当Redux中状态的另一部分发生更改时，更新一部分状态

import { combineReducers } from 'redux';

// 定义初始状态
const initialState = {
  counter: 0,
  username: '',
  isLoggedIn: false
};

// 定义处理counter相关操作的reducer函数
const counterReducer = (state = initialState.counter, action) => {
  switch (action.type) {
    case 'INCREMENT':
      return state + 1;
    case 'DECREMENT':
      return state - 1;
    default:
      return state;
  }
};

// 定义处理username相关操作的reducer函数
const usernameReducer = (state = initialState.username, action) => {
  switch (action.type) {
    case 'SET_USERNAME':
      return action.payload;
    default:
      return state;
  }
};

// 定义处理isLoggedIn相关操作的reducer函数
const isLoggedInReducer = (state = initialState.isLoggedIn, action) => {
  switch (action.type) {
    case 'LOGIN':
      return true;
    case 'LOGOUT':
      return false;
    default:
      return state;
  }
};

// 使用combineReducers将多个reducer函数合并成一个
const rootReducer = combineReducers({
  counter: counterReducer,
  username: usernameReducer,
  isLoggedIn: isLoggedInReducer
});

export default rootReducer;

store.dispatch({ type: 'INCREMENT' });