: this.state.count + 1,
		})
		console.log(this.state.count) // 1
	}
	render() {
		return (
			<div>
				<h1>计数器：{this.state.count}</h1>
				<button onClick={this.handleClick}>+1</button>
			</div>
		)
	}
}
ReactDOM.render(<Opp />, document.getElementById('root'))

import React from 'react'
import ReactDOM from 'react-dom'
class Opp extends React.Component {
	state = {
		count: 1,
	}
	handleClick = () => {
		/* // 异步更新数据
		this.setState({
			count: this.state.count + 1,
		})
		console.log(this.state.count) //1
    this.setState({
			count: this.state.count + 1,
		})
		console.log(this.state.count) //1
    */
		// 推荐语法
		this.setState((state, props) => {
			return {
				count: state.count + 1,
			}
		})
		this.setState((state, props) => {
			console.log('第二次调用：', state) //2
			return {
				count: state.count + 1,
			}
		})
		console.log(this.state.count) // 3
	}
	render() {
		return (
			<div>
				<h1>计数器：{this.state.count}</h1>
				<button onClick={this.handleClick}>+1</button>
			</div>
		)
	}
}
ReactDOM.render(<Opp />, document.getElementById('root'))

import React from 'react'
import ReactDOM from 'react-dom'
class Opp extends React.Component {
	state = {
		count: 1,
	}
	handleClick = () => {
		this.setState(
			(state, props) => {
				return {
					count: state.count + 1,
				}
			},
			// 状态更新后并且重新渲染后，立即执行
			() => {
				console.log('状态更新完成：', this.state.count) // 2
				console.log(document.getElementById('title').innerText) // 计数器：2
				document.title = '更新后的 count 为：' + this.state.count
			}
		)
		console.log(this.state.count) //1
	}
	render() {
		return (
			<div>
				<h1 id='title'>计数器：{this.state.count}</h1>
				<button onClick={this.handleClick}>+1</button>
			</div>
		)
	}
}
ReactDOM.render(<Opp />, document.getElementById('root'))

import React from 'react'
import ReactDOM from 'react-dom'
// JSX 语法的转化过程
// const element = <h1 className='greeting'>Hello JSX</h1>
const element = React.createElement(
	'h1',
	{
		className: 'greeting',
	},
	'Hello JSX'
)
console.log(element)
ReactDOM.render(element, document.getElementById('root'))

import React from 'react'
import ReactDOM from 'react-dom'
class Opp extends React.Component {
	state = {
		count: 0,
	}
	handleClick = () => {
		this.setState((state) => {
			return {
				count: this.state.count + 1,
			}
		})
	}
	// 钩子函数
	shouldComponentUpdate(nextProps, nextState) {
		// 返回 false,阻止组件重新渲染
		// return false
		// 最新的状态
		console.log('最新的state', nextState)
		// 更新前的状态
		console.log(this.state)
		// 返回 true,组件重新渲染
		return true
	}
	render() {
		console.log('组件更新了')
		return (
			<div>
				<h1>计数器：{this.state.count}</h1>
				<button onClick={this.handleClick}>+1</button>
			</div>
		)
	}
}
ReactDOM.render(<Opp />, document.getElementById('root'))

import React from 'react'
import ReactDOM from 'react-dom'
// 生成随机数
class Opp extends React.Component {
	state = {
		number: 0,
	}
	handleClick = () => {
		this.setState((state) => {
			return {
				number: Math.floor(Math.random() * 3),
			}
		})
	}
	// 两次生成的随机数可能相同，则没必要重新渲染
	shouldComponentUpdate(nextState) {
		console.log('最新状态：', nextState, '当前状态：', this.state)
		return nextState.number !== this.state.number
		/* if ( nextState.number ！== this.state.number) {
			return true
		}
		return false*/
	}
	render() {
		console.log('render')
		return (
			<div>
				<h1>随机数：{this.state.number}</h1>
				<button onClick={this.handleClick}>重新生成</button>
			</div>
		)
	}
}
ReactDOM.render(<Opp />, document.getElementById('root'))

import React from 'react'
import ReactDOM from 'react-dom'
// 生成随机数
class Opp extends React.Component {
	state = {
		number: 0,
	}
	handleClick = () => {
		this.setState((state) => {
			return {
				number: Math.floor(Math.random() * 3),
			}
		})
	}
	render() {
		return (
			<div>
				<NumberBox number={this.state.number} />
				<button onClick={this.handleClick}>重新生成</button>
			</div>
		)
	}
}
class NumberBox extends React.Component {
	shouldComponentUpdate(nextProps) {
		console.log('最新props：', nextProps, '当前props：', this.props)
		return nextProps.number !== this.props.number
	}
	render() {
		console.log('子组件render')
		return <h1>随机数：{this.props.number}</h1>
	}
}
ReactDOM.render(<Opp />, document.getElementById('root'))