让react用起来更得心应手系列文章:
react API
import React from 'react'
import ReactDom from 'react-dom'
let el=<h1><span>hello</span><span>wrold</span></h1>
console.log(el)
ReactDom.render(el,window.root);
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- 利用babel中的babel-preset-react进行转译,调用React.createElement(type,props,children)生成一个虚拟dom节点:
- React.createElement原理简析:
function createElement(type,props,children){
let obj={};
obj.type=type;
obj.props={};
obj.props.children=null;
for(let key in props){
obj.props[key]=props[key]
}
children.forEach(child => {
let {type,props,subchildren} = child
obj.props.children.push(createElement(type,props,subchildren))
})
return obj
}
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3. ReactDom.render(vnode,root)会将其渲染出到页面root中:
4. ReactDom.render原理简析:
function render(vnode,container){
if(typeof vnode ==='string') return container.appendChild(document.createTextNode(vnode));
let{type,props} = vnode;
let tag = document.createElement(type);
for(let key in props){
if(key==='children'){
Array.from(props[key]).forEach(child => {
render(child,tag)
});
}else{
tag.setAttribute(key,props[key]);
}
}
container.appendChild(tag);
}
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react组件
函数声明组件
- 函数式声明组件:函数返回一个JSX语法的vnode
import React from 'react'
import ReactDom from 'react-dom'
function test(){
let state = {
title:'标题',
context:'内容'
}
return (
<div>
<h3>{state.title}</h3>
<p>{state.context}</p>
</div>
)
}
ReactDom.render(
<div>
{test()}
{test()}
{test()}
</div>,window.root);
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- 将上面的函数声明组件进行一定的包装
import React from 'react'
import ReactDom from 'react-dom'
function Test(props){
let state={
title:"标题"
}
return (
<div>
<h3>{state.title}</h3>
<p>{props.context}</p>
</div>
)
}
ReactDom.render(
<div>
<Test context='123'></Test>
<Test context='456'></Test>
<Test context='789'></Test>
</div>,window.root);
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函数声明组件存在一些问题:
- 没有this
- 没有状态,state无法进行动态更改
- 没有生命周期
类组件
类组件消除了函数声明组件的问题,也是现在写react的正常语法。
import React, { Component } from 'react';
import ReactDOM, { render } from 'react-dom';
import PropTypes from 'prop-types'; //属性校验插件
class Clock extends Component {
state = {
time: new Date().toLocaleString()
}
static propTypes = {
name: PropTypes.string.isRequired //name必填,校验是否填写name属性
}
// 组件挂载后调用
componentDidMount() {
this.timer = setInterval(() => {
// 只会覆盖以前的属性类似 Object.assign()
this.setState({ time: new Date().toLocaleString() })
}, 1000)
}
handleClick = () => { //箭头函数,this用最外层的this,指向当前组件
ReactDOM.unmountComponentAtNode(window.root)
}
// 组件卸载前调用,一般用于解绑事件和方法
componentWillUnmount() {
clearInterval(this.timer)
}
// 默认渲染这个组件会调用render方法
// 只是在上面函数式声明基础上包了一层函数,这样可以控制其执行时间,添加声明周期
render() {
let {name} = this.props //一般会解构props
return <div>
{name} <span>{this.state.time}</span>
<button onClick={this.handleClick}>删除</button>
</div>
}
}
render(<Clock name='test-clock'/>, window.root);
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react 生命周期
// React16.3 推出了新的声明周期
import React, { Component } from 'react';
import ReactDOM from 'react-dom';
class Counter extends Component {
static defaultProps = {
a: 1
}
state = {
num: 0
}
constructor(props) {
console.log('parent-constructor')
super();
}
// react16.3中标识了这个方法会被废弃掉
// 后期有需要的话 可以放在constructor中替代掉
componentWillMount() {
console.log('parent-componentWillMount');
}
// react的性能优化 immutablejs
shouldComponentUpdate(){
console.log('parent-shouldComponentUpdate');
return true
}
componentWillReceiveProps(){
console.log('parent-componentWillReceiveProps');
}
componentWillUpdate(){
console.log('parent-componentWillUpdate');
}
componentDidUpdate() {
console.log('parent-componentDidUpdate');
}
handleClick = () => {
this.setState({ num: this.state.num + 0 });
}
render() {
console.log('parent-render');
return <div>
<button onClick={this.handleClick}>+</button>
{this.state.num}
<ChildCounter n={this.state.num}></ChildCounter>
</div>
}
componentDidMount() {
console.log('parent-didmount');
}
componentWillUnmount() {
console.log('parent-组件卸载')
}
}
class ChildCounter extends Component{
componentWillMount(){
console.log('child-componentWillMount')
}
render(){
console.log('child-render');
return <div>child counter {this.props.n}</div>
}
componentDidMount() {
console.log('child-componentDidMount')
}
shouldComponentUpdate(){
console.log('child-shouldComponentUpdate');
return true
}
componentWillUpdate(){
console.log('child-componentWillUpdate');
}
componentDidUpdate() {
console.log('child-componentDidUpdate');
}
//16.3中这个方法废弃了
componentWillReceiveProps(){
console.log('child-componentWillReceiveProps');
}
}
ReactDOM.render(<Counter></Counter>, window.root);
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- 初次渲染时打印的结果为:
