CAKE-296 | removed xmr to xrp and xmr to xlm exchange pairs from ChangeNow pair list; disabled exchange button when pair is not exists

lib/exchange/changenow/changenow_exchange_provider.dart

@@ -17,12 +17,23 @@ import 'package:cake_wallet/exchange/trade_not_created_exeption.dart';
 class ChangeNowExchangeProvider extends ExchangeProvider {
   ChangeNowExchangeProvider()
       : super(
-            pairList: CryptoCurrency.all
+            /*pairList: CryptoCurrency.all
                 .map((i) => CryptoCurrency.all
                     .map((k) => ExchangePair(from: i, to: k, reverse: true))
                     .where((c) => c != null))
                 .expand((i) => i)
-                .toList());
+                .toList());*/
+      pairList: CryptoCurrency.all
+          .map((i) => CryptoCurrency.all
+          .map((k) {
+            if ((i != CryptoCurrency.xmr)||
+                ((k != CryptoCurrency.xrp)&&(k != CryptoCurrency.xlm))) {
+              return ExchangePair(from: i, to: k, reverse: true);
+            }
+           })
+          .where((c) => c != null))
+          .expand((i) => i)
+          .toList());
 
   static const apiUri = 'https://changenow.io/api/v1';
   static const apiKey = secrets.changeNowApiKey;

lib/src/screens/exchange/exchange_page.dart

@@ -464,6 +464,7 @@ class ExchangePage extends BasePage {
                           },
                           color: Theme.of(context).accentTextTheme.body2.color,
                           textColor: Colors.white,
+                          isDisabled: exchangeViewModel.isDisabledExchangeButton,
                           isLoading:
                               exchangeViewModel.tradeState is TradeIsCreating)),
                 ]),

lib/view_model/exchange/exchange_view_model.dart

@@ -60,6 +60,7 @@ abstract class ExchangeViewModelBase with Store {
     _defineIsReceiveAmountEditable();
     isFixedRateMode = false;
     isReceiveAmountEntered = false;
+    isDisabledExchangeButton = false;
     loadLimits();
   }
 
@@ -113,6 +114,9 @@ abstract class ExchangeViewModelBase with Store {
   @observable
   bool isFixedRateMode;
 
+  @observable
+  bool isDisabledExchangeButton;
+
   @computed
   SyncStatus get status => wallet.syncStatus;
 
@@ -369,19 +373,20 @@ abstract class ExchangeViewModelBase with Store {
             pair.from == depositCurrency && pair.to == receiveCurrency)
         .isNotEmpty;
 
-    if (!isPairExist) {
+    if (isPairExist) {
       final provider =
           _providerForPair(from: depositCurrency, to: receiveCurrency);
 
       if (provider != null) {
+        isDisabledExchangeButton = false;
         changeProvider(provider: provider);
       }
+    } else {
+      isDisabledExchangeButton = true;
+      depositAmount = '';
+      receiveAmount = '';
+      limitsState = LimitsLoadedFailure(error: 'Pair is not exists');
     }
-
-    _defineIsReceiveAmountEditable();
-    depositAmount = '';
-    receiveAmount = '';
-    loadLimits();
   }
 
   ExchangeProvider _providerForPair({CryptoCurrency from, CryptoCurrency to}) {