Add addToCart function

lib/data/cart.dart

@@ -0,0 +1,20 @@
+import 'dart:convert';
+import 'package:shared_preferences/shared_preferences.dart';
+
+Future<void> addToCart(productId) async {
+  final prefs = await SharedPreferences.getInstance();
+
+  var cart = jsonDecode(prefs.getString('cart') ?? '[]');
+  var filteredCart = cart.where((x) => x["product_id"] == productId).toList();
+  if (filteredCart.length == 0) {
+    cart.add({
+      "product_id": productId,
+      "quantity": 1
+    });
+  } else {
+    filteredCart[0]["quantity"]++;
+    cart = cart.where((x) => x["product_id"] != productId).toList();
+    cart.add(filteredCart[0]);
+  }
+  await prefs.setString('cart', jsonEncode(cart).toString());
+}