React优化子组件render的使用

class A extends React.Component {
  render() {
    console.log('render')
    return <div>这是A组件</div>
  }
}

class Main extends React.Component {
  render() {
    return (
      <div>
        // 点击button会让A不断调用render
        <button onClick={() => this.setState({ a: 1 })}>Main</button>
        <A />
      </div>
    )
  }
}

class A extends React.Component {
  shouldComponentUpdate(nextProps, nextState) {
    //两次props对比
    return nextProps.a === this.props.a ? false : true
  }
  render() {
    console.log('render')
    return <div>这是A组件</div>
  }
}

class Main extends React.Component {
  // ...
  render() {
    return (
      <div>
        <button onClick={() => this.setState({ a: 1 })}>Main</button>
        <A a={this.state.a} />
      </div>
    )
  }
}

//PureComponent
class A extends React.PureComponent {
  render() {
    console.log('render')
    return <div>这是A组件</div>
  }
}

class Main extends React.Component {
  state = {
    a: 1
  }
  render() {
    return (
      <div>
        <button onClick={() => this.setState({ a: 1 })}>Main</button>
        <A a={this.state.a} />
      </div>
    )
  }
}

const A = props => {
  console.log('render A')
  return <div>这是A组件</div>
}
// React.memo包裹A
const B = React.memo(A)

const Main = props => {
  const [a, setA] = useState(1)
  console.log('render Main')

  return (
    <div>
      // 通过setA(a + 1)让父组件重新render
      <button onClick={() => setA(a + 1)}>Main</button>
      // 一直传入相同的props不会让子组件重新render
      <B a={1} />
    </div>
  )
}

// extends React.Component
class A extends React.Component {
  render() {
    console.log('render A')
    console.log(this.props)
    return <div>这是组件A</div>
  }
}

class Main extends React.Component {
  test = [1, 2, 3]
  render() {
    console.log('render Main')
    return (
      <div>
        <button
          onClick={() => {
            // 父组件render
            this.setState({})
            this.test.push(4)
          }}
        >
          Main
        </button>
        <A test={this.test} />
      </div>
    )
  }
}