How to change live data(String) in kotlin? You are using a viewmodel

I want to change the live data of the view model and output it to the view. I set the string as the initial value, and after that, I try to change the string by executing a function in the view model, but it does not change. Is there something wrong??

ViewModel.kt

class MainViewModel: ViewModel() {
    private var _title_1 = MutableLiveData<String>()

init {
    _title_1.value = "PISO NOBLE"
    }


val title_1: MutableLiveData<String>
    get() = _title_1

fun changeCat () {
    _title_1.value = "PISO NOBRE" }

fragement_page.xml

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<TextView
                    android:id="@+id/pageOneTitle_1"
                    android:layout_width="match_parent"
                    android:layout_height="wrap_content"
                    android:textColor="@color/white"
                    android:textSize="24dp" />


<com.google.android.material.button.MaterialButton
            android:id="@+id/langCat"
            android:layout_width="160dp"
            android:layout_height="match_parent"
            style="@style/Widget.MaterialComponents.ExtendedFloatingActionButton"
            app:cornerRadius="28dp"
            android:onClick="@{()->mainViewModel.changeCat()}"
            android:text="CAT"
            android:textSize="24dp" />

PageFragement.kt

class PageOne : Fragment() {
private var _binding: FragmentPageOneBinding? = null
private val binding get() = _binding!!
private val mainViewModel: MainViewModel by viewModels()


override fun onCreate(savedInstanceState: Bundle?) {
    super.onCreate(savedInstanceState)
}

override fun onCreateView(
    inflater: LayoutInflater, container: ViewGroup?,
    savedInstanceState: Bundle?
): View? {
    _binding = FragmentPageOneBinding.inflate(inflater, container, false)

    mainViewModel.title_1.observe(viewLifecycleOwner, Observer {
        binding.pageOneTitle1.text = it.toString()
    })        

    val view = binding.root
    val vFlipper: ViewFlipper = binding.viewFlipper as ViewFlipper

    return view
}

>Solution :

Is there something wrong??

My first guess is that you didn’t set the viewmodel on the binding. You have:

app:cornerRadius="28dp"
android:onClick="@{()->mainViewModel.changeCat()}"
android:text="CAT"

Which implies you have a variable in the layout named mainViewModel. So you should set that on the binding. And since you’re using LiveData you should set the LifeCycleOwner as well.

_binding = FragmentPageOneBinding.inflate(inflater, container, false)

// v--- MISSING THIS -------

_binding.setMainViewModel(mainViewModel) // <- Set the variable
_binding.setLifeCycleOwner(this) // <- Also need this to observe LiveData

// ^--- MISSING THIS -------

mainViewModel.title_1.observe(viewLifecycleOwner, Observer {
    binding.pageOneTitle1.text = it.toString()
})        

val view = binding.root
val vFlipper: ViewFlipper = binding.viewFlipper as ViewFlipper

return view

This is all covered in the documentation for databinding.