How to write and read data anywhere by shared_preferences on Flutter 3.7 background isolates? - flutter

Question

On Flutter 3.7 platform channels can run on any isolate. So I tried this sample,
import ‘package:flutter/services.dart’;
import ‘package:shared_preferences/shared_preferences.dart’;
void main() {
// Identify the root isolate to pass to the background isolate.
// (API introduced in Flutter 3.7)
RootIsolateToken rootIsolateToken = RootIsolateToken.instance!;
Isolate.spawn(_isolateMain, rootIsolateToken);
}
void _isolateMain(RootIsolateToken rootIsolateToken) async {
// Register the background isolate with the root isolate.
BackgroundIsolateBinaryMessenger
.ensureInitialized(rootIsolateToken);
// You can now use the shared_preferences plugin.
SharedPreferences sharedPreferences =
await SharedPreferences.getInstance();
print(sharedPreferences.getBool(‘isDebug’));
}
I can read from data on shared_preferences in this sample okey. But how can I use this feature anywhere in my app? How can I set or read data using this isolate on initState for example?

Answer 1

Basically you need to implement communication between isolates. You can read more about it here
Here is an example, you can change flutter_secure_storage that i used with shared_preferences package
import 'dart:async';
import 'dart:isolate';
import 'package:flutter/services.dart';
import 'package:flutter/widgets.dart';
import 'package:flutter_secure_storage/flutter_secure_storage.dart';
class CreationEvent {
final RootIsolateToken isolateToken;
final SendPort sendPort;
CreationEvent(this.isolateToken, this.sendPort);
}
class DeletetionEvent {}
class ReadEvent {
final String key;
const ReadEvent(this.key);
}
class ReadResult {
final String key;
final String? content;
const ReadResult(this.key, this.content);
}
class IsolateIO {
IsolateIO._();
final _toBgPort = Completer();
final Map<Object, Completer> _completerMap = {};
Isolate? _isolate;
StreamSubscription? _fromBgListener;
void start() async {
RootIsolateToken rootIsolateToken = RootIsolateToken.instance!;
ReceivePort fromBG = ReceivePort();
_fromBgListener = fromBG.listen((message) {
// setup process
if (message is SendPort) {
_toBgPort.complete(message);
return;
}
if (message is ReadResult) {
_completerMap['read:${message.key}']?.complete(message.content);
_completerMap.remove('read:${message.key}');
}
});
_isolate = await Isolate.spawn(
(CreationEvent data) {
final worker = IsolateWorker(data.isolateToken, data.sendPort);
worker.listen();
},
CreationEvent(rootIsolateToken, fromBG.sendPort),
);
}
Future<String?> readFromStorage(String key) async {
// make sure isolate created with ports
final port = await _toBgPort.future;
// store completer
final completer = Completer<String?>();
_completerMap['read:$key'] = completer;
// send key to be read
port.send(ReadEvent(key));
// return result
return completer.future;
}
void stop() async {
if (_toBgPort.isCompleted) {
final port = await _toBgPort.future;
port.send(DeletetionEvent());
}
_fromBgListener?.cancel();
_isolate?.kill(priority: Isolate.immediate);
}
static final i = IsolateIO._();
}
class IsolateWorker {
final RootIsolateToken rootIsolateToken;
final SendPort toMain;
final FlutterSecureStorage storage;
StreamSubscription? subs;
IsolateWorker(
this.rootIsolateToken,
this.toMain, {
this.storage = const FlutterSecureStorage(
aOptions: AndroidOptions(
encryptedSharedPreferences: true,
),
),
}) {
// Register the background isolate with the root isolate.
BackgroundIsolateBinaryMessenger.ensureInitialized(rootIsolateToken);
}
void listen() {
ReceivePort fromMain = ReceivePort();
toMain.send(fromMain.sendPort);
subs = fromMain.listen((message) => onMessage(message));
}
void onMessage(dynamic message) async {
if (message is DeletetionEvent) {
subs?.cancel();
return;
}
if (message is ReadEvent) {
final rawJson = await storage.read(key: message.key);
toMain.send(ReadResult(message.key, rawJson));
}
}
}
class View extends StatefulWidget {
const View({super.key});
#override
State<View> createState() => _ViewState();
}
class _ViewState extends State<View> {
String username = '';
#override
void initState() {
super.initState();
IsolateIO.i.start();
WidgetsBinding.instance.addPostFrameCallback((_) async {
final name = await IsolateIO.i.readFromStorage('username');
setState(() {
username = name ?? '';
});
});
}
#override
void dispose() {
IsolateIO.i.stop();
super.dispose();
}
#override
Widget build(BuildContext context) {
return SizedBox(
child: Text(username),
);
}
}