ViewModelProvider 构造函数有什么区别

Question

I am learning about ViewModel and I was wondering is there any difference between theses methods to get a ViewModelProvider instance?

method A:

ViewModelProvider viewModelProvider = new ViewModelProvider(getViewModelStore(),
                ViewModelProvider.AndroidViewModelFactory.getInstance(getApplication()));
mViewModel = viewModelProvider.get(NoteActivityViewModel.class);

method B:

mViewModel = new ViewModelProvider(this).get(NoteActivityViewModel.class);

Answer 1

As per the ViewModelProvider(ViewmodelStoreOwner) documentation :

This method will use the default factory if the owner implements HasDefaultViewModelProviderFactory . Otherwise, a ViewModelProvider.NewInstanceFactory will be used.

And as per the Lifecycle 2.2.0 release notes (which was when that constructor was added):

You can pass a Fragment or FragmentActivity to the new ViewModelProvider(ViewModelStoreOwner) constructor to achieve the same functionality when using Fragment 1.2.0 .

And the Fragment 1.2.0 release notes state that:

SavedStateViewModelFactory is now the default factory used when using by viewModels() , by activityViewModels() , the ViewModelProvider constructor, or ViewModelProviders.of() with a Fragment.

So new ViewModelProvider(this) uses SavedStateViewModelFactory , which provides everything that AndroidViewModelFactory provides in addition to support for the Saved State module for ViewModel .

There would be no reason to use the ViewModelProvider with one of the standard factories when using Fragment 1.2.0 or higher - that constructor would only be useful if you have your own custom factory.