Task: Add my cart code example to explain a local cart mutable

example/my_cart_example.dart

@@ -0,0 +1,77 @@
+import 'dart:convert';
+
+import 'package:cubetiq/interfaces.dart';
+
+void main(List<String> args) {
+  MyCart.addCart(
+    Cart(1, 'Apple', 1),
+  );
+
+  MyCart.show();
+
+  MyCart.addCart(
+    Cart(1, 'Apple', 2),
+  );
+
+  MyCart.show();
+
+  MyCart.inc(1, qty: 2);
+
+  MyCart.show();
+
+  MyCart.dec(1, qty: 4);
+
+  MyCart.show();
+}
+
+class Cart implements ToJson {
+  final int id;
+  final String name;
+  double qty;
+
+  Cart(this.id, this.name, this.qty);
+
+  @override
+  Map toJson() {
+    return {
+      'id': id,
+      'name': name,
+      'qty': qty,
+    };
+  }
+}
+
+class MyCart {
+  static final Map<int, Cart> carts = {};
+
+  static void inc(int id, {double qty = 1}) {
+    if (carts.containsKey(id)) {
+      carts[id]!.qty += qty;
+    }
+  }
+
+  static void dec(int id, {double qty = 1}) {
+    if (carts.containsKey(id)) {
+      carts[id]!.qty -= qty;
+    }
+  }
+
+  static void addCart(Cart cart) {
+    if (carts.containsKey(cart.id)) {
+      carts[cart.id]!.qty += cart.qty;
+    } else {
+      carts[cart.id] = cart;
+    }
+  }
+
+  static void removeCart(Cart cart) {
+    if (carts.containsKey(cart.id)) {
+      carts.remove(cart.id);
+    }
+  }
+
+  static void show() {
+    var data = carts.map((key, value) => MapEntry(key, jsonEncode(value)));
+    print('$data');
+  }
+}