How to initialize ViewModel with/without arguments

ViewModelInit.kt

// Reference: https://dev.to/vtsen/recommended-ways-to-create-viewmodel-or-androidviewmodel-5e7k

// MyViewModel (no argument)
class MyViewModel: ViewModel() {
    // Do something
}

// MyAndroidViewModel (Application argument)
class MyAndroidViewModel (app: Application)
    : AndroidViewModel(app) {
    // Do something
}

// How to initialize ViewModel without arguments
// 1. local variables
val viewModel: MyViewModel = ViewModelProvider(this).get(MyViewModel::class.java)
val androidViewModel: MyAndroidViewModel = ViewModelProvider(this).get(MyAndroidViewModel::class.java)

// 2. lateinit var with ViewModelProvider
// Activity / Fragment variables
private lateinit var viewModel: MyViewModel
private lateinit var androidViewModel: MyAndroidViewModel

// In Activity-onCreate() callback, Fragment-onCreateView callback method
override fun onCreateView(inflater: LayoutInflater, container: ViewGroup?, savedInstanceState: Bundle?): View {
    viewModel = ViewModelProvider(this).get(MyViewModel::class.java)
    androidViewModel = ViewModelProvider(this).get(MyAndroidViewModel::class.java)
}

// 3. by lazy with ViewModelProvider
// Activity / Fragment variables
private val viewModel: MyViewModel by lazy {
    ViewModelProvider(this).get(MyViewModel::class.java)
}

private val androidViewModel: MyAndroidViewModel by lazy {
    ViewModelProvider(this).get(MyAndroidViewModel::class.java)
}

// 4. by viewModels / activityViewModels (elegant way)
// library dependecy for viewModels() extension function
implementation 'androidx.fragment:fragment-ktx:1.5.5'

private val viewModel: MyViewModel by viewModels()
private val androidViewModel: MyAndroidViewModel by viewModels()

// If you want to share your ViewModel across different fragments within the same activity. 
// You can use by activityViewModels.
private val viewModel: MyViewModel by activityViewModels()
private val androidViewModel: MyAndroidViewModel by activityViewModels()


// MyViewModel (custom constructor parameter)
class MyViewModel(private val repository: Repository) : ViewModel() {
  // Do something
}

// MyViewModeFactory which has custom constructor parameter
class MyViewModelFactory(private val repository: Repository)
    : ViewModelProvider.Factory {
    override fun <T : ViewModel?> create(modelClass: Class<T>): T {
        if (modelClass.isAssignableFrom(MyViewModel::class.java)) {
            return MyViewModel(repository) as T
        }
        throw IllegalArgumentException("Unknown ViewModel class")
    }
}

class MyAndroidViewModel(app: Application, repository: Repository) : AndroidViewModel(app) {
  // Do something
}

// MyAndroidViewModelFactory which have custom constructor parameters
class MyAndroidViewModelFactory(
    private val app: Application,
    private val repository: Repository)
    : ViewModelProvider.Factory {
    override fun <T : ViewModel?> create(modelClass: Class<T>): T {
        if (modelClass.isAssignableFrom(
                MyAndroidViewModel::class.java)) {
            return MyAndroidViewModel(app, repository) as T
        }
        throw IllegalArgumentException("Unknown ViewModel class")
    }
}

// Activity / Fragment variables using by viewModels
private val viewModel: MyViewModel by viewModels {
    MyViewModelFactory(Repository())
}

private val androidViewModel: MyAndroidViewModel by viewModels {
    MyAndroidViewModelFactory(
        requireActivity().application,
        Repository())
}

// Activity / Fragment variables using by activityViewModels
private val viewModel: MyAndroidViewModel by activityViewModels {
    MyViewModelFactory(Repository())
}

private val androidViewModel: MyAndroidViewModel by activityViewModels {
    MyAndroidViewModelFactory(requireActivity().application, Repository())
}

// Activity / Fragment variables using by lazy
private val viewModel: MyViewModel by lazy {
    val factory = MyViewModelFactory(Repository())
    ViewModelProvider(this, factory).get(MyAndroidViewModel::class.java)
}
private val viewModel: MyAndroidViewModel by lazy {
    val factory = MyAndroidViewModelFactory(
        requireActivity().application, 
        Repository())
    ViewModelProvider(this, factory).get(MyAndroidViewModel::class.java)
}