- 组件状态更新时打印的结果为: 从上面可以得出以下结论:
- 生命周期分为初始化生命周期和组件运行生命周期,初始化生命周期为:constructor->componentWillMount->render->componentDidMount
- 组件运行生命周期为:(存在props时:componentWillReceiveProps)->shouldComponentUpdate->componentWillUpdate->render->componentDidUpdate
- 子组件的生命周期嵌在父组件render后,DidMount和DidUpdate前
- shouldComponentUpdate返回false,组件将不会更新
- 只有componentWillReceiveProps、componentWillMount、componentDidMount中可以调用setState,不应该在componentWillReceiveProps中调用setState(但是大家还是这么用)
受控组件和非受控组件
- 非受控组件:ref操作dom,很方便;可以和一些地三方库结合使用
import React,{Component} from 'react';
import {render} from 'react-dom';
class UnControl extends Component{
b = React.createRef(); // 16.3的api React.createRef()
handleClick = () =>{
alert(this.a.value); // 写法1
alert(this.b.current.value) // 写法2
}
render(){
return (<div>
<input type="text" id="username" ref={dom=>this.a=dom}/>
<input type="text" id="password" ref={this.b}/>
<button onClick={this.handleClick}>点击</button>
</div>)
}
}
render(<UnControl></UnControl>,window.root);
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- 非受控组件:利用e.target和setState来修改状态并控制dom
import React from 'react';
import ReactDOM from 'react-dom';
class Control extends React.Component{
state = {
a:'hello',
b:'world'
}
changeHandler = (e)=>{
let val = e.target.name
this.setState({
[val]:e.target.value
})
}
render(){
return (
<div>
{this.state.a}
{this.state.b}
<input type="text" name="a" value={this.state.a} onChange={this.changeHandler}/>
<input type="text" name="b" value={this.state.b} onChange={this.changeHandler}/>
</div>
)
}
}
ReactDOM.render(<Control></Control>,window.root);
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react数据传递
- 逐级传递:当组件嵌套的层级不超过3层时,使用这种方式传递数据简单,但是嵌套过深时,会很难管理的问题
//index.js
import React, { Component } from 'react';
import ReactDOM from 'react-dom';
import Second1 from './Second1';
import Second from './Second';
class App extends Component {
state = {
count:0
}
handleCountChange = (num)=>{
this.setState({count:num})
}
render(){
return(
<div>
<Second count={this.state.count}></Second>
<Second1 count={this.state.count} countChange={this.handleCountChange}></Second1>
</div>
)
}
}
ReactDOM.render(<App></App>, window.root);
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//second.js
import React, { Component } from 'react'
export default class Second extends Component {
render() {
let {count} = this.props
return (
<div>{count}</div>
)
}
}
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//second1.js
import React, { Component } from 'react'
import Third from './Third';
export default class Second1 extends Component {
render() {
let {count,countChange} = this.props
return (
<Third num={count} numChange={countChange}></Third>
)
}
}
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//third.js
import React, { Component } from 'react';
export default class Third extends Component {
handleClick =()=>{
console.log(this.props.num+1);
this.props.numChange(this.props.num+1);
}
render(){
return (
<span onClick={this.handleClick}>+</span>
)
}
}
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- 采用context
import React, { Component } from 'react';
import {Provider} from './context';
import ReactDOM from 'react-dom';
import Second1 from './Second1';
import Second from './Second';
class App extends Component {
state = {
count:0
}
handleCountChange = (num)=>{
this.setState({count:num})
}
render(){
return(
// 提供一个contex,上面挂载了全局状态
<Provider value={{numChange:this.handleCountChange,num:this.state.count}}>
<Second></Second>
<Second1></Second1>
</Provider>
)
}
}
ReactDOM.render(<App></App>, window.root);
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// context.js
import React from 'react';
let { Provider, Consumer } = React.createContext(); //react提供的API
export {Provider,Consumer}
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// second.js
import React, { Component } from 'react';
import {Consumer} from './context';
export default class Second extends Component {
render() {
return (
<Consumer>
{(value)=>{
return <div>{value.num}</div>
}}
</Consumer>
)
}
}
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// second1.js
import React, { Component } from 'react'
import Third from './Third';
export default class Second1 extends Component {
render() {
return (
<Third></Third>
)
}
}
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//third.js
import React, { Component } from 'react';
import {Consumer} from './context';
export default class Third extends Component {
render(){
return (<Consumer>
{(value)=>{ //固定语法value就是Provider中的value
return <span onClick={()=>{value.numChange(value.num + 1)}}>+</span>
}}
</Consumer>)
}
}
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结语:
个人使用一种框架时总有一种想知道为啥这样用的强迫症,不然用框架用的不舒服,不要求从源码上知道其原理,但是必须得从心理上说服自己。