How should i use ViewModel in two fragments?

Question

I have an app with one activity and two fragments, in the first fragment, I should be able to insert data to the database, in the second I should be able to see the added items in a recyclerView.

So I've made the Database, my RecyclerView Adapter, and the ViewModel,

the issue is now how should I manage all that?

Should I initialize the ViewModel in the activity and call it in some way from the fragment to use the insert?

Should I initialize the viewmodel twice in both fragments?

My code looks like this:

Let's assume i initialize the viewholder in my Activity:

class MainActivity : AppCompatActivity() {
     private val articoliViewModel: ArticoliViewModel by viewModels {
        ArticoliViewModelFactory((application as ArticoliApplication).repository)
    }
}

Then my FirstFragments method where i should add the data to database using the viewModel looks like this:

class FirstFragment : Fragment() {
    private val articoliViewModel: ArticoliViewModel by activityViewModels()
    private fun addArticolo(barcode: String, qta: Int) { // function which add should add items on click
      // here i should be able to do something like this

        articoliViewModel.insert(Articolo(barcode, qta))
    }
}

And my SecondFragment

class SecondFragment : Fragment() {    
    private lateinit var recyclerView: RecyclerView
    private val articoliViewModel: ArticoliViewModel by activityViewModels()
    override fun onViewCreated(view: View, savedInstanceState: Bundle?) {
        super.onViewCreated(view, savedInstanceState)

        recyclerView = view.findViewById(R.id.recyclerView)
        val adapter = ArticoliListAdapter()
        recyclerView.adapter = adapter
        recyclerView.layoutManager = LinearLayoutManager(activity)
        // HERE I SHOULD BE ABLE DO THIS   
        articoliViewModel.allWords.observe(viewLifecycleOwner) { articolo->
            articolo.let { adapter.submitList(it) }
        }

    } 
}

EDIT:

My ViewModel looks like this:

class ArticoliViewModel(private val repository: ArticoliRepository): ViewModel() {
    val articoli: LiveData<List<Articolo>> = repository.articoli.asLiveData()

    fun insert(articolo: Articolo) = viewModelScope.launch {
        repository.insert(articolo)
    }
}

class ArticoliViewModelFactory(private val repository: ArticoliRepository): ViewModelProvider.Factory {
    override fun <T : ViewModel?> create(modelClass: Class<T>): T {
        if (modelClass.isAssignableFrom(ArticoliViewModel::class.java)) {
            @Suppress("UNCHECKED_CAST")
            return ArticoliViewModel(repository) as T
        }
        throw IllegalArgumentException("Unknown ViewModel class")
    }

}

Answer 1

Whether multiple fragments should share a ViewModel depends on whether they are showing the same data. If they show the same data, I think it usually makes sense to share a ViewModel so the data doesn't have to be pulled from the repository when you switch between them, so the transition is faster. If either of them also has significant amount of unique data, you might consider breaking that out into a separate ViewModel so it doesn't take up memory when it doesn't need to.

Assuming you are using a shared ViewModel, you can do it one of at least two different ways, depending on what code style you prefer. There's kind of a minor trade-off between encapsulation and code duplication, although it's not really encapsulated anyway since they are looking at the same instance. So personally, I prefer the second way of doing it.

1. Each ViewModel directly creates the ViewModel. If you use by activityViewModels(), then the ViewModel will be scoped to the Activity, so they will both receive the same instance. But since your ViewModel requires a custom factory, you have to specify it in both Fragments, so there is a little bit of code duplication:

// In each Fragment:
private val articoliViewModel: ArticoliViewModel by activityViewModels {
    ArticoliViewModelFactory((application as ArticoliApplication).repository)
}

2. Specify the ViewModel once in the MainActivity and access it in the Fragments by casting the activity.

// In Activity: The same view model code you already showed in your Activity, but not private

// In Fragments:
private val articoliViewModel: ArticoliViewModel
    get() = (activity as MainActivity).articoliViewModel

Or to avoid code duplication, you can create an extension property for your Fragments so they don't have to have this code duplication:

val Fragment.articoliViewModel: ArticoliViewModel
    get() = (activity as MainActivity).articoliViewModel