ViewModelProviders.get(...) in base class

Question

I'm trying to cut some of Dagger's boilerplate by moving some of my ViewModel instantiation in an abstract base class but can't find quite a good way to do this. My intent is to instantiate all my ViewModels from my base fragment for them to be ready to consume by all child fragments without having them do their own instantiation. My issue lies in retrieving the ViewModel using a generic (VM)- specifically here: .get(viewModel::class.java) . I also attempted .get(VM::class.java) that is not permitted

BaseFragment

abstract class BaseFragment<VM : ViewModel> : Fragment() {

    @Inject lateinit var viewModelFactory: ViewModelProvider.Factory
    lateinit var viewModel : VM

    override fun onAttach(context: Context?) {
        super.onAttach(context)
        viewModel = ViewModelProviders.of(this, viewModelFactory).get(viewModel::class.java)
    }
}

ViewModelProviders.get(...) method signature

public <T extends ViewModel> T get(@NonNull Class<T> modelClass)

Is this even possible?

check out this blog post https://mag.n26.com/reducing-viewmodel-provision-boilerplate-in-googles-githubbrowsersample-549818ee72f0 — LordRaydenMK, Feb 20 '18 at 20:48

score 7 · Answer 1 · answered Aug 31 '18 at 00:47

7

val viewModel: T by lazy {
    ViewModelProviders.of(this).get(getTClass())
}

//获取泛型T的实际类型
private fun getTClass(): Class<T> {
    return (javaClass.genericSuperclass as ParameterizedType).actualTypeArguments[0] as Class<T>
}

answered Aug 31 '18 at 00:47

王大大

71
1
1

score 5 · Accepted Answer · answered Feb 21 '18 at 19:26

If you have a BaseFragment defined like this:

public class BaseFragment<T extends ViewModel> extends Fragment {

    @Inject
    protected T viewModel;

}

You can use the ViewModelUtils class from the Architecture Components's Github browser sample to create ViewModel factories.

/**
 * Creates a one off view model factory for the given view model instance.
 */
public class ViewModelUtil {
    private ViewModelUtil() {}
    public static <T extends ViewModel> ViewModelProvider.Factory createFor(T model) {
        return new ViewModelProvider.Factory() {
            @Override
            public <T extends ViewModel> T create(Class<T> modelClass) {
                if (modelClass.isAssignableFrom(model.getClass())) {
                    return (T) model;
                }
                throw new IllegalArgumentException("unexpected model class " + modelClass);
            }
        };
    }
}

In the onActivityCreated method of the BaseFragment you can add

    @Override
    public void onActivityCreated(@Nullable final Bundle savedInstanceState) {
        super.onActivityCreated(savedInstanceState);
        ViewModelProvider.Factory viewModelFactory = ViewModelUtil.createFor(viewModel);
        ViewModelProviders.of(this, viewModelFactory).get(viewModel.getClass());
    }

The code for this technique is from this blog post.

But this way all the fragments will have the same context as they have extend the same fragment, and the values in the viewmodel will be live forever. Is this right?, I have this problem how to avoid this. — Andrew Hossam, Mar 07 '19 at 22:27
Looks good, but the injected ViewModels are not the same instance after orientation changes, because they are generated every time :( — PSK, Oct 21 '19 at 23:35

score 0 · Answer 3 · answered May 07 '18 at 21:17

I had the same issue but fix it with a bit of Reflection; I really don't like to use Reflection on Android, but didn't have other option.

private void initViewModel() {
    final Type[] types = ((ParameterizedType) this.getClass().getGenericSuperclass()).getActualTypeArguments();
    mViewModel = ViewModelProviders.of(this).get((Class<M>) types[0]);
}

Hope this is useful to someone.

ViewModelProviders.get(...) in base class

3 Answers3

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