讓react用起來更得心應手——(React 基礎簡析)

摘要： import React from 'react' import ReactDom from 'react-dom' let el=<h1><span>hello</span><span>wrold</spa...

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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1. 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:

   

2. 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
}
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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);
}
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react元件

函式宣告元件

1. 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);
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2. 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);
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函式宣告元件存在一些問題:

1. 沒有this
2. 沒有狀態，state無法進行動態更改
3. 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);
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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);
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• 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：

1. 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
2. 元件執行生命週期為：(存在props時:componentWillReceiveProps)->shouldComponentUpdate->componentWillUpdate->render->componentDidUpdate
3. 子元件的生命週期嵌在父元件render後，DidMount和DidUpdate前
4. shouldComponentUpdate返回false，元件將不會更新
5. 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);
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• 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);

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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);
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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 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>
)
}
}
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• 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);
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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 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>)
}
}
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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。