Adds change detection mechanism for document changes

lib/core/notifier/document_changed_notifier.dart

@@ -0,0 +1,38 @@
+import 'dart:async';
+
+import 'package:paperless_api/paperless_api.dart';
+import 'package:rxdart/subjects.dart';
+
+typedef DocumentChangedCallback = void Function(DocumentModel document);
+
+class DocumentChangedNotifier {
+  final Subject<DocumentModel> _updated = PublishSubject();
+  final Subject<DocumentModel> _deleted = PublishSubject();
+
+  void notifyUpdated(DocumentModel updated) {
+    _updated.add(updated);
+  }
+
+  void notifyDeleted(DocumentModel deleted) {
+    _deleted.add(deleted);
+  }
+
+  List<StreamSubscription> listen({
+    DocumentChangedCallback? onUpdated,
+    DocumentChangedCallback? onDeleted,
+  }) {
+    return [
+      _updated.listen((value) {
+        onUpdated?.call(value);
+      }),
+      _updated.listen((value) {
+        onDeleted?.call(value);
+      }),
+    ];
+  }
+
+  void close() {
+    _updated.close();
+    _deleted.close();
+  }
+}