Add saved payout destination UX and service contracts (#54)

frontend/packages/sxchnge_wallet/lib/src/providers/transfer.dart

@@ -6,6 +6,7 @@ import 'package:sxchnge_analytics/sxchnge_analytics.dart';
 
 import 'package:sxchnge_auth/sxchnge_auth.dart';
 
+import '../destinations/models.dart';
 import '../models/wallet.dart';
 import 'wallet.dart';
 
@@ -40,16 +41,18 @@ class TransferProvider extends ChangeNotifier {
     if (!accounts.any((a) => a.id == destination) || destination == source) {
       destination = accounts.where((a) => a.id != source).firstOrNull?.id;
     }
-    if (!wallet.service.isSimulated && kind == TransferKind.payout) {
-      recipient = wallet.account(source)?.payoutMethod ?? '';
-    }
   }
 
   void _walletChanged() {
     if (_disposed) return;
     final previous = source;
     _syncAccounts();
-    if (source != previous) notifyListeners();
+    if (source != previous) {
+      payoutDestination = null;
+      payoutLabel = '';
+      recipient = '';
+      notifyListeners();
+    }
   }
 
   final WalletProvider wallet;
@@ -58,6 +61,8 @@ class TransferProvider extends ChangeNotifier {
   String source;
   String? destination;
   String amount = '', recipient = '';
+  DestinationInput? payoutDestination;
+  String payoutLabel = '';
   Quote? quote;
   WalletOperation? result;
   WalletError? error;
@@ -72,7 +77,7 @@ class TransferProvider extends ChangeNotifier {
     TransferKind.payout => JourneyKind.payout,
   };
   bool get hasDraft =>
-      amount.isNotEmpty || (wallet.service.isSimulated && recipient.isNotEmpty);
+      amount.isNotEmpty || payoutDestination != null || recipient.isNotEmpty;
   bool get expired =>
       quote != null && !DateTime.now().isBefore(quote!.expiresAt);
   void change({
@@ -80,8 +85,17 @@ class TransferProvider extends ChangeNotifier {
     String? destination,
     String? amount,
     String? recipient,
+    DestinationInput? payoutDestination,
+    String? payoutLabel,
   }) {
     if (busy || uncertain) return;
+    if (source != null && source != this.source) {
+      this.payoutDestination = null;
+      this.payoutLabel = '';
+      recipient = '';
+    }
+    this.payoutDestination = payoutDestination ?? this.payoutDestination;
+    this.payoutLabel = payoutLabel ?? this.payoutLabel;
     this.source = source ?? this.source;
     this.destination = destination ?? this.destination;
     this.amount = amount ?? this.amount;
@@ -96,6 +110,36 @@ class TransferProvider extends ChangeNotifier {
     notifyListeners();
   }
 
+  Future<void> selectPayout(DestinationInput input) async {
+    if (busy || uncertain || wallet.destinations == null) return;
+    busy = true;
+    error = null;
+    notifyListeners();
+    try {
+      final service = wallet.destinations!;
+      final assessment = await service.assess(
+        input,
+        PayoutContext(source, amount: wallet.account(source)?.parse(amount)),
+      );
+      if (!assessment.eligible)
+        throw const DestinationException(DestinationIssue.unsupported);
+      final details = input is OneTimeDestination
+          ? input.details
+          : (await service.get((input as SavedReference).id)).details;
+      if (_disposed) return;
+      busy = false;
+      change(payoutDestination: input, payoutLabel: details.display);
+    } catch (_) {
+      if (_disposed) return;
+      payoutDestination = null;
+      payoutLabel = '';
+      error = WalletError.destinationChanged;
+    }
+    if (_disposed) return;
+    busy = false;
+    notifyListeners();
+  }
+
   void edit() {
     if (!busy && !uncertain) {
       quote = null;
@@ -133,6 +177,7 @@ class TransferProvider extends ChangeNotifier {
           amount: money,
           destination: kind == TransferKind.exchange ? destination : null,
           recipient: recipient.trim(),
+          payoutDestination: payoutDestination,
         ),
       );
       if (_disposed || revision != _revision) return;
@@ -142,6 +187,10 @@ class TransferProvider extends ChangeNotifier {
     } catch (e) {
       if (_disposed) return;
       error = _code(e);
+      if (error == WalletError.destinationChanged) {
+        payoutDestination = null;
+        payoutLabel = '';
+      }
       recorder.finish(op, failure: _failure(error!));
     }
     if (_disposed) return;
@@ -225,6 +274,11 @@ class TransferProvider extends ChangeNotifier {
         uncertain = false;
       } else {
         error = _code(e);
+        if (error == WalletError.destinationChanged) {
+          quote = null;
+          payoutDestination = null;
+          payoutLabel = '';
+        }
       }
       // All transport/unexpected failures are ambiguous after execution started.
       uncertain = [
@@ -247,6 +301,7 @@ class TransferProvider extends ChangeNotifier {
     error = null;
     factorError = null;
     recipient = '';
+    payoutDestination = null;
     recorder.finish(op, simulated: wallet.service.isSimulated);
     wallet.track(value, recorder, journey);
     _handedOff = true;