CAKE-278 | added isReceiveAmountEditable and isFixedRateMode to exchange_view_model.dart; fixed changeReceiveAmount() in the exchange_view_model.dart; applied StandardCheckbox to exchange_page.dart for checking fixed rate mode; applied alert dialog to exchange_page.dart when receive amount has focus and fixed rate mode is not checked; added reaction for isFixedRateMode in the exchange_page.dart; added key and changeValue() to standard_checkbox.dart

lib/src/screens/exchange/exchange_page.dart

@@ -1,6 +1,8 @@
 import 'dart:ui';
 import 'package:cake_wallet/entities/sync_status.dart';
 import 'package:cake_wallet/entities/wallet_type.dart';
+import 'package:cake_wallet/exchange/changenow/changenow_exchange_provider.dart';
+import 'package:cake_wallet/src/widgets/standard_checkbox.dart';
 import 'package:dotted_border/dotted_border.dart';
 import 'package:flutter/cupertino.dart';
 import 'package:flutter/material.dart';
@@ -37,6 +39,7 @@ class ExchangePage extends BasePage {
   final ExchangeViewModel exchangeViewModel;
   final depositKey = GlobalKey<ExchangeCardState>();
   final receiveKey = GlobalKey<ExchangeCardState>();
+  final checkBoxKey = GlobalKey<StandardCheckboxState>();
   final _formKey = GlobalKey<FormState>();
   final _depositAmountFocus = FocusNode();
   final _receiveAmountFocus = FocusNode();
@@ -240,9 +243,7 @@ class ExchangePage extends BasePage {
                                           ? exchangeViewModel.wallet.address
                                           : exchangeViewModel.receiveAddress,
                                       initialIsAmountEditable: exchangeViewModel
-                                              .provider is XMRTOExchangeProvider
-                                          ? true
-                                          : false,
+                                          .isReceiveAmountEditable,
                                       initialIsAddressEditable:
                                           exchangeViewModel
                                               .isReceiveAddressEnabled,
@@ -271,6 +272,21 @@ class ExchangePage extends BasePage {
                         ],
                       ),
                     ),
+                    Padding(
+                      padding: EdgeInsets.only(top: 12, left: 24),
+                      child: Row(
+                        mainAxisAlignment: MainAxisAlignment.start,
+                        children: [
+                          StandardCheckbox(
+                            key: checkBoxKey,
+                            value: exchangeViewModel.isFixedRateMode,
+                            caption: 'Fixed rate', // FIXME
+                            onChanged: (value) =>
+                            exchangeViewModel.isFixedRateMode = value,
+                          ),
+                        ],
+                      )
+                    ),
                     Padding(
                       padding: EdgeInsets.only(top: 30, left: 24, bottom: 24),
                       child: Row(
@@ -567,7 +583,7 @@ class ExchangePage extends BasePage {
     });
 
     reaction((_) => exchangeViewModel.provider, (ExchangeProvider provider) {
-      provider is XMRTOExchangeProvider
+      provider is ChangeNowExchangeProvider
           ? receiveKey.currentState.isAmountEditable(isEditable: true)
           : receiveKey.currentState.isAmountEditable(isEditable: false);
     });
@@ -647,6 +663,39 @@ class ExchangePage extends BasePage {
       }
     });
 
+    _receiveAmountFocus.addListener(() {
+      if (_receiveAmountFocus.hasFocus && !exchangeViewModel.isFixedRateMode) {
+        showPopUp<void>(
+            context: context,
+            builder: (BuildContext context) {
+              return AlertWithTwoActions(
+                  alertTitle: S.of(context).exchange,
+                  alertContent: 'You will be able to enter receive amount when fixed rate is checked. Do you want to check fixed rate?', //FIXME
+                  leftButtonText: S.of(context).cancel,
+                  rightButtonText: S.of(context).ok,
+                  actionLeftButton: () {
+                    FocusScope.of(context).unfocus();
+                    receiveAmountController.text = '';
+                    Navigator.of(context).pop();
+                  },
+                  actionRightButton: () {
+                    exchangeViewModel.isFixedRateMode = true;
+                    checkBoxKey.currentState
+                        .changeValue(exchangeViewModel.isFixedRateMode);
+                    Navigator.of(context).pop();
+                  });
+            });
+      }
+    });
+
+    reaction((_) => exchangeViewModel.isFixedRateMode, (bool isFixedRateMode) {
+      if ((_receiveAmountFocus.hasFocus ||
+           exchangeViewModel.isReceiveAmountEntered) && !isFixedRateMode) {
+        FocusScope.of(context).unfocus();
+        receiveAmountController.text = '';
+      }
+    });
+
     _isReactionsSet = true;
   }
 

lib/src/widgets/standard_checkbox.dart

@@ -4,9 +4,10 @@ import 'package:flutter/material.dart';
 
 class StandardCheckbox extends StatefulWidget {
   StandardCheckbox({
+    Key key,
     @required this.value,
     this.caption = '',
-    @required this.onChanged});
+    @required this.onChanged}) : super(key: key);
 
   final bool value;
   final String caption;
@@ -24,6 +25,10 @@ class StandardCheckboxState extends State<StandardCheckbox> {
   String caption;
   Function(bool) onChanged;
 
+  void changeValue(bool newValue) {
+    setState(() => value = newValue);
+  }
+
   @override
   Widget build(BuildContext context) {
     return GestureDetector(

lib/view_model/exchange/exchange_view_model.dart

@@ -58,6 +58,9 @@ abstract class ExchangeViewModelBase with Store {
       }
     });
 
+    isReceiveAmountEditable = provider is ChangeNowExchangeProvider;
+    isFixedRateMode = false;
+
     isReceiveAmountEntered = false;
     loadLimits();
   }
@@ -106,6 +109,12 @@ abstract class ExchangeViewModelBase with Store {
   @observable
   bool isReceiveAmountEntered;
 
+  @observable
+  bool isReceiveAmountEditable;
+
+  @observable
+  bool isFixedRateMode;
+
   @computed
   SyncStatus get status => wallet.syncStatus;
 
@@ -127,6 +136,7 @@ abstract class ExchangeViewModelBase with Store {
     this.provider = provider;
     depositAmount = '';
     receiveAmount = '';
+    isReceiveAmountEditable = provider is ChangeNowExchangeProvider;
     loadLimits();
   }
 
@@ -160,8 +170,8 @@ abstract class ExchangeViewModelBase with Store {
 
     provider
         .calculateAmount(
-            from: depositCurrency,
-            to: receiveCurrency,
+            from: receiveCurrency,
+            to: depositCurrency,
             amount: _amount,
             isReceiveAmount: true)
         .then((amount) => _cryptoNumberFormat