- Add whether this provide supports fixed rate or not

- Fix Change now fixed rate exchange due to wrong rate id

lib/exchange/changenow/changenow_exchange_provider.dart

@@ -11,7 +11,6 @@ import 'package:cake_wallet/exchange/trade_request.dart';
 import 'package:cake_wallet/exchange/trade_state.dart';
 import 'package:cake_wallet/exchange/changenow/changenow_request.dart';
 import 'package:cake_wallet/exchange/exchange_provider_description.dart';
-import 'package:cake_wallet/exchange/trade_not_created_exeption.dart';
 
 class ChangeNowExchangeProvider extends ExchangeProvider {
   ChangeNowExchangeProvider()
@@ -21,8 +20,7 @@ class ChangeNowExchangeProvider extends ExchangeProvider {
                 .where((i) => i != CryptoCurrency.xhv)
                 .map((i) => CryptoCurrency.all
                     .where((i) => i != CryptoCurrency.xhv)
-                    .map((k) => ExchangePair(from: i, to: k, reverse: true))
-                    .where((c) => c != null))
+                    .map((k) => ExchangePair(from: i, to: k, reverse: true)))
                 .expand((i) => i)
                 .toList());
 
@@ -43,6 +41,9 @@ class ChangeNowExchangeProvider extends ExchangeProvider {
   @override
   bool get isEnabled => true;
 
+  @override
+  bool get supportsFixedRate => true;
+
   @override
   ExchangeProviderDescription get description =>
       ExchangeProviderDescription.changeNow;
@@ -109,6 +110,15 @@ class ChangeNowExchangeProvider extends ExchangeProvider {
     };
 
     if (isFixedRateMode) {
+      // since we schedule to calculate the rate every 5 seconds we need to ensure that
+      // we have the latest rate id with the given inputs before creating the trade
+      await calculateAmount(
+        from: _request.to,
+        to: _request.from,
+        amount: double.tryParse(_request.toAmount) ?? 0,
+        isFixedRateMode: true,
+        isReceiveAmount: true,
+      );
       body['rateId'] = _lastUsedRateId;
     }
 
@@ -180,9 +190,7 @@ class ChangeNowExchangeProvider extends ExchangeProvider {
     final extraId = responseJSON['payinExtraId'] as String;
     final outputTransaction = responseJSON['payoutHash'] as String;
     final expiredAtRaw = responseJSON['validUntil'] as String;
-    final expiredAt = expiredAtRaw != null
-        ? DateTime.parse(expiredAtRaw).toLocal()
-        : null;
+    final expiredAt = DateTime.tryParse(expiredAtRaw)?.toLocal();
 
     return Trade(
         id: id,

lib/exchange/exchange_provider.dart

@@ -1,4 +1,3 @@
-import 'package:flutter/foundation.dart';
 import 'package:cw_core/crypto_currency.dart';
 import 'package:cake_wallet/exchange/trade_request.dart';
 import 'package:cake_wallet/exchange/exchange_pair.dart';
@@ -14,6 +13,7 @@ abstract class ExchangeProvider {
   ExchangeProviderDescription get description;
   bool get isAvailable;
   bool get isEnabled;
+  bool get supportsFixedRate;
 
   @override
   String toString() => title;

lib/exchange/morphtoken/morphtoken_exchange_provider.dart

@@ -66,6 +66,9 @@ class MorphTokenExchangeProvider extends ExchangeProvider {
   @override
   bool get isEnabled => true;
 
+  @override
+  bool get supportsFixedRate => false;
+
   @override
   ExchangeProviderDescription get description =>
       ExchangeProviderDescription.morphToken;

lib/exchange/sideshift/sideshift_exchange_provider.dart

@@ -12,7 +12,6 @@ import 'package:cw_core/crypto_currency.dart';
 import 'package:cake_wallet/exchange/trade_request.dart';
 import 'package:cake_wallet/exchange/trade.dart';
 import 'package:cake_wallet/exchange/limits.dart';
-import 'package:flutter/foundation.dart';
 import 'package:http/http.dart';
 
 class SideShiftExchangeProvider extends ExchangeProvider {
@@ -48,8 +47,7 @@ class SideShiftExchangeProvider extends ExchangeProvider {
 
     return supportedCurrencies
         .map((i) => supportedCurrencies
-            .map((k) => ExchangePair(from: i, to: k, reverse: true))
-            .where((c) => c != null))
+            .map((k) => ExchangePair(from: i, to: k, reverse: true)))
         .expand((i) => i)
         .toList();
   }
@@ -257,8 +255,7 @@ class SideShiftExchangeProvider extends ExchangeProvider {
     state = TradeState.deserialize(raw: status ?? 'created');
 
     final expiredAtRaw = responseJSON['expiresAtISO'] as String;
-    final expiredAt =
-        expiredAtRaw != null ? DateTime.parse(expiredAtRaw).toLocal() : null;
+    final expiredAt = DateTime.tryParse(expiredAtRaw)?.toLocal();
 
     return Trade(
       id: id,
@@ -278,6 +275,9 @@ class SideShiftExchangeProvider extends ExchangeProvider {
   @override
   bool get isEnabled => true;
 
+  @override
+  bool get supportsFixedRate => true;
+
   @override
   String get title => 'SideShift';
 

lib/exchange/simpleswap/simpleswap_exchange_provider.dart

@@ -209,6 +209,9 @@ class SimpleSwapExchangeProvider extends ExchangeProvider {
   @override
   bool get isEnabled => true;
 
+  @override
+  bool get supportsFixedRate => false;
+
   @override
   String get title => 'SimpleSwap';
 

lib/exchange/xmrto/xmrto_exchange_provider.dart

@@ -48,6 +48,9 @@ class XMRTOExchangeProvider extends ExchangeProvider {
   @override
   bool get isEnabled => true;
 
+  @override
+  bool get supportsFixedRate => false;
+
   @override
   ExchangeProviderDescription get description =>
       ExchangeProviderDescription.xmrto;

lib/view_model/exchange/exchange_view_model.dart

@@ -306,10 +306,11 @@ abstract class ExchangeViewModelBase with Store {
   }
 
   Future<void> _calculateBestRate() async {
-    final amount = double.tryParse(depositAmount) ?? 1;
+    final amount = double.tryParse(isFixedRateMode ? receiveAmount : depositAmount) ?? 1;
 
     final result = await Future.wait<double>(
         _tradeAvailableProviders
+            .where((element) => element.supportsFixedRate)
             .map((element) => element.calculateAmount(
                 from: depositCurrency,
                 to: receiveCurrency,