Flutter background service: onStart method must be a top-level or static function

Question

E/flutter (28533): [ERROR:flutter/runtime/dart_vm_initializer.cc(41)] Unhandled Exception: onStart method must be a top-level or static function E/flutter (28533): #0 FlutterBackgroundServiceAndroid.configure (package:flutter_background_service_android/flutter_background_service_android.dart:60:7) E/flutter (28533): #1 FlutterBackgroundService.configure (package:flutter_background_service/flutter_background_service.dart:20:17) E/flutter (28533): #2 initializeService (package:park_space/tow_home/tow_home_screen.dart:68:17) E/flutter (28533): #3 _TowHomeScreenState.getData.. (package:park_space/tow_home/tow_home_screen.dart:126:15) E/flutter (28533):

Here's my code:

Future<void> initializeService() async {
  print('sulod');
  final service = FlutterBackgroundService();

  @pragma('vm:entry-point')
  void onStart(ServiceInstance serviceInstance) async {
    DartPluginRegistrant.ensureInitialized();
    if (serviceInstance is AndroidServiceInstance) {
      serviceInstance.on('setAsForeground').listen((event) {
        serviceInstance.setAsForegroundService();
      });

      serviceInstance.on('setAsBackground').listen((event) {
        serviceInstance.setAsBackgroundService();
      });
    }

    serviceInstance.on('stopService').listen((event) {
      serviceInstance.stopSelf();
    });

    if (serviceInstance is AndroidServiceInstance) {
      serviceInstance.setForegroundNotificationInfo(
        title: "Background Service",
        content: "Park Space background service.",
      );
    }
  }

  await service.configure(
    androidConfiguration: AndroidConfiguration(
      onStart: onStart,
      autoStart: true,
      isForegroundMode: true,
    ),
    iosConfiguration: IosConfiguration(
      autoStart: true,
      onForeground: onStart,
      onBackground: (ServiceInstance serviceInstance) {
        return true;
      },
    ),

  );
  

  service.startService();
}

I formatted the code like this because I want to pass a callBack function to initializeService function. But I'm getting this error. Is there a way to pass a callBack function into onStart from 'service.configure'? Any suggestions will greatly be appreciated, thank you.

Answer 1

When using plugins that require a callback to be registered for background execution, you must define the callback as a static method in one of the two ways:

// Top-level static methods
void onStart(ServiceInstance serviceInstance) {
  // Your code here
}

bool onBackground(ServiceInstance serviceInstance) {
  return true;
}

class Foo {
  // Static methods in a class
  static void onStart(ServiceInstance serviceInstance) {
    // Your code here
  }

  static bool onBackground(ServiceInstance serviceInstance) {
    return true;
  }
}

Future<void> initializeService() async {
  final service = FlutterBackgroundService();
  await service.configure(
    androidConfiguration: AndroidConfiguration(
      onStart: onStart,
      autoStart: true,
      isForegroundMode: true,
    ),
    iosConfiguration: IosConfiguration(
      autoStart: true,
      onForeground: onStart,
      onBackground: onBackground,
    ),
  );
  service.startService();
}

This is because the Flutter engine needs to be able to look up the callback by name, which isn't possible for callbacks associated with a specific instance of an object or non-static context (e.g., functions declared within other functions). I go into this in more detail in the Medium article on Flutter background execution, which I wrote when I implemented background execution support in Flutter a few years back.