Flutter: Unable to execute database CRUD inside dart Isolate (Android)

[dineshrajbhar777]

I am trying to do data syncing from server in separate dart isolate/compute and also need to dump downloaded data to database using SQFlite but I am getting error that I can't figure out what was gone wrong.

here is the code

import "dart:isolate";
import "package:flutter/foundation.dart";
import "package:flutter/widgets.dart";
//import 'package:path_provider/path_provider.dart';
import "package:sqflite/sqflite.dart";

Future<Database> getDB() async{
  //var dir= await getExternalStorageDirectory();
  //var dbPath= "${dir.path}youtility.db";
  var dbPath= "/storage/emulated/0/Android/data/com.example.sampleapp/files/test.db";
  var database= await openDatabase(
    dbPath,
    version: 1,
    onCreate: (db, version) {
      return db.execute(
        "CREATE TABLE typeassist ("
          "taid   INTEGER PRIMARY KEY,"
          "tacode TEXT,"
          "taname TEXT"
        ");"
      );
    }
  );
  return database;
}

Future<int> upsert() async {
  print("upsert() -");
  Database db= await getDB();
  var taMapList= [
    { "taid": 1, "tacode": "ASSET", "taname": "ASSET" },
    { "taid": 2, "tacode": "SMARTPLACE", "taname": "SMARTPLACE" },
    { "taid": 3, "tacode": "CHECKPOINT", "taname": "CHECKPOINT" },
  ];
  for(var i= 0; i < taMapList.length; i++) {
    print("upsert() taMapList[$i] ${taMapList[i]}");
    await db.insert("typeassist", taMapList[i], conflictAlgorithm: ConflictAlgorithm.replace);
  }
  await db.close();
  print("upsert() db closed");
  db= null;
  print("upsert() +");
  return 0;
}

Future<int> fetch(String name) async {
  print("fetch(name $name) -");
  Database db= await getDB();
  var list= await db.rawQuery("select * from typeassist");
  for(var i= 0; i < list.length; i++) {
    print("fetch() [$i] ${list[i]}");
  }
  await db.close();
  print("fetch() db closed");
  db= null;
  print("fetch() +");
  return 0;
}

//  NORMAL
Future<int> dbNormal() async {
  print("dbNormal() -");
  await upsert();
  await fetch("normal");
  print("dbNormal() +");
  return 0;
}

//  COMPUTE
Future<int> dbCompute() async{
  print("dbCompute() -");
  await upsert();
  compute(fetch, "compute");
  print("dbCompute() +");
  return 0;
}

//  ISOLATE
Isolate isolate;
Future<int> start() async {
  print("start() -");
  ReceivePort receivePort= new ReceivePort();
  isolate= await Isolate.spawn(fetchData, receivePort.sendPort);
  receivePort.listen((data) {
    print("start() RECEIVE: $data");
  });
  print("start() +");
  return 0;
}

int stop() {
  print("stop() -");
  if(isolate != null) {
    isolate.kill(priority: Isolate.immediate);
    isolate= null;
  }
  print("stop() +");
  return 0;
}

int fetchData(SendPort sendPort) {
  print("fetchDB(sendPort $sendPort) -");
  var f= fetch("isolate");
  f.then((rc) {
    print("fetchDB() SEND: 0");
    sendPort.send(0);
  });
  print("fetchDB() +");
  return 0;
}

Future<int> dbIsolate() async {
  print("dbIsolate() -");
  start();
  print("dbIsolate() +");
  return 0;
}

Future<int> main() async {
  print("main() -");
  WidgetsFlutterBinding.ensureInitialized();

  print("main()  normal execution -");
  var n= dbNormal();
  n.then((rc){
    print("main() normal execution +");
  });

  await Future.delayed(new Duration(seconds: 1));
  /*print("main() compute execution -");
  var c= dbCompute();
  c.then((rc){
    print("main() compute execution +");
  });*/

  print("main() isolate execution -");
  var i= dbIsolate();
  i.then((rc){
    print("main() isolate execution +");
  });
  print("main() +");
  return 0;
}

