How to merge one LiveData to another

Question

I am trying, following the principles of UDF, to display/change the state of the screen through the State class. I get data from the database(Room) -> repository -> useCase in the ViewModel and try to map this data there, but it doesn't work. Tell me what's wrong. I tried to make a test variable categoryTest and subscribe to it - everything works, the data comes to the fragment and the list is filled in. From this I conclude that the problem lies somewhere inside MediatorLiveData().apply, but maybe somewhere else.

` class MenuViewModel @Inject constructor( private val loadCategoryUseCase: LoadCategoryUseCase, private val getListCategoryUseCase: GetListCategoryUseCase ) : ViewModel() {

private val _categories = MutableLiveData<StateMenuFragment>()
val categories: LiveData<StateMenuFragment>
    get() = _categories

val categoryTest = getListCategoryUseCase() // Test variable

init {
    viewModelScope.launch {
        loadCategoryUseCase()
    }
    getCategories()
}

private fun getCategories() {
    _categories.value = StateMenuFragment(loading = true)
    viewModelScope.launch {
        try {
            val result = getListCategoryUseCase() 

            MediatorLiveData<StateMenuFragment>().apply {
                addSource(result) { categoryList ->
                    val updatedState = _categories.value?.copy(result = categoryList )
                        ?: StateMenuFragment(result = categoryList )

                    _categories.value = updatedState
                }
            }

        } catch (e: IOException) {
            if (_categories.value == StateMenuFragment(result = emptyList())) {
                _categories.value = StateMenuFragment(
                    error = ErrorMessage(isError = true, errorMessage = "error message")
                )
            }
        }
    }

}

}

I subscribe to them in the fragment. But the list doesn't come

class MenuFragment : Fragment() {

private fun updateState() {
    viewModel.categories.observe(viewLifecycleOwner) {
        binding.progressBar.visibility = View.GONE
        if (it.loading) {
            binding.progressBar.visibility = View.VISIBLE
        }
        if (it.error.isError) {
            Toast.makeText(requireContext(), it.error.errorMessage, Toast.LENGTH_SHORT). apply {
                setGravity(Gravity.CENTER, 0, 0)
            }.show()
        }
        menuAdapter.submitList(it.result)
    }
   // I subscribe directly, I pass the State class - the data is coming
   viewModel.categoryTest.observe(viewLifecycleOwner) {
        menuAdapter.submitList(it)
    }
}
}



data class StateMenuFragment(
var result: List<Category> = emptyList(),
val loading: Boolean = false,
val error: ErrorMessage = ErrorMessage()
)
data class ErrorMessage(
val isError: Boolean = false,
val errorMessage: String = ""
)

data class Category (
val id: Int,
val title: String,
val imageUrl: String
)`