I am following this documentation to learn about LiveData and ViewModel. In the doc, the ViewModel class has constructor as such,
public class UserModel extends ViewModel {
private MutableLiveData<User> user;
@Inject UserModel(MutableLiveData<User> user) {
this.user = user;
}
public void init() {
if (this.user != null) {
return;
}
this.user = new MutableLiveData<>();
}
public MutableLiveData<User> getUser() {
return user;
}
}
However, when I run the code, I get exception:
final UserViewModelviewModel = ViewModelProviders.of(this).get(UserViewModel.class);
Caused by: java.lang.RuntimeException: Cannot create an instance of class UserViewModel Caused by: java.lang.InstantiationException: java.lang.Class has no zero argument constructor
While initializing subclasses of ViewModel
using ViewModelProviders
, by default it expects your UserModel
class to have zero argument constructor.
In your case your constructor has argument MutableLiveData<User> user
One way to fix this is to have a default no arg constructor for your UserModel
Otherwise, if you want to have a non zero argument constructor for your ViewModel class, you may have to create a custom ViewModelFactory
class to initialise your ViewModel instance, which will implement ViewModelProvider.Factory
interface.
I have not tried this yet, but here is the link to excellent sample from google for the same: github.com/googlesamples/android-architecture-components. Specifically, checkout this class GithubViewModelFactory.java for Java code and this class GithubViewModelFactory.kt for corresponding Kotlin code