Android ViewModel with ArgsConstructor via Custom ViewModelFactory

Android ViewModel is very useful.
However ViewModel has no args constructor by default.

Typical usage of ViewModel looks like:

public class UserModel extends ViewModel {
} 
final UserModel viewModel = ViewModelProviders.of(this).get(UserModel.class);

Let's look the definition of ViewModelProviders.of method.

    /**
     * Creates a {@link ViewModelProvider}, which retains ViewModels while a scope of given Activity
     * is alive. More detailed explanation is in {@link ViewModel}.
     * 

* It uses the given {@link Factory} to instantiate new ViewModels. * * @param activity an activity, in whose scope ViewModels should be retained * @param factory a {@code Factory} to instantiate new ViewModels * @return a ViewModelProvider instance */ @NonNull @MainThread public static ViewModelProvider of(@NonNull FragmentActivity activity, @Nullable Factory factory) {

Have you found it? We can pass a Factory to create the ViewModel.
Implement your own Factory then you can have args constructor for your ViewModel.

Here we set up a ViewModel with 1 argument construcgtor.
Example code:

    public QASViewModel(@NonNull FragmentActivity activity) {
        this.activity = activity;
    }
    
QASViewModel qasViewModel = ViewModelProviders.of(getActivity(),
                new QASViewModelFactory(getActivity())).get(QASViewModel.class);
import androidx.annotation.NonNull;
import androidx.fragment.app.FragmentActivity;
import androidx.lifecycle.ViewModel;
import androidx.lifecycle.ViewModelProvider;

import java.lang.reflect.InvocationTargetException;

public class QASViewModelFactory extends ViewModelProvider.NewInstanceFactory {

    private final FragmentActivity activity;

    /**
     * Creates a {@code AndroidViewModelFactory}
     *
     * @param activity an AssetManager to pass in {@link QASViewModel}
     */
    public QASViewModelFactory(@NonNull FragmentActivity activity) {
        this.activity = activity;
    }

    @NonNull
    @Override
    public  T create(@NonNull Class modelClass) {
        if (QASViewModel.class.isAssignableFrom(modelClass)) {
            try {
                return modelClass.getConstructor(FragmentActivity.class).newInstance(activity);
            } catch (NoSuchMethodException e) {
                throw new RuntimeException("Cannot create an instance of " + modelClass, e);
            } catch (IllegalAccessException e) {
                throw new RuntimeException("Cannot create an instance of " + modelClass, e);
            } catch (InstantiationException e) {
                throw new RuntimeException("Cannot create an instance of " + modelClass, e);
            } catch (InvocationTargetException e) {
                throw new RuntimeException("Cannot create an instance of " + modelClass, e);
            }
        }
        return super.create(modelClass);
    }
}

LengErrong

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