here is the error log

Performing hot restart...
Syncing files to device XT1706...
Restarted application in 1,801ms.
I/flutter (15820): main() -
I/flutter (15820): main()  normal execution -
I/flutter (15820): dbNormal() -
I/flutter (15820): upsert() -
I/flutter (15820): upsert() taMapList[0] {taid: 1, tacode: ASSET, taname: ASSET}
I/flutter (15820): upsert() taMapList[1] {taid: 2, tacode: SMARTPLACE, taname: SMARTPLACE}
I/flutter (15820): upsert() taMapList[2] {taid: 3, tacode: CHECKPOINT, taname: CHECKPOINT}
I/flutter (15820): upsert() db closed
I/flutter (15820): upsert() +
I/flutter (15820): fetch(name normal) -
I/flutter (15820): fetch() [0] {taid: 1, tacode: ASSET, taname: ASSET}
I/flutter (15820): fetch() [1] {taid: 2, tacode: SMARTPLACE, taname: SMARTPLACE}
I/flutter (15820): fetch() [2] {taid: 3, tacode: CHECKPOINT, taname: CHECKPOINT}
I/flutter (15820): fetch() db closed
I/flutter (15820): fetch() +
I/flutter (15820): dbNormal() +
I/flutter (15820): main() normal execution +
I/flutter (15820): main() isolate execution -
I/flutter (15820): dbIsolate() -
I/flutter (15820): start() -
I/flutter (15820): dbIsolate() +
I/flutter (15820): main() +
I/flutter (15820): main() isolate execution +
I/flutter (15820): fetchDB(sendPort SendPort) -
I/flutter (15820): start() +
I/flutter (15820): fetch(name isolate) -
I/flutter (15820): fetchDB() +
E/flutter (15820): [ERROR:flutter/runtime/dart_isolate.cc(915)] Unhandled exception:
E/flutter (15820): ServicesBinding.defaultBinaryMessenger was accessed before the binding was initialized.
E/flutter (15820): If you're running an application and need to access the binary messenger before `runApp()` has been called (for example, during plugin initialization), then you need to explicitly call the `WidgetsFlutterBinding.ensureInitialized()` first.
E/flutter (15820): If you're running a test, you can call the `TestWidgetsFlutterBinding.ensureInitialized()` as the first line in your test's `main()` method to initialize the binding.
E/flutter (15820): #0      defaultBinaryMessenger.<anonymous closure> (package:flutter/src/services/binary_messenger.dart:76:7)
E/flutter (15820): #1      defaultBinaryMessenger (package:flutter/src/services/binary_messenger.dart:89:4)
E/flutter (15820): #2      MethodChannel.binaryMessenger (package:flutter/src/services/platform_channel.dart:140:62)
E/flutter (15820): #3      MethodChannel.invokeMethod (package:flutter/src/services/platform_channel.dart:314:35)
E/flutter (15820): #4      invokeMethod (package:sqflite/src/sqflite_impl.dart:17:13)
E/flutter (15820): #5      SqfliteDatabaseFactoryImpl.invokeMethod (package:sqflite/src/factory_impl.dart:82:7)
E/flutter (15820): #6      SqfliteDatabaseMixin.invokeMethod (package:sqflite_common/src/database_mixin.dart:287:15)
E/flutter (15820): #7      SqfliteDatabaseMixin.safeInvokeMethod.<anonymous closure> (package:sqflite_common/src/database_mixin.dart:208:43)
E/flutter (15820): #8      wrapDatabaseException (package:sqflite/src/exception_impl.dart:7:32)
E/flutter (15820): #9      SqfliteDatabaseFactoryImpl.wrapDatabaseException (package:sqflite/src/factory_impl.dart:78:7)
E/flutter (15820): #10     SqfliteDatabaseMixin.safeInvokeMethod (package:sqflite_common/src/database_mixin.dart:208:15)
E/flutter (15820): #11     SqfliteDatabaseMixin.openDatabase (package:sqflite_common/src/database_mixin.dart:542:15)
E/flutter (15820): #12     SqfliteDatabaseMixin.doOpen (package:sqflite_common/src/database_mixin.dart:635:28)
E/flutter (15820): #13     SqfliteDatabaseOpenHelper.openDatabase (package:sqflite_common/src/database.dart:46:22)
E/flutter (15820): #14     SqfliteDatabaseFactoryMixin.openDatabase.<anonymous closure> (package:sqflite_common/src/factory_mixin.dart:104:43)
E/flutter (15820): <asynchronous suspension>
E/flutter (15820): #15     ReentrantLock.synchronized.<anonymous closure>.<anonymous closure> (package:synchronized/src/reentrant_lock.dart:35:24)
E/flutter (15820): #16     _rootRun (dart:async/zone.dart:1126:13)
E/flutter (15820): #17     _CustomZone.run (dart:async/zone.dart:1023:19)
E/flutter (15820): #18     _runZoned (dart:async/zone.dart:1518:10)
E/flutter (15820): #19     runZoned (dart:async/zone.dart:1465:12)
E/flutter (15820): #20     ReentrantLock.synchronized.<anonymous closure> (package:synchronized/src/reentrant_lock.dart:34:24)
E/flutter (15820): #21     BasicLock.synchronized (package:synchronized/src/basic_lock.dart:32:26)
E/flutter (15820): #22     ReentrantLock.synchronized (package:synchronized/src/reentrant_lock.dart:30:17)
E/flutter (15820): #23     SqfliteDatabaseFactoryMixin.openDatabase (package:sqflite_common/src/factory_mixin.dart:71:17)
E/flutter (15820): #24     openDatabase (package:sqflite/sqflite.dart:152:26)
E/flutter (15820): #25     getDB (package:sampleapp/main.dart:11:23)
E/flutter (15820): #26     fetch (package:sampleapp/main.dart:48:22)
E/flutter (15820): #27     fetchData (package:sampleapp/main.dart:103:10)
E/flutter (15820): #28     _startIsolate.<anonymous closure> (dart:isolate-patch/isolate_patch.dart:310:17)
E/flutter (15820): #29     _RawReceivePortImpl._handleMessage (dart:isolate-patch/isolate_patch.dart:174:12)

[alextekartik]

Access should be done in the main isolate only.

• sqflite native access already happens in a background native thread
• Transaction mechanism is not cross-isolate safe
• sqflite_common_ffi access is made in a separate isolate.

Some related discussions here:

• Cannot access database instance from another Isolate
• Problem tunning Sqflite in Isolate
• Multi-Isolate access to Sqflite (iOS)
• MissingPluginException when using sqflite via flutter_isolate

[iMro0t]

I faced the same issue. I managed to get data from isolated with help of Stream. Here is the snippet

// create stream controller with whatever data type you want to communicate
var controller = StreamController<Map<String, String>>();
controller.stream.listen((data) {
     // process Map<String, String> data here
});

// pass controller to isolated function
await compute(fetch, controller);
controller.close();

static fetch(StreamController<Map<String, String>> controller) {
    // compute logic

    // send data main isolated
    controller.add({"foo": "bar"})
}

[rytisder]

To pass persistent data between isolates in Flutter Dart, consider using the Drift database. Drift offers built-in threading support, allowing easy database operations across isolates without extra effort.

More about Isolates - https://drift.simonbinder.eu/docs/advanced-features/isolates/