subscribe static method
Implementation
static Future<void> subscribe(
{String? notificationIcon, Color? backgroundIcon}) async {
FirebaseMessaging.onBackgroundMessage(_firebaseMessagingBackgroundHandler);
await _requestPermission();
final fcmToken = await _firebaseMessaging.getToken();
await InngageService.registerSubscriber(fcmToken);
await receiveNotificationForeground(backgroundIcon);
await receiveNotificationClosed();
await _config(notificationIcon);
}