Can anyone check if I'm doing something wrong with...

# android-architecture

[Can Korkmaz]

Can anyone check if I'm doing something wrong with instantiating a viewmodel? here is the code:

@Preview
@Composable
fun LoginScreenPreview(){
    val navController = rememberNavController()
    val main: MainActivity = MainActivity()
    val myfactory = MainViewModelFactory(Database.getInstance(main.applicationContext).Dao, main.application)
    val viewModel: MainViewModel = viewModel(
        factory =myfactory
    )
    //TextField(value ="xxx", onValueChange ={} )
    LoginScreen(navController,  viewmodel = viewModel)
}

and viewmodel factory:

class MainViewModelFactory(
    private val dataSource: Dao,
    private val application: Application
): ViewModelProvider.Factory{
    @Suppress("unchecked_cast")
    override fun <T : ViewModel> create(modelClass: Class<T>): T {
        if (modelClass.isAssignableFrom(MainViewModel::class.java)) {
            return MainViewModel(dataSource, application) as T
        }
        throw IllegalArgumentException("Unknown ViewModel class")
    }
}

The login screen only has basic fields such as textfields and a button, but uses data from viewmodel. Can't preview anything as of this state.

[Francesc]

You can't instantiate an Activity by calling its constructor, you should not pass a navigation controller to your composables (pass a lambda), and you should be providing fakes for the dependencies (viewmodel)

[Can Korkmaz]

Thank you for answering. Why should I not pass the navController, isn't it more convenient this way? I'll learn about the last part, creating fakes for the dependencies.

[Francesc]

It makes your previews and tests that much harder, it's best practice to pass a lambda, check this https://developer.android.com/jetpack/compose/navigation#testing

[Can Korkmaz]

So all of a sudden clean architecture makes sense, I've been struggling (failing) to get my viewmodels with repo or dao arguments to work in preview. Probably unrelated.

[Francesc]

I have a sample project here that you may find useful for reference https://github.com/fvilarino/Weather-Sample/tree/feature/compose