This is how the guides recommend retrieving the ViewModel of an activity for the first time, which normally happens inside the onCreate:

public class FooActivity extends AppCompatActivity {

    @Inject
    ViewModelProvider.Factory viewModelFactory;

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        //...
        
        FooViewModel viewModel = new ViewModelProvider(this, viewModelFactory)
            .get(FooViewModel.class);
    }

}

Now that has to be a costly operation, since we are creating a new instance of ViewModelProvider. It is also somewhat verbose. What if I need the ViewModel again outside the onCreate?

I initially though of caching it in a private variable inside the activity, but can’t that leak the ViewModel? E.g.:

public class FooActivity extends AppCompatActivity {
    FooViewModel viewModel;

    @Inject
    ViewModelProvider.Factory viewModelFactory;

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        //...
        
        viewModel = new ViewModelProvider(this, viewModelFactory)
            .get(FooViewModel.class);
    }

    @Override
    protected void onStop() {
        //...
        
        viewModel.someMethod();
    }

}

Would this approach be safer if I manually set the viewModel variable to null in the onDestroy?

Another alternative approach would be to somehow retrieve the associated ViewModel from the activity. AppCompatActivity has a ViewModelStore, but calling getViewModelStore() returns an object where only a clear method is available.

So what is the recommended way of getting the associated ViewModel for a second time in a fragmentless activity?