add prompt confirmation and update carton

lib/pages/discount/discount_editor.dart

@@ -98,9 +98,13 @@ class _DiscountEditorState extends State<DiscountEditor> {
             leading: new IconButton(
               icon: new Icon(CupertinoIcons.back),
               onPressed: () {
-                showConfirmDialog(context, "back.button_confirm", () {
+                if (isDataChanged()) {
+                  showConfirmDialog(context, "back.button_confirm", () {
+                    Navigator.of(context).pop();
+                  });
+                } else {
                   Navigator.of(context).pop();
-                });
+                }
               },
             ),
             backgroundColor: primaryColor,
@@ -192,4 +196,19 @@ class _DiscountEditorState extends State<DiscountEditor> {
       });
     }
   }
+
+  isDataChanged() {
+    if (_isNew) {
+      return _codeController.text != "" ||
+          _amountController.text != "" ||
+          customerName != "";
+    } else {
+      Discount _discount = Discount(
+          code: _codeController.text,
+          customerName: customerName,
+          customerId: customerId,
+          amount: double.parse(_amountController.text));
+      return widget.discount.isChangedForEdit(_discount);
+    }
+  }
 }