Jetpack Compose 和 ViewModel-Instantiation

Question

Can someone explain me this line of code:

fun NotesApp(noteViewModel: NoteViewModel = viewModel()) {

The default parameter confuses me. The class, which inherits from ViewModel, is called NoteViewModel. Wouldn't the default parameter be: NoteViewModel()?

But it works nevertheless. How is that possible?

Answer 1

It is the default initializer for view models inside a Composable. It takes in a type parameter, so I don't think what you've posted above would be enough.

For example, if you wanted to initialise a MainViewModel , it'll be something like

val vm = viewModel<MainViewModel>()

It is supposed to return the same instance of a viewmodel, of it had been created in the past.

Answer 2

It works because the function viewModel() provides that viewmodel. I assume it's from the androidx.lifecycle.viewmodel.compose package.

Returns an existing ViewModel or creates a new one in the given owner (usually, a fragment or an activity), defaulting to the owner provided by LocalViewModelStoreOwner.