React 函数组件

函数组件

const Hello = (props) =>{
    return <div>{props.message)}</div>
}

const Hello = props => <div>{props.message}</div>

function Hello(props){
    return <div>{props.message}</div>
}

useState

const App = () => {
  const [n, setN] = useState(0);
  const onClick = () => {
    setN(n + 1);
  };
  return (
    <div>
      {n}
      <button onClick={onClick}>+1</button>
    </div>
  );
};

useEffect

模拟 componentDidMount:

useEffect( ()=> {console.log('第一次渲染')}, [] )

模拟 componentDidUpdate:

useEffect( ()=> {console.log('任意属性变更')} )
useEffect( ()=>{console.log('n 变了')}, [n])

// 示例：
  useEffect(() => {
    console.log("state 变了");
  });

  useEffect(() => {
    console.log("n 变了");
  }, [n]);

模拟 componentWillUnmount:

useEffect( ()=> {
    console.log('第一次渲染')
    return ()=>{
        console.log('组件要死了')
    }
})
// 示例
const App = () => {
  let [childVisible, setChildVisible] = useState(true);
  const show = () => {
    setChildVisible(true);
  };
  return (
    <div>
      {childVisible ? (
        <button onClick={() => setChildVisible(false)}>hide</button>
      ) : (
        <button onClick={show}>show</button>
      )}
      {childVisible ? <Child /> : null}
    </div>
  );
};
const Child = (props) => {
  useEffect(() => {
    console.log("渲染或变化了");
    return () => {
      console.log("Child 销毁了");
    };
  });
  return <div>Child</div>;
};

constructor

函数组件执行的时候，就相当于 constructor

shouldComponentUpdate

React.memo

useMemo

render

函数组件的返回值 就是 render 的返回值。

自定义 Hook 之 useUpdate

const useUpdate = (fn, dep) => {
    const [count, setCount] = useState(0);
    useEffect(() => {
        setCount((x) => x + 1);
    }, [dep]);

    useEffect(() => {
        if (count > 1) {
            fn();
        }
    }, [count, fn]);
};



const App = () => {
    const [n, setN] = useState(0);
    const onClick = () => {
        setN(n + 1);
    };
    // 使用
    useUpdate(() => {
        console.log("n变了");
    }, n);

    return (
        <div>
            {n}
            <button onClick={onClick}>点击+1</button>
        </div>
    );
};

useRef demo

useContext demo

能用函数组件就用函数组件

因为简单。