类组件-shouldComponentUpdate

注：变化需要在render中打印，在component中检测不到
核心代码: 子组件中用shouldComponentUpdate防止子组件重渲染

// 性能优化
shouldComponentUpdate(nextProps, nextState) {
  if (nextProps.num === this.props.num) return false;
  return true;
}

完整例子
父组件

export default class Index extends Component {
  state = {
    num1: 111,
    num2: 222,
  };
  render() {
    return (
      <div>
        <button onClick={() => this.setState({ num1: this.state.num1 + 1 })}>
          修改num1
        </button>
        <button onClick={() => this.setState({ num2: this.state.num2 + 1 })}>
          修改num2
        </button>
        // 子组件
        <Schild num={this.state.num1} />
      </div>
    );
  }
}

子组件

import { Component } from "react";
export default class Index extends Component {
  constructor(props) {
    super(props);
  }

  // 性能优化
  shouldComponentUpdate(nextProps, nextState) {
    if (nextProps.num === this.props.num) return false;
    return true;
  }

  render() {
    // 父组件的状态变更会触发子组件的渲染
    console.log("渲染子组件");
    return (
      <div>
        <h3>s子组件</h3>
        <div>来自父组件的num1：{this.props.num}</div>
      </div>
    );
  }
}

函数组件-memo

可阻止子组件页面重渲染
核心代码：export default memo(Child);
详细代码
子组件

import { useEffect, memo } from "react";

const Child = ({ num }) => {
  useEffect(() => {
    console.log("渲染子组件");
  });

  return (
    <div>
      <h1>我是子组件</h1>
      <div>来自父组件的参数：{num}</div>
    </div>
  );
};
export default memo(Child);

父组件

import { useEffect, useState, useMemo } from "react";

import Child from "./child";
const Home = () => {
  const [num1, setNum1] = useState(111);
  const [num2, setNum2] = useState(222);

  return (
    <div>
      <h1>性能优化-usemome</h1>
      <div>
        num1: {num1};num2: {num2};
      </div>
      <button onClick={() => setNum1(num1 + 1)}>修改num1</button>
      <button onClick={() => setNum2(num2 + 1)}>修改num2</button>
      <Child num={num1} />
    </div>
  );
};
export default Home;