React 父组件重新渲染，子组件不需要渲染的三种性能优化方式，PureComponent,memo,shouldComponentUpdate;

//使用React普通函数时，可以使用两种优化方式，PureComponent和shouldComponentUpdate

//shouldComponentUpdate

//shouldComponentUpdate
class Foo extends Component {
  shouldComponentUpdate(nextProps,nextState){
    if(nextProps.count===this.props.count){ //传入的count与组件当前props的count比较,count没改变，return false，不渲染
      return false    //不渲染
    }
    return true;  //渲染
  }
  render() {
    console.log("组件渲染");  //可以看到，当父组件的name改变时，子组件不会打印，只有count改变，才会打印，优化性能
    return null
  }
}

class App extends Component {
  state = {
    count: 0,
    name: 0
  }
  render() {
    return (
      <Fragment>
        <Foo count={this.state.count} ></Foo>
        <button onClick={() => { this.setState(() => ({ count:this.state.count+1 })) }}>加count</button>
        <button onClick={() => { this.setState(() => ({ name: this.state.count+1 })) }}>加name</button>
      </Fragment>

    )
  }
}

//PureComponent

//引入PureComponent
import React, { Component, Fragment, PureComponent} from 'react';
//PureComponent，自动比较组件数据是否改变，注意只能比较一层，比如一个对象，对象中的属性改变，他不会重新渲染，只有对象改变，才重新渲染。
class Foo extends PureComponent {
  render() {
    console.log("组件渲染");
    return null
  }
}

class App extends Component {
  state = {
    count: 0,
    name: 0
  }
  render() {
    return (
      <Fragment>
        <Foo count={this.state.count} ></Foo>
        <button onClick={() => { this.setState(() => ({ count:this.state.count+1 })) }}>加count</button>
        <button onClick={() => { this.setState(() => ({ name: this.state.count+1 })) }}>加name</button>
      </Fragment>

    )
  }
}

//hooks 独有优化方式memo

//引入memo
import React, { Component, Fragment,  memo } from 'react';
//用memo把hooks包裹即可
const Foo = memo(function Foo(props) {
  console.log("组件渲染");
  return <div>count:{props.count}</div>
})

class App extends Component {
  state = {
    count: 0,
    name: 0
  }
  render() {
    return (
      <Fragment>
        <Foo count={this.state.count} ></Foo>
        <button onClick={() => { this.setState(() => ({ count:this.state.count+1 })) }}>加count</button>
        <button onClick={() => { this.setState(() => ({ name: this.state.count+1 })) }}>加name</button>
      </Fragment>

    )
  }
}