amugofjava#94 Downloads failing immediately correctly update UI

Add a Lock to DownloadService. This ensures that download status updates are not processed while a download start/stop is requested (they are processed right after).

lib/services/download/mobile_download_service.dart

@@ -15,6 +15,7 @@ import 'package:collection/collection.dart' show IterableExtension;
 import 'package:logging/logging.dart';
 import 'package:mp3_info/mp3_info.dart';
 import 'package:rxdart/rxdart.dart';
+import 'package:synchronized/synchronized.dart';
 
 /// An implementation of a [DownloadService] that handles downloading
 /// of episodes on mobile.
@@ -24,17 +25,27 @@ class MobileDownloadService extends DownloadService {
   final log = Logger('MobileDownloadService');
   final Repository repository;
   final DownloadManager downloadManager;
+  late final StreamSubscription _downloadProgressSubscription;
+
+  /// Lock ensures we wait for task creation and local save
+  /// before handling subsequent [Download update events].
+  final _downloadProgressLock = Lock();
 
   MobileDownloadService({required this.repository, required this.downloadManager}) {
-    downloadManager.downloadProgress.pipe(downloadProgress);
-    downloadProgress.listen((progress) {
-      _updateDownloadProgress(progress);
-    });
+    _downloadProgressSubscription = downloadManager.downloadProgress.listen(
+      (event) async => await _downloadProgressLock.synchronized(
+        () {
+          downloadProgress.add(event);
+          _updateDownloadProgress(event);
+        },
+      ),
+    );
   }
 
   @override
   void dispose() {
     downloadManager.dispose();
+    _downloadProgressSubscription.cancel();
   }
 
   @override
@@ -91,16 +102,22 @@ class MobileDownloadService extends DownloadService {
         /// the URL before calling download and ensure it is https.
         var url = await resolveUrl(episode.contentUrl!, forceHttps: true);
 
-        final taskId = await downloadManager.enqueueTask(url, downloadPath, filename);
-
-        // Update the episode with download data
-        episode.filepath = episodePath;
-        episode.filename = filename;
-        episode.downloadTaskId = taskId;
-        episode.downloadState = DownloadState.downloading;
-        episode.downloadPercentage = 0;
-
-        await repository.saveEpisode(episode);
+        await _downloadProgressLock.synchronized(() async {
+          final taskId = await downloadManager.enqueueTask(
+            url,
+            downloadPath,
+            filename!,
+          );
+
+          // Update the episode with download data
+          episode.filepath = episodePath;
+          episode.filename = filename;
+          episode.downloadTaskId = taskId;
+          episode.downloadState = DownloadState.downloading;
+          episode.downloadPercentage = 0;
+
+          await repository.saveEpisode(episode);
+        });
 
         return Future.value(true);
       }