reactjs - 为什么 createStore 调用每个 reducer 3 次？

Question

我有 Redux，我注意到 createStore 调用每个 reducer 3 次，为什么？

// pageReducer.js
const initialState = {
  year: 2020,
  events: [],
};

const pageReducer = (state = initialState, action) => {
  console.log(state); //spy for reducer
 
  return { ...state }; 
};

// rootReducer.js
export default combineReducers({ page: pageReducer });

const store = createStore(rootReducer));

const App = () => {
  const page = useSelector((store) => store.page);
  return (
    <Provider store={store}>
      <div>{page.year}</div>
    </Provider>
  );
};

我在控制台中看到，createStore()并combineReducers()调用了pageReducer()3 次。

为什么？为了什么？我认为创建全局状态和设置初始状态，但是 3 次...

标签： reactjsredux