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 publicT 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); } }
