reactjs - 如果随后释放并设置状态，React hook useRender 会调用两次

Question

我不确定这是否是预期的行为，但是如果您在使用 useReducer Hook 时退出调度 ( https://reactjs.org/docs/hooks-reference.html#bailing-out-of-a-dispatch )，则如果后面跟着渲染，则该操作会发生两次。让我解释一下:

// bailing out to prevent re-rendering
const testReducer = (state, action) => {
  switch (action.type) {
    case "ADD":
      state.test += 1
      return state;
  }
};

const myComponent = () => {
  let [totalClicks, setClicks] = useState(0);
    const [state, setState] = useReducer(testReducer, {
      test: 0,
    });

  const clickHandler = () => {
    setState({type: 'ADD'});
    setClicks((totalClicks += 1));
  };

  return (
    <div>
      <button onClick={clickHandler}>+</button>
      <p>{totalClicks}</p>
      <p>test count: {state.test}</p>
    </div>
  );
}

当您单击按钮时，state.test 增加 2，而 TotalClicks 增加 1。 但是，如果我要更改 reducer ，使其不会像下面这样，它们都会增加增加 1。

// non-bailing reducer
const testReducer = (state, action) => {
  switch (action.type) {
    case "ADD":
      return {
        test: state.test + 1,
      };
  }
};

这是为什么呢？这是预期的行为还是错误？沙箱示例:https://codesandbox.io/s/sad-robinson-dds63?file=/src/App.js

<小时/>

更新:进行一些调试后，看起来这种行为仅在用 React.StrictMode

包装时才会发生

有谁知道这是什么原因吗？？？

Answer 1

根据doc of StrictMode ，react 故意使用相同的操作调用 reducer 函数两次，以便暴露未被注意到的潜在有害副作用，这正是您的情况所发生的情况。

Strict mode can’t automatically detect side effects for you, but it can help you spot them by making them a little more deterministic. This is done by intentionally double-invoking the following functions: […] Functions passed to useState, useMemo, or useReducer