useReducer 通过 dispatch 分发 action，然后由 reducer 函数根据 action 类型决定如何更新状态，适合复杂的状态更新逻辑：

const reducer = (state, action) => {
  switch (action.type) {
    case "increment":
      return { count: state.count + 1 };
    case "decrement":
      return { count: state.count - 1 };
    default:
      throw new Error();
  }
};