React hook useRender 调用两次，如果 bailing 和之后设置状态

Question

我不确定这是否是预期的行为，但如果您在使用 useReducer 挂钩时退出分派 (https://reactjs.org/docs/hooks-reference.html#bailing-out-of-a-dispatch)，则该操作会在其后跟渲染时发生两次。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，使其不会保释到 1如下所示，它们都会增加 1.

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

这是为什么？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

更新： 在进行了一些调试之后，看起来这种行为只有在用 React.StrictMode

包装时才会发生

有谁知道这是什么原因造成的？？？

Answer 1

根据 doc of StrictMode，React 故意使用相同的操作两次调用 reducer 函数，以暴露未被注意的潜在有害副作用，这正是您的案例所发生的情况。

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

React hook useRender 调用两次，如果 bailing 和之后设置状态

React hook useRender called twice if bailing and setting state afterward

reactjs

react-hooks

react-lifecycle-hooks

use-reducer

use-state