useReducer للحالة المعقّدة

عندما لا يكون useState كافيًا

نمط دالة Reducer

type Action =
  | { type: 'increment' }
  | { type: 'decrement' }
  | { type: 'reset'; payload: number };

function counterReducer(state: number, action: Action): number {
  switch (action.type) {
    case 'increment': return state + 1;
    case 'decrement': return state - 1;
    case 'reset':     return action.payload;
    default: return state;
  }
}

صيغة useReducer

function Counter() {
  const [count, dispatch] = useReducer(counterReducer, 0);

  return (
    <div>
      <p>{count}</p>
      <button onClick={() => dispatch({ type: 'increment' })}>+</button>
      <button onClick={() => dispatch({ type: 'decrement' })}>-</button>
      <button onClick={() => dispatch({ type: 'reset', payload: 0 })}>Reset</button>
    </div>
  );
}

الإجراءات ذات الأنواع — الاتحادات المميّزة

type CartAction =
  | { type: 'ADD_ITEM'; item: CartItem }
  | { type: 'REMOVE_ITEM'; id: string }
  | { type: 'SET_QUANTITY'; id: string; qty: number }
  | { type: 'CLEAR' };

مثال على حالة معقدة: عربة التسوق

interface CartState {
  items: CartItem[];
  total: number;
}

function cartReducer(state: CartState, action: CartAction): CartState {
  switch (action.type) {
    case 'ADD_ITEM': {
      const exists = state.items.find(i => i.id === action.item.id);
      if (exists) {
        const items = state.items.map(i =>
          i.id === action.item.id ? { ...i, qty: i.qty + 1 } : i
        );
        return { ...state, items, total: calcTotal(items) };
      }
      const items = [...state.items, { ...action.item, qty: 1 }];
      return { ...state, items, total: calcTotal(items) };
    }
    case 'CLEAR': return { items: [], total: 0 };
    default: return state;
  }
}

dispatch ثابت

دمج useReducer وuseContext

const CartContext = createContext<{ state: CartState; dispatch: Dispatch<CartAction> } | null>(null);

function CartProvider({ children }: { children: React.ReactNode }) {
  const [state, dispatch] = useReducer(cartReducer, { items: [], total: 0 });
  return <CartContext.Provider value={{ state, dispatch }}>{children}</CartContext.Provider>;
}

useReducer أم useState

اختبار Reducers بشكل مستقل

test('increment action increases count', () => {
  expect(counterReducer(0, { type: 'increment' })).toBe(1);
  expect(counterReducer(5, { type: 'increment' })).toBe(6);
});

test('reset action sets count to payload', () => {
  expect(counterReducer(42, { type: 'reset', payload: 0 })).toBe(0);
});

استخدام Immer للتحديثات غير القابلة للتغيير

تحقق سريع

مراجعة: useReducer