feat(architecture): add obsevers for alternative usecase scenarions (#38)

packages/wyatt_architecture/lib/src/domain/usecases/observers.dart

@@ -0,0 +1,44 @@
+// Copyright (C) 2022 WYATT GROUP
+// Please see the AUTHORS file for details.
+//
+// This program is free software: you can redistribute it and/or modify
+// it under the terms of the GNU General Public License as published by
+// the Free Software Foundation, either version 3 of the License, or
+// any later version.
+//
+// This program is distributed in the hope that it will be useful,
+// but WITHOUT ANY WARRANTY; without even the implied warranty of
+// MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
+// GNU General Public License for more details.
+//
+// You should have received a copy of the GNU General Public License
+// along with this program. If not, see <https://www.gnu.org/licenses/>.
+
+import 'dart:async';
+
+import 'package:wyatt_architecture/wyatt_architecture.dart';
+
+/// Usecase observers
+mixin Observer<Parameters, ReturnType> {
+  /// Called before usecase is runned.
+  /// Usefull to check the preconditions
+  FutureOr<void> onStart(Parameters? params) {}
+
+  /// Called when error occures during main scenario
+  /// Usefull to run alternative scenario
+  FutureOr<void> onError(AppException? error) {}
+}
+
+/// Specific observer for classic usecase
+mixin AsyncObserver<ReturnType> {
+  FutureOr<void> onComplete(ReturnType? data) {}
+}
+
+/// Specific observer for stream case usecase
+mixin StreamObserver<ReturnType> {
+  /// Replaces the data event handler of this subscription.
+  void onDone() {}
+
+  ///  Replaces the done event handler of this subscription.
+  void onData(ReturnType? data) {}
+}