How to read data and listen to changes from Room database in a Service?

Question

i am storing user information in a local room database. In activities and fragments I use AndroidViewModel and LiveData to listen to changes made to the database and update the UI.

Now I want to analyze all of the past user data to give recommendations for future decisions. My recommendations change on every change to the database made by the user so I need to update my reommendations frequently while doing the same calculations over and over again.

I was thinking about starting a service on app start that listens to database changes via ViewModel and LiveData and updates my recommendations (which are also stored in the same database). But somehow a Service cannot

1. get a ViewModel via viewModel = ViewModelProviders.of(this).get(DataViewModel.class);
2. observe a LiveData object as it is not a LifecycleOwner.

Basically I simply need to read all entries from the database, analyze the data and update 5-10 values every time the database content changes.

How and where should I do my calculations if not in a service? Maybe I am trapped in a wrong thought and a service is not the right way to do it so any idea on how to do this is very much appreciated!

Answer 1

I ended up using a Service and solved my problem as follows:

In the onCreate method of my Application object I bind MyService to it:

serviceConnection = new ServiceConnection() {
        @Override
        public void onServiceConnected(ComponentName name, IBinder iBinder) {
            service = ((MyService.MyLocalBinder) iBinder ).getService();
        }
        @Override
        public void onServiceDisconnected(ComponentName name) {
        }
    };
Intent intent = new Intent(this, MyService.class);
getApplicationContext().bindService(intent, serviceConnection, BIND_AUTO_CREATE);

Binding a Service to the application context should keep the Service alive as long as the application is not destroyed. In MyService I get an instance of ViewModel via AndroidViewModelFactory like this

MyViewModel myViewModel = ViewModelProvider.AndroidViewModelFactory.getInstance(getApplication()).create(MyViewModel.class);

and I am able to observe the fetched LiveData from the ViewModel via observeForever like this

Observer<List<Entry>> obsEntries = new Observer<List<Entry>>() {
        @Override
        public void onChanged(@Nullable List<Entry> entries) {
            //perform calculations with entries in here
        }
    };
    viewModel.getEntries().observeForever(obsEntries);

Important: Remove the observer from the LiveData reference in onDestroy of the Service (that is why I keep a local reference to the Observer object):

@Override
public void onDestroy() {
    super.onDestroy();
    viewModel.getEntries().removeObserver(obsEntries);
}

Thanks everybody!