How to use SqlitePersistentStorage on Android

[FluffyDiscord]

I am trying to use the SqlitePersistentStorage since I will have thousands of tasks to track and I think the default local_storage will be a bottleneck.

I created FileDownloader in main() with SqlitePersistentStorage, but it's throwing error about "db" not being initialized. When I manually call the initialize() before passing it to FileDownloader, it complains about "db" already being initlized. I have no clue what to do.

[781flyingdutchman]

Hi, sorry this is causing problems. You should call FileDownloader(persistentStorage: SqlitePersistentStorage()); before the first use of the FileDownloader, and this statement can only be executed once. In the example app therefore it is the first statement in the initState of the main screen (it is commented out, but you can see it there), but for a typical app it maybe even better to do it before runApp.

If this is causing problems, can you share the code snippet you use, and also the Flutter logs around that time (with the apparent reference to the db not being initialized)?

[FluffyDiscord]

My main.dart looks like this:

import "package:background_downloader/background_downloader.dart";

void main() async {
  WidgetsFlutterBinding.ensureInitialized();

  await FileDownloader(persistentStorage: SqlitePersistentStorage()).trackTasks();

  runApp(const MyApp());
}

and this throws:

Debug service listening on ws://127.0.0.1:41195/QD38PIx0Dhs=/ws
Syncing files to device sdk gphone64 x86 64...
E/flutter (10026): [ERROR:flutter/runtime/dart_vm_initializer.cc(41)] Unhandled Exception: LateInitializationError: Field 'db' has not been initialized.
E/flutter (10026): #0      SqlitePersistentStorage.db (package:background_downloader/src/sqlite_storage.dart)
E/flutter (10026): #1      SqlitePersistentStorage.retrieveAllTaskRecords (package:background_downloader/src/sqlite_storage.dart:186:15)
E/flutter (10026): #2      Database.allRecords (package:background_downloader/src/database.dart:36:39)
E/flutter (10026): #3      BaseDownloader.trackTasks (package:background_downloader/src/base_downloader.dart:321:38)
E/flutter (10026): #4      FileDownloader.trackTasks (package:background_downloader/src/file_downloader.dart:689:23)
E/flutter (10026): #5      main (package:app/main.dart:27:70)
E/flutter (10026): #6      _runMain.<anonymous closure> (dart:ui/hooks.dart:159:23)
E/flutter (10026): #7      _delayEntrypointInvocation.<anonymous closure> (dart:isolate-patch/isolate_patch.dart:296:19)
E/flutter (10026): #8      _RawReceivePort._handleMessage (dart:isolate-patch/isolate_patch.dart:189:12)
E/flutter (10026): 

[781flyingdutchman]

I suggest separating the trackTasks call, and running it with a slight delay:
Future.delayed(const Duration(milliseconds: 50), (_) => FileDownloader().trackTasks())

The database initialization is async, but not awaited so I think what's happening here is that you call trackTasks when the DB initialization is still running. I'll look into a more elegant solution, but please try this for now

[FluffyDiscord]

I didn't think of that :D Thanks

[781flyingdutchman]

V8.0.0 adds a ready getter for FileDownloader and trackTasks waits for the downloader to be ready before manipulating the database, so I believe this should now be fixed, and your original (more elegant!) code await FileDownloader(persistentStorage: SqlitePersistentStorage()).trackTasks(); should work.

However, SqlitePersistentStorage has moved out of the background_downloader into a separate package background_downloader_sql which is a small extension package with the sqflite dependency, so you need to add this to your pubspec.yaml alongside background_downloader. The reason is that sqflite adds quite a bit of size to an app, and the default downloader does not require it, so it adds overhead for many, to benefit only those who use SqlitePersistentStorage.