Fun with flows, getting null when converting to live data

Issue

I am trying out flows and trying to see how they can be converted to mvvm with android view models. Here is what I tried first to test it out :

class HomeViewModel : ViewModel() {

    private lateinit var glucoseFlow: LiveData<Int>
    var _glucoseFlow = MutableLiveData<Int>()


    fun getGlucoseFlow() {
        glucoseFlow = flowOf(1,2).asLiveData()
        _glucoseFlow.value = glucoseFlow.value
    }
}


class HomeFragment : Fragment() {

    private lateinit var viewModel: HomeViewModel

    override fun onCreateView (
        inflater: LayoutInflater, container: ViewGroup?,
        savedInstanceState: Bundle?
    ): View? {
        return inflater.inflate(R.layout.home_fragment, container, false)
    }

    override fun onActivityCreated(savedInstanceState: Bundle?) {
        super.onActivityCreated(savedInstanceState)
        viewModel = ViewModelProviders.of(this).get(HomeViewModel::class.java)

        viewModel._glucoseFlow.observe(this, Observer {
            handleUpdate(it)
        })

        viewModel.getGlucoseFlow()
    }

    private fun handleUpdate(reading : Int) {
        glucose_reading.text = reading.toString()
    }
}

I get a null for the reading number however any ideas ?

Solution

This happens because you are trying to assign glucoseFlow.value to _glucoseFlow.value directly, I guess you should use a MediatorLiveData<Int>, however this is not my final suggestion.

You can solve it if you collect flow items and then assign them to your private variable.

// For private variables, prefer use underscore prefix, as well MutableLiveData for assignable values.
private val _glucoseFlow = MutableLiveData<Int>()
// For public variables, prefer use LiveData just to read values.
val glucoseFlow: LiveData<Int> get() = _glucoseFlow 

fun getGlucoseFlow() {
    viewModelScope.launch {
        flowOf(1, 2)
            .collect {
                _glucoseFlow.value = it
            }
    }
}

Having the before implementation over the HomeViewModel, start to observe your public glucoseFlow from HomeFragment and you will be able to receive non-null sequence values (1 and then 2).

Answered By - Julio Lemus