How to finish activity from ViewModel using Android's architecture components?

Question

I try to figure out how to in best way finish an Activity from ViewModel. I found the one way to do this using LiveData object and emitting "signal".

I have a doubt that this solution has a overhead. So is it right solution or I should use more accurate?

So go to example: let's suppose that in an app is an activity MainActivity and view model like below:

 class MainActivity : AppCompatActivity() {
    override fun onCreate(savedInstanceState: Bundle?) {
        super.onCreate(savedInstanceState)

        val model = ViewModelProviders.of(this).get(MainViewModel::class.java)

        model.shouldCloseLiveData.observe(this, Observer { finish() })

    }
 }

and as a companion to the MainActivity is a MainViewModel like below:

class MainViewModel(app: Application) : AndroidViewModel(app) {

  val shouldCloseLiveData = MutableLiveData<Void>()

  fun someAction(){
    shouldCloseLiveData.postValue(null)
  }

}

Answer 1

I share your feelings that this solution does not look tidy for two reasons. First using a MutableLiveData object to signal an event is a workaround. No data is changed. Second exposing LiveData to the outside of the view model violates the principle of encapsulation in general.

I am still surprised about this ugly concept of android. They should provide an option to observe the view model instead of it's internal LiveData objects.

I experimented with WeakReferences to implement the observer pattern. This was unstable. In an unpredictable manner the referent of the WeakReference was lost (null), in wich cases it was not possible to call finish(). This was surprising as I don't think the activity is garbage collected while running.

So this is an partly answer by exclusion. The observer pattern implemented as WeakReference seems to be no alternative to your suggestion.

A wonder if it is legitimate implement the observer pattern by hard references, if I remove the references during onStop(), or onDestroy(). I asked this question here.