传递一个空数组作为useEffect()的第二个参数,仅在mount上运行回调.

function ComponentDidMount() {
  const [count,setCount] = React.useState(0);
  React.useEffect(() => {
    console.log('componentDidMount');
  },[]);

  return (
    <div>
      <p>componentDidMount: {count} times</p>
      <button
        onClick={() => {
          setCount(count + 1);
        }}
      >
        Click Me
      </button>
    </div>
  );
}

ReactDOM.render(
  <div>
    <ComponentDidMount />
  </div>,document.querySelector("#app")
);

<script src="https://unpkg.com/react@16.7.0-alpha.0/umd/react.development.js"></script>
<script src="https://unpkg.com/react-dom@16.7.0-alpha.0/umd/react-dom.development.js"></script>

<div id="app"></div>

componentDidUpdate

更新发生后立即调用componentDidUpdate().初始渲染不会调用此方法. useEffect在每个渲染上运行,包括第一个渲染.因此,如果您希望具有严格的等效作为componentDidUpdate,则必须使用useRef来确定组件是否已安装一次.如果你想更严格,请使用useLayoutEffect(),但它会同步触发.在大多数情况下,useEffect()应该足够了.

这answer is inspired by Tholle,归功于他.

function ComponentDidUpdate() {
  const [count,setCount] = React.useState(0);

  const isFirstUpdate = React.useRef(true);
  React.useEffect(() => {
    if (isFirstUpdate.current) {
      isFirstUpdate.current = false;
      return;
    }

    console.log('componentDidUpdate');
  });

  return (
    <div>
      <p>componentDidUpdate: {count} times</p>
      <button
        onClick={() => {
          setCount(count + 1);
        }}
      >
        Click Me
      </button>
    </div>
  );
}

ReactDOM.render(
  <ComponentDidUpdate />,document.getElementById("app")
);

<script src="https://unpkg.com/react@16.7.0-alpha.0/umd/react.development.js"></script>
<script src="https://unpkg.com/react-dom@16.7.0-alpha.0/umd/react-dom.development.js"></script>

<div id="app"></div>

componentWillUnmount

在useEffect的回调参数中返回一个回调函数,它将在卸载之前调用.

function ComponentWillUnmount() {
  function ComponentWillUnmountInner(props) {
    React.useEffect(() => {
      return () => {
        console.log('componentWillUnmount');
      };
    },[]);

    return (
      <div>
        <p>componentWillUnmount</p>
      </div>
    );
  }
  
  const [count,setCount] = React.useState(0);

  return (
    <div>
      {count % 2 === 0 ? (
        <ComponentWillUnmountInner count={count} />
      ) : (
        <p>No component</p>
      )}
      <button
        onClick={() => {
          setCount(count + 1);
        }}
      >
        Click Me
      </button>
    </div>
  );
}

ReactDOM.render(
  <div>
    <ComponentWillUnmount />
  </div>,document.querySelector("#app")
);

<script src="https://unpkg.com/react@16.7.0-alpha.0/umd/react.development.js"></script>
<script src="https://unpkg.com/react-dom@16.7.0-alpha.0/umd/react-dom.development.js"></script>

<div id="app"></div>