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

frontend/packages/sxchnge_wallet/lib/src/destinations/search.dart

@@ -0,0 +1,65 @@
+import 'dart:async';
+
+import 'package:flutter/foundation.dart';
+
+import 'models.dart';
+import 'service.dart';
+
+class BankSearchProvider extends ChangeNotifier {
+  BankSearchProvider(this.service);
+  final DestinationService service;
+  Timer? _timer;
+  Completer<void>? _abort;
+  int _generation = 0;
+  bool _disposed = false, loading = false;
+  String query = '';
+  List<Bank> results = [];
+  DestinationIssue? error;
+  void search(String value) {
+    _timer?.cancel();
+    if (_abort != null && !_abort!.isCompleted) _abort!.complete();
+    _abort = null;
+    final generation = ++_generation;
+    query = value;
+    results = [];
+    error = null;
+    loading = false;
+    if (value.trim().length >= 2) {
+      _timer = Timer(
+        const Duration(milliseconds: 350),
+        () => _lookup(value, generation),
+      );
+    }
+    notifyListeners();
+  }
+
+  Future<void> _lookup(String value, int generation) async {
+    loading = true;
+    notifyListeners();
+    try {
+      final abort = Completer<void>();
+      _abort = abort;
+      final rows = await service.banks(value, abortTrigger: abort.future);
+      if (_disposed || generation != _generation) return;
+      results = rows;
+      if (rows.isEmpty) error = DestinationIssue.invalid;
+    } on DestinationException catch (e) {
+      if (!_disposed && generation == _generation) error = e.issue;
+    } catch (_) {
+      if (!_disposed && generation == _generation)
+        error = DestinationIssue.unavailable;
+    }
+    if (_disposed || generation != _generation) return;
+    loading = false;
+    notifyListeners();
+  }
+
+  @override
+  void dispose() {
+    _disposed = true;
+    _generation++;
+    _timer?.cancel();
+    if (_abort != null && !_abort!.isCompleted) _abort!.complete();
+    super.dispose();
+  }
+}