CAKE-185 | added validation of amount for xmr.to provider in the exchange_page.dart; added isValidAmount() to exchange_view_model.dart

lib/src/screens/exchange/exchange_page.dart

@@ -411,7 +411,18 @@ class ExchangePage extends BasePage {
                                   buttonAction: () => Navigator.of(context).pop());
                               });
                           } else {
-                            exchangeViewModel.createTrade();
+                            exchangeViewModel.isValidAmount()
+                            ? exchangeViewModel.createTrade()
+                            : showPopUp<void>(
+                                context: context,
+                                builder: (BuildContext context) {
+                                  return AlertWithOneAction(
+                                      alertTitle: 'XMR.TO error',
+                                      alertContent: 'Invalid amount. Maximum limit 8 digits after the decimal point ',
+                                      buttonText: S.of(context).ok,
+                                      buttonAction: () =>
+                                          Navigator.of(context).pop());
+                                });
                           }
                         }
                       },

lib/view_model/exchange/exchange_view_model.dart

@@ -306,6 +306,27 @@ abstract class ExchangeViewModelBase with Store {
   void removeTemplate({ExchangeTemplate template}) =>
       _exchangeTemplateStore.remove(template: template);
 
+  bool isValidAmount() {
+    bool isValid = true;
+
+    if (provider is XMRTOExchangeProvider) {
+      final amount = isReceiveAmountEntered
+          ? receiveAmount
+          : depositAmount;
+
+      final amountParts = amount.replaceAll(',', '.')
+          .split('.');
+
+      final numberOfFractionDigits =
+      amountParts.length > 1
+          ? amountParts[1].length : 0;
+
+      isValid = !(numberOfFractionDigits > 8);
+    }
+
+    return isValid;
+  }
+
   List<ExchangeProvider> providersForCurrentPair() {
     return _providersForPair(from: depositCurrency, to: receiveCurrency);
   }