How to `setState()` in a global function? - flutter

Question

Here's a simple code block for detecting transactions from SMS :
FlutterLocalNotificationsPlugin notificationsPlugin =
FlutterLocalNotificationsPlugin();
onBackgroundMessage(SmsMessage message) async {
log("Message recieved in background");
_TransactionState().onMessage(message);
}
class Transaction extends StatefulWidget {
const Transaction({Key? key}) : super(key: key);
#override
State<Transaction> createState() => _TransactionState();
}
class _TransactionState extends State<Transaction> {
String _message = "";
var debit = 0.0;
var credit = 0.0;
var last = 0.0;
var transact = 0.0;
final Telephony telephony = Telephony.instance;
#override
void initState() {
super.initState();
initPlatformState();
}
onMessage(SmsMessage message) async {
setState(() {
bool mesFlag = false;
bool debFlag = false;
if (message.body!.contains('debit')) {
debFlag = true;
} //check whether debit or credit
for (var bank in banks.entries) {
if (message.address!.contains(bank.key)) {
_message = message.body?.split(
' ')[(debFlag ? bank.value.item2 : bank.value.item1)] ??
"Error reading transaction!";
_message = _message.replaceAll(',', ''); //remove comma
if (_message.contains("Rs.")) {
_message = _message.substring(3);
} else if (_message.contains("Rs")) {
_message = _message.substring(2);
} // remove Rs. and Rs
showNotification("Last transaction amount: $_message");
transact = double.parse(_message);
mesFlag = true;
if (debFlag) {
debit += transact;
last = -transact;
} else {
credit += transact;
last = transact;
}
limit += last;
if (limit < 0) {
limit = 0;
showNotification("You are over-spending!");
}
transList.add(TransactionTile(trans: last));
break;
}
}
if (!mesFlag) _message = ''; //if not a transaction
});
}
Here's the exception it throws when I receive a SMS in the background :
setState() called in constructor: _TransactionState#f0921(lifecycle state: created, no widget, not mounted)
This happens when you call setState() on a State object for a widget that hasn't been inserted into the widget tree yet. It is not necessary to call setState() in the constructor, since the state is already assumed to be dirty when it is initially created.
The telephony package requires onBackgroundMessage to be a global function. Is there any work around by means of which I can call the onMessage in the global function or setState() cause I need them to be updated even when the app isn't open & it should function exactly like the onMessage fucntion.
Any help appreciated.

Answer 1

From the Firebase Cloud Messaging Flutter documentation on background messages:
Since the handler runs in its own isolate outside your applications context, it is not possible to update application state or execute any UI impacting logic. You can, however, perform logic such as HTTP requests, perform IO operations (e.g. updating local storage), communicate with other plugins etc.
You can execute a logic that affects the UI only when the user decides to click on a notification that contains a message, and the app comes from background to foreground.
If you want't to silently receive some data and update the UI when the app is activated again, you can for example:
save the data in shared preferences which you can access from onBackgroundMessage,
implement a didChangeAppLifecycleState override that checks the data in shared preferences when the app is activated and updated the UI accordingly.