Add reusable two-factor verification for login and money operations (#32)

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

@@ -4,6 +4,8 @@ import 'package:flutter/foundation.dart';
 
 import 'package:sxchnge_analytics/sxchnge_analytics.dart';
 
+import 'package:sxchnge_auth/sxchnge_auth.dart';
+
 import '../models/wallet.dart';
 import 'wallet.dart';
 
@@ -33,6 +35,7 @@ class TransferProvider extends ChangeNotifier {
   Quote? quote;
   WalletOperation? result;
   WalletError? error;
+  FactorError? factorError;
   bool noSubmission = false;
   bool busy = false, uncertain = false, _disposed = false, _handedOff = false;
   int _revision = 0;
@@ -59,6 +62,7 @@ class TransferProvider extends ChangeNotifier {
     quote = null;
     noSubmission = false;
     error = null;
+    factorError = null;
     _key = null;
     _revision++;
     notifyListeners();
@@ -68,6 +72,7 @@ class TransferProvider extends ChangeNotifier {
     if (!busy && !uncertain) {
       quote = null;
       error = null;
+      factorError = null;
       _key = null;
       notifyListeners();
     }
@@ -84,6 +89,7 @@ class TransferProvider extends ChangeNotifier {
     busy = true;
     noSubmission = false;
     error = null;
+    factorError = null;
     notifyListeners();
     final revision = ++_revision;
     final op = recorder.begin(
@@ -115,7 +121,45 @@ class TransferProvider extends ChangeNotifier {
     notifyListeners();
   }
 
-  Future<void> submit() async {
+  Future<FactorChallenge?> challenge() async {
+    if (busy || uncertain || quote == null) return null;
+    if (expired) {
+      error = WalletError.expiredQuote;
+      notifyListeners();
+      return null;
+    }
+    busy = true;
+    error = null;
+    factorError = null;
+    notifyListeners();
+    FactorChallenge? result;
+    final op = recorder.begin(
+      journey,
+      JourneyStep.factorChallenge,
+      simulated: wallet.service.isSimulated,
+    );
+    try {
+      result = await wallet.factors?.begin(
+        FactorPurpose.values.byName(kind.name),
+        quote!.id,
+      );
+    } catch (e) {
+      if (!_disposed) {
+        factorError = e is FactorException ? e.code : FactorError.unavailable;
+      }
+    }
+    if (_disposed) return null;
+    busy = false;
+    notifyListeners();
+    recorder.finish(
+      op,
+      simulated: wallet.service.isSimulated,
+      failure: factorError == null ? null : JourneyFailure.unavailable,
+    );
+    return result;
+  }
+
+  Future<void> submit({FactorGrant? approval}) async {
     if (busy || quote == null || uncertain) return;
     if (expired) {
       error = WalletError.expiredQuote;
@@ -125,6 +169,7 @@ class TransferProvider extends ChangeNotifier {
     busy = true;
     noSubmission = false;
     error = null;
+    factorError = null;
     notifyListeners();
     _key ??= List.generate(
       16,
@@ -136,18 +181,31 @@ class TransferProvider extends ChangeNotifier {
       simulated: wallet.service.isSimulated,
     );
     try {
-      final r = await wallet.service.submit(quote!.id, _key!);
+      final r = await wallet.service.submit(
+        quote!.id,
+        _key!,
+        approval: approval?.token,
+      );
       if (_disposed) return;
       _accepted(r, op);
     } catch (e) {
       if (_disposed) return;
-      error = _code(e);
+      if (e is FactorException) {
+        error = null;
+        factorError = e.code;
+        uncertain = false;
+      } else {
+        error = _code(e);
+      }
       // All transport/unexpected failures are ambiguous after execution started.
       uncertain = [
         WalletError.unavailable,
         WalletError.uncertain,
       ].contains(error);
-      recorder.finish(op, failure: _failure(error!));
+      recorder.finish(
+        op,
+        failure: error == null ? JourneyFailure.invalid : _failure(error!),
+      );
     }
     if (_disposed) return;
     busy = false;
@@ -158,6 +216,7 @@ class TransferProvider extends ChangeNotifier {
     result = value;
     uncertain = false;
     error = null;
+    factorError = null;
     recipient = '';
     recorder.finish(op, simulated: wallet.service.isSimulated);
     wallet.track(value, recorder, journey);
@@ -169,6 +228,7 @@ class TransferProvider extends ChangeNotifier {
     busy = true;
     noSubmission = false;
     error = null;
+    factorError = null;
     notifyListeners();
     final op = recorder.begin(
       journey,