feat: full local assets / album sync

mobile-v2/lib/domain/models/device_asset_download.model.dart

@@ -0,0 +1,56 @@
+import 'dart:async';
+import 'dart:io';
+
+enum DeviceAssetRequestStatus {
+  preparing,
+  downloading,
+  success,
+  failed,
+}
+
+class DeviceAssetDownloadHandler {
+  DeviceAssetDownloadHandler() : stream = const Stream.empty() {
+    assert(
+      Platform.isIOS || Platform.isMacOS,
+      '$runtimeType should only be used on iOS or macOS.',
+    );
+  }
+
+  /// A stream that provides information about the download status and progress of the asset being downloaded.
+  Stream<DeviceAssetDownloadState> stream;
+}
+
+class DeviceAssetDownloadState {
+  final double progress;
+  final DeviceAssetRequestStatus status;
+
+  const DeviceAssetDownloadState({
+    required this.progress,
+    required this.status,
+  });
+
+  DeviceAssetDownloadState copyWith({
+    double? progress,
+    DeviceAssetRequestStatus? status,
+  }) {
+    return DeviceAssetDownloadState(
+      progress: progress ?? this.progress,
+      status: status ?? this.status,
+    );
+  }
+
+  @override
+  String toString() {
+    return 'DeviceAssetDownloadState(progress: $progress, status: $status)';
+  }
+
+  @override
+  bool operator ==(covariant DeviceAssetDownloadState other) {
+    return other.progress == progress && other.status == status;
+  }
+
+  @override
+  int get hashCode {
+    return progress.hashCode ^ status.hashCode;
+  }
+}