讓react用起來更得心應手——(React 基礎簡析)
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); 複製程式碼
- 利用babel中的babel-preset-react進行轉譯,呼叫React.createElement(type,props,children)生成一個虛擬dom節點:
- React.createElement原理簡析:
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 } 複製程式碼
3. ReactDom.render(vnode,root)會將其渲染出到頁面root中:
4. ReactDom.render原理簡析:
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); } 複製程式碼
react元件
函式宣告元件
- 函式式宣告元件:函式返回一個JSX語法的vnode
import React from 'react' import ReactDom from 'react-dom' function test(){ let state = { title:'標題', context:'內容' } return ( <div> <h3>{state.title}</h3> <p>{state.context}</p> </div> ) } ReactDom.render( <div> {test()} {test()} {test()} </div>,window.root); 複製程式碼
- 將上面的函式宣告元件進行一定的包裝
import React from 'react' import ReactDom from 'react-dom' function Test(props){ let state={ title:"標題" } 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); 複製程式碼
函式宣告元件存在一些問題:
- 沒有this
- 沒有狀態,state無法進行動態更改
- 沒有生命週期
類元件
類元件消除了函式宣告元件的問題,也是現在寫react的正常語法。
import React, { Component } from 'react'; import ReactDOM, { render } from 'react-dom'; import PropTypes from 'prop-types'; //屬性校驗外掛 class Clock extends Component { state = { time: new Date().toLocaleString() } static propTypes = { name: PropTypes.string.isRequired//name必填,校驗是否填寫name屬性 } // 元件掛載後呼叫 componentDidMount() { this.timer = setInterval(() => { // 只會覆蓋以前的屬性類似 Object.assign() this.setState({ time: new Date().toLocaleString() }) }, 1000) } handleClick = () => { //箭頭函式,this用最外層的this,指向當前元件 ReactDOM.unmountComponentAtNode(window.root) } // 元件解除安裝前呼叫,一般用於解綁事件和方法 componentWillUnmount() { clearInterval(this.timer) } // 預設渲染這個元件會呼叫render方法 // 只是在上面函式式聲明基礎上包了一層函式,這樣可以控制其執行時間,新增宣告週期 render() { let {name} = this.props//一般會解構props return <div> {name} <span>{this.state.time}</span> <button onClick={this.handleClick}>刪除</button> </div> } } render(<Clock name='test-clock'/>, window.root); 複製程式碼
react 生命週期
// React16.3 推出了新的宣告週期 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中標識了這個方法會被廢棄掉 // 後期有需要的話 可以放在constructor中替代掉 componentWillMount() { console.log('parent-componentWillMount'); } // react的效能優化 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-元件解除安裝') } } 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中這個方法廢棄了 componentWillReceiveProps(){ console.log('child-componentWillReceiveProps'); } } ReactDOM.render(<Counter></Counter>, window.root); 複製程式碼
- 初次渲染時列印的結果為:
- 元件狀態更新時列印的結果為: 從上面可以得出以下結論:
- 生命週期分為初始化生命週期和元件執行生命週期,初始化生命週期為:constructor->componentWillMount->render->componentDidMount
- 元件執行生命週期為:(存在props時:componentWillReceiveProps)->shouldComponentUpdate->componentWillUpdate->render->componentDidUpdate
- 子元件的生命週期嵌在父元件render後,DidMount和DidUpdate前
- shouldComponentUpdate返回false,元件將不會更新
- 只有componentWillReceiveProps、componentWillMount、componentDidMount中可以呼叫setState,不應該在componentWillReceiveProps中呼叫setState(但是大家還是這麼用)
受控元件和非受控元件
- 非受控元件:ref操作dom,很方便;可以和一些地三方庫結合使用
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);// 寫法1 alert(this.b.current.value)// 寫法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}>點選</button> </div>) } } render(<UnControl></UnControl>,window.root); 複製程式碼
- 非受控元件:利用e.target和setState來修改狀態並控制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); 複製程式碼
react資料傳遞
- 逐級傳遞:當元件巢狀的層級不超過3層時,使用這種方式傳遞資料簡單,但是巢狀過深時,會很難管理的問題
//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); 複製程式碼
//second.js import React, { Component } from 'react' export default class Second extends Component { render() { let {count} = this.props return ( <div>{count}</div> ) } } 複製程式碼
//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> ) } } 複製程式碼
//third.js import React, { Component } from 'react'; export defaultclass Third extends Component { handleClick =()=>{ console.log(this.props.num+1); this.props.numChange(this.props.num+1); } render(){ return ( <span onClick={this.handleClick}>+</span> ) } } 複製程式碼
- 採用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( // 提供一個contex,上面掛載了全域性狀態 <Provider value={{numChange:this.handleCountChange,num:this.state.count}}> <Second></Second> <Second1></Second1> </Provider> ) } } ReactDOM.render(<App></App>, window.root); 複製程式碼
// context.js import React from 'react'; let { Provider, Consumer } = React.createContext(); //react提供的API export {Provider,Consumer} 複製程式碼
// 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> ) } } 複製程式碼
// second1.js import React, { Component } from 'react' import Third from './Third'; export default class Second1 extends Component { render() { return ( <Third></Third> ) } } 複製程式碼
//third.js import React, { Component } from 'react'; import {Consumer} from './context'; export defaultclass Third extends Component { render(){ return (<Consumer> {(value)=>{//固定語法value就是Provider中的value return <span onClick={()=>{value.numChange(value.num + 1)}}>+</span> }} </Consumer>) } } 複製程式碼
結語:
個人使用一種框架時總有一種想知道為啥這樣用的強迫症,不然用框架用的不舒服,不要求從原始碼上知道其原理,但是必須得從心理上說服自己。