Invalid reference to 'this' expression when trying to open database in Flutter - database

Question

I am trying to access a static database that is stored in assets using this answer. I therefore set up my main() function like this:
main() async {
// Construct a file path to copy database to
Directory documentsDirectory = await getApplicationDocumentsDirectory();
String path = join(documentsDirectory.path, "asset_worldcities.db");
// Only copy if the database doesn't exist
if (FileSystemEntity.typeSync(path) == FileSystemEntityType.notFound) {
// Load database from asset and copy
ByteData data = await rootBundle.load(join('assets', 'worldcities.db'));
List<int> bytes = data.buffer.asUint8List(
data.offsetInBytes, data.lengthInBytes);
// Save copied asset to documents
await new File(path).writeAsBytes(bytes);
}
Directory appDocDir = await getApplicationDocumentsDirectory();
String databasePath = join(appDocDir.path, 'asset_database.db');
this.db = await openDatabase(databasePath);
initialized = true;
runApp(Globeye());
}
However this seems not to be allowed as Android Studio marks this as illegal reference and also complains about that the name initialize is undefined. How can I correctly set this up?

Answer 1

You can copy paste run full code below
You can use DatabaseHelper to do this
code snippet
class DatabaseHelper {
static final DatabaseHelper _instance = DatabaseHelper.internal();
factory DatabaseHelper() => _instance;
static Database _db;
Future<Database> get db async {
if (_db != null) {
return _db;
}
_db = await initDb();
return _db;
}
DatabaseHelper.internal();
initDb() async {
var databasesPath = await getDatabasesPath();
var path = join(databasesPath, "asset_database.db");
print(path);
// delete existing if any
await deleteDatabase(path);
// Make sure the parent directory exists
try {
await Directory(dirname(path)).create(recursive: true);
} catch (_) {}
// Copy from asset
ByteData data = await rootBundle.load(join("assets", "worldcities.db"));
List<int> bytes =
data.buffer.asUint8List(data.offsetInBytes, data.lengthInBytes);
await File(path).writeAsBytes(bytes, flush: true);
// open the database
var db = await openDatabase(path);
return db;
}
}
void main() async {
WidgetsFlutterBinding.ensureInitialized();
Database db = await DatabaseHelper().db;
print(db.path);
runApp(MyApp());
}
output
I/flutter ( 6569): /data/user/0/yourdomain.projecct/databases/asset_database.db
I/flutter ( 6569): /data/user/0/yourdomain.projecct/databases/asset_database.db
full code
import 'dart:io';
import 'package:flutter/material.dart';
import 'package:flutter/services.dart';
import 'package:sqflite/sqflite.dart';
import 'package:path/path.dart';
class DatabaseHelper {
static final DatabaseHelper _instance = DatabaseHelper.internal();
factory DatabaseHelper() => _instance;
static Database _db;
Future<Database> get db async {
if (_db != null) {
return _db;
}
_db = await initDb();
return _db;
}
DatabaseHelper.internal();
initDb() async {
var databasesPath = await getDatabasesPath();
var path = join(databasesPath, "asset_database.db");
print(path);
// delete existing if any
await deleteDatabase(path);
// Make sure the parent directory exists
try {
await Directory(dirname(path)).create(recursive: true);
} catch (_) {}
// Copy from asset
ByteData data = await rootBundle.load(join("assets", "worldcities.db"));
List<int> bytes =
data.buffer.asUint8List(data.offsetInBytes, data.lengthInBytes);
await File(path).writeAsBytes(bytes, flush: true);
// open the database
var db = await openDatabase(path);
return db;
}
}
void main() async {
WidgetsFlutterBinding.ensureInitialized();
Database db = await DatabaseHelper().db;
print(db.path);
runApp(MyApp());
}
class MyApp extends StatelessWidget {
#override
Widget build(BuildContext context) {
return MaterialApp(
title: 'Flutter Demo',
theme: ThemeData(
primarySwatch: Colors.blue,
visualDensity: VisualDensity.adaptivePlatformDensity,
),
home: MyHomePage(title: 'Flutter Demo Home Page'),
);
}
}
class MyHomePage extends StatefulWidget {
MyHomePage({Key key, this.title}) : super(key: key);
final String title;
#override
_MyHomePageState createState() => _MyHomePageState();
}
class _MyHomePageState extends State<MyHomePage> {
int _counter = 0;
void _incrementCounter() {
setState(() {
_counter++;
});
}
#override
Widget build(BuildContext context) {
return Scaffold(
appBar: AppBar(
title: Text(widget.title),
),
body: Center(
child: Column(
mainAxisAlignment: MainAxisAlignment.center,
children: <Widget>[
Text(
'You have pushed the button this many times:',
),
Text(
'$_counter',
style: Theme.of(context).textTheme.headline4,
),
],
),
),
floatingActionButton: FloatingActionButton(
onPressed: _incrementCounter,
tooltip: 'Increment',
child: Icon(Icons.add),
),
);
}
}