Why do I need ViewModelFactory in android?

Question

I am trying to pass a constructor parameter in the ViewModel class. But I don't understand the best approach to do this. Please take a look at the code and comment and let me know the easiest and appropriate way.

class MainActivity : AppCompatActivity() {
    private lateinit var binding: ActivityMainBinding

    // Question 1: How can I pass constructor parameter if I use this delegate feature?
    // private lateint var viewModel: MainViewModel by viewModels()

    private lateinit var viewModel: MainViewModel

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

        binding = ActivityMainBinding.inflate(layoutInflater)
        setContentView(binding.root)

        val repository = Repository()

        // Passing constructor directly works.
        viewModel = MainViewModel(repository)

        // Question 2: Why do I need this factory class / method as it works without these two lines.

//        val viewModelFactory = MainViewModelFactory(repository)
//        viewModel = ViewModelProvider(this, viewModelFactory).get(MainViewModel::class.java)


        viewModel.getPost()
        viewModel.mResponse.observe(this, Observer { response ->
            if (response.isSuccessful) {
                Log.d("Response: ", response.body()?.userId.toString())
                Log.d("Response: ", response.body()?.id.toString())
                binding.textView.text = response.body()?.title.toString()
                Log.d("Response: ", response.body()?.body.toString())
            } else {
                Log.d("Response: ", response.errorBody().toString())
                binding.textView.text = response.code().toString()
            }
        })

    }
}

Answer 1

// Question 1: How can I pass constructor parameter if I use this delegate feature?
// private lateint var viewModel: MainViewModel by viewModels()

You would pass your ViewModelFactory as a parameter to viewModels(). Or, depending on the nature of the parameter, use a dependency inversion framework (Dagger/Hilt, Koin, etc.). In your case, you seem to be passing some sort of repository, in which case a dependency inversion framework is a popular solution.

   // Question 2: Why do I need this factory class / method as it works without these two lines.

Because it does not actually work. In particular, that MainViewModel instance does not survive a configuration change (e.g., screen rotation). If you want to use the Jetpack ViewModel system correctly, the ViewModel system has to instantitate your MainViewModel.