Why LiveData not updating? (Android, Kotlin)

Question

How to expose MutableLiveData as LiveData?.

Code below not working.

SimpleMixerFragment.kt

private lateinit var binding: MixerSimpleFragmentBinding
private lateinit var viewModel: SimpleMixerViewModel

override fun onCreateView(
    inflater: LayoutInflater, container: ViewGroup?,
    savedInstanceState: Bundle?
): View? {

    // Inflate view and obtain an instance of the binding class
    binding = DataBindingUtil.inflate(
        inflater,
        R.layout.mixer_simple_fragment,
        container,
        false
    )

    viewModel = ViewModelProvider(this).get(SimpleMixerViewModel::class.java)

    binding.simpleMixerViewModel = viewModel

    binding.lifecycleOwner = viewLifecycleOwner

    return binding.root
}

SimpleMixerViewModel.kt

class SimpleMixerViewModel : ViewModel() {

   private val _etDate = MutableLiveData<String>()
   val etDate: LiveData<String>
       get() = _etDate
}

mixer_simple_fragment.kt

<variable name="simpleMixerViewModel" type="....SimpleMixerViewModel" />

<com.google.android.material.textfield.TextInputEditText
                                android:id="@+id/etDate"                
                                android:text="@{simpleMixerViewModel.etDate}"/>

This works instead.

SimpleMixerViewModel.kt

class SimpleMixerViewModel : ViewModel() {

   val etDate = MutableLiveData<String>()
}

mixer_simple_fragment.xml

<variable name="simpleMixerViewModel" type="....SimpleMixerViewModel" />

<com.google.android.material.textfield.TextInputEditText
                                android:id="@+id/etDate"                
                                android:text="@={simpleMixerViewModel.etDate}"/>

Answer 1

@{} registers a one-way binding, meaning if you modify MutableLiveData from code, then it would propagate the changes to the EditText .

@={} registers a two-way binding, meaning if you modify either the MutableLiveData OR the android:text property, then it will update MutableLiveData -> android:text OR android:text -> MutableLiveData (whichever was modified).

To use two-way binding, you must expose the mutable version of LiveData so that the binding can write the new value of the EditText into the MutableLiveData.