add alot

lib/services/product_service.dart

@@ -21,6 +21,9 @@ abstract class ProductService {
   Future<void> updateProduct(Product product);
 
   Future<void> deleteProduct(String id);
+
+  Future<void> decreaseStock(String productId, int amount);
+  Future<void> increaseStock(String productId, int amount);
 }
 
 class DriftProductService implements ProductService {
@@ -118,4 +121,39 @@ class DriftProductService implements ProductService {
 
     await query.go();
   }
+
+  @override
+  Future<void> decreaseStock(String productId, int amount) async {
+    final product = await getProductById(productId);
+
+    if (product == null) {
+      throw Exception('Product not found');
+    }
+
+    if (product.stockQuantity < amount) {
+      throw Exception('Not enough stock');
+    }
+
+    await updateProduct(
+      product.copyWith(
+        stockQuantity: product.stockQuantity - amount,
+      ),
+    );
+  }
+
+  @override
+  Future<void> increaseStock(String productId, int amount) async {
+    final product = await getProductById(productId);
+
+    if (product == null) {
+      throw Exception('Product not found');
+    }
+
+    await updateProduct(
+      product.copyWith(
+        stockQuantity: product.stockQuantity + amount,
+      ),
+    );
+  }
+
 }