React 元素渲染简单演示

react更新元素渲染：计时器案例

src/index.js

import React from 'react';
import ReactDOM from 'react-dom';
import './index.css';
import App from './App';
import * as serviceWorker from './serviceWorker';

function tick(){
  const element=(
    <div>现在是{new Date().toLocaleTimeString()}</div>
  )

  ReactDOM.render(
    element,
    document.getElementById('example')
  );

}
setInterval(tick,1000);

// If you want your app to work offline and load faster, you can change
// unregister() to register() below. Note this comes with some pitfalls.
// Learn more about service workers: https://bit.ly/CRA-PWA
serviceWorker.unregister();

以上代码可以改为函数式封装

import React from 'react';
import ReactDOM from 'react-dom';
import './index.css';
import App from './App';
import * as serviceWorker from './serviceWorker';

function Clock(props){
  return(
    <div>现在是{props.date.toLocaleTimeString()}</div>
  )
}

function tick(){
  ReactDOM.render(
    <Clock date={new Date()}/>,
    document.getElementById('example')
  );
}
setInterval(tick,1000);

// If you want your app to work offline and load faster, you can change
// unregister() to register() below. Note this comes with some pitfalls.
// Learn more about service workers: https://bit.ly/CRA-PWA
serviceWorker.unregister();

也可以改为类封装

import React, { Component } from 'react';
import ReactDOM from 'react-dom';
import './index.css';
import App from './App';
import * as serviceWorker from './serviceWorker';

class Clock extends React.Component {

  render(){
    return(
      <div>现在是{this.props.date.toLocaleTimeString()}</div>
    )
  }

}

function tick(){
  ReactDOM.render(
    <Clock date={new Date()}/>,
    document.getElementById('example')
  );
}
setInterval(tick,1000);

// If you want your app to work offline and load faster, you can change
// unregister() to register() below. Note this comes with some pitfalls.
// Learn more about service workers: https://bit.ly/CRA-PWA
serviceWorker.unregister();

React 只会更新必要的部分
值得注意的是 React DOM 首先会比较元素内容先后的不同，而在渲染过程中只会更新改变了的部分。