Why a viewmodel factory is needed in Android?

Question

We have been discussing about this but we don't know the reason of creating a viewmodel factory to create a viewmodel instead of instantiate the viewmodel directly. What is the gain of creating a factory that just creates the viewmodel?

I just put a simple example of how I did it without Factory

here is the kodein module:

val heroesRepositoryModel = Kodein {     bind<HeroesRepository>() with singleton {         HeroesRepository()     }      bind<ApiDataSource>() with singleton {         DataModule.create()     }      bind<MainViewModel>() with provider {         MainViewModel()     } } 

The piece of the Activity where I instantiate the viewmodel without using the factory

class MainActivity : AppCompatActivity() {     private lateinit var heroesAdapter: HeroAdapter     private lateinit var viewModel: MainViewModel     private val heroesList = mutableListOf<Heroes.MapHero>()     private var page = 0     private var progressBarUpdated = false      override fun onCreate(savedInstanceState: Bundle?) {         super.onCreate(savedInstanceState)         setContentView(R.layout.activity_main)         viewModel = ViewModelProviders.of(this)                 .get(MainViewModel::class.java)         initAdapter()         initObserver()         findHeroes()     } 

The ViewModel where I instantiate the usecase directly without having it in the constructor

class MainViewModel : ViewModel(), CoroutineScope {      private val heroesRepository: HeroesRepository = heroesRepositoryModel.instance()     val data = MutableLiveData<List<Heroes.MapHero>>()      private var job: Job = Job()     override val coroutineContext: CoroutineContext         get() = uiContext + job      fun getHeroesFromRepository(page: Int) {         launch {             try {                 val response = heroesRepository.getHeroes(page).await()                 data.value = response.data.results.map { it.convertToMapHero() }             } catch (e: HttpException) {                 data.value = null             } catch (e: Throwable) {                 data.value = null             }         }     }      override fun onCleared() {         super.onCleared()         job.cancel()     } } 

So here a example using factory

class ListFragment : Fragment(), KodeinAware, ContactsAdapter.OnContactListener {      override val kodein by closestKodein()      private lateinit var adapterContacts: ContactsAdapter      private val mainViewModelFactory: MainViewModelFactory by instance()     private val mainViewModel: MainViewModel by lazy {         activity?.run {             ViewModelProviders.of(this, mainViewModelFactory)                 .get(MainViewModel::class.java)         } ?: throw Exception("Invalid Activity")     }      override fun onCreateView(             inflater: LayoutInflater, container: ViewGroup?, savedInstanceState: Bundle?): View? {         return inflater.inflate(R.layout.fragment_list, container, false)     } 

The viewmodelfactory:

class MainViewModelFactory (private val getContacts: GetContacts) : ViewModelProvider.Factory {      override fun <T : ViewModel?> create(modelClass: Class<T>): T {         if (modelClass.isAssignableFrom(MainViewModel::class.java)) {             return MainViewModel(getContacts) as T         }         throw IllegalArgumentException("Unknown ViewModel class")     } } 

And the viewmodel:

class MainViewModel(private val getContacts: GetContacts) : BaseViewModel() {     lateinit var gamesList: LiveData<PagedList<Contact>>     var contactsSelectedData: MutableLiveData<List<Contact>> = MutableLiveData()     var contactsSelected: ArrayList<Contact> = ArrayList()     private val pagedListConfig by lazy {         PagedList.Config.Builder()                 .setEnablePlaceholders(false)                 .setInitialLoadSizeHint(PAGES_CONTACTS_SIZE)                 .setPageSize(PAGES_CONTACTS_SIZE)                 .setPrefetchDistance(PAGES_CONTACTS_SIZE*2)                 .build()     } 

Here is the complete first example:

https://github.com/ibanarriolaIT/Marvel/tree/mvvm

And the complete second example:

https://github.com/AdrianMeizoso/Payment-App

Answer 1

We can not create ViewModel on our own. We need ViewModelProviders utility provided by Android to create ViewModels.

But ViewModelProviders can only instantiate ViewModels with no arg constructor.

So if I have a ViewModel with multiple arguments, then I need to use a Factory that I can pass to ViewModelProviders to use when an instance of MyViewModel is required.

For example -

public class MyViewModel extends ViewModel {     private final MyRepo myrepo;     public MyViewModel(MyRepo myrepo) {          this.myrepo = myrepo;     } } 

To instantiate this ViewModel, I need to have a factory which ViewModelProviders can use to create its instance.

ViewModelProviders Utility can not create instance of a ViewModel with argument constructor because it does not know how and what objects to pass in the constructor.