Android ViewModel has no zero argument constructor

问题

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

回答1:

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

回答2:

ViewModelFactory that will provide us a right ViewModel from ViewModelModule

public class ViewModelFactory implements ViewModelProvider.Factory {
    private final Map<Class<? extends ViewModel>, Provider<ViewModel>> viewModels;

    @Inject
    public ViewModelFactory(Map<Class<? extends ViewModel>, Provider<ViewModel>> viewModels) {
        this.viewModels = viewModels;
    }

    @Override
    public <T extends ViewModel> T create(Class<T> modelClass) {
        Provider<ViewModel> viewModelProvider = viewModels.get(modelClass);

        if (viewModelProvider == null) {
            throw new IllegalArgumentException("model class " + modelClass + " not found");
        }

        return (T) viewModelProvider.get();
    }
}

ViewModelModule is responsible for binding all over ViewModel classes into
Map<Class<? extends ViewModel>, Provider<ViewModel>> viewModels

@Module
public abstract class ViewModelModule {

    @Binds
    abstract ViewModelProvider.Factory bindViewModelFactory(ViewModelFactory viewModelFactory); 
    //You are able to declare ViewModelProvider.Factory dependency in another module. For example in ApplicationModule.

    @Binds
    @IntoMap
    @ViewModelKey(UserViewModel.class)
    abstract ViewModel userViewModel(UserViewModel userViewModel);

    //Others ViewModels
}

ViewModelKey is an annotation for using as a key in the Map and looks like

@Target(ElementType.METHOD)
@Retention(RetentionPolicy.RUNTIME)
@MapKey
@interface ViewModelKey {
    Class<? extends ViewModel> value();
}

Now you are able to create ViewModel and satisfy all necessary dependencies from the graph

public class UserViewModel extends ViewModel {
    private UserFacade userFacade;

    @Inject
    public UserViewModel(UserFacade userFacade) { // UserFacade should be defined in one of dagger modules
        this.userFacade = userFacade;
    }
} 

Instantiating ViewModel

public class MainActivity extends AppCompatActivity {

    @Inject
    ViewModelFactory viewModelFactory;
    UserViewModel userViewModel;

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);

        ((App) getApplication()).getAppComponent().inject(this);

        userViewModel = ViewModelProviders.of(this, viewModelFactory).get(UserViewModel.class);

    }
}

And do not forger to add ViewModelModule into modules list

@Singleton
@Component(modules = {ApplicationModule.class, ViewModelModule.class})
public interface ApplicationComponent {
    //
}

回答3:

I wrote a library that should make achieving this more straightforward and way cleaner, no multibindings or factory boilerplate needed, while also giving the ability to further parametrise the ViewModel at runtime: https://github.com/radutopor/ViewModelFactory

@ViewModelFactory
class UserViewModel(@Provided repository: Repository, userId: Int) : ViewModel() {

    val greeting = MutableLiveData<String>()

    init {
        val user = repository.getUser(userId)
        greeting.value = "Hello, $user.name"
    }    
}

In the view:

class UserActivity : AppCompatActivity() {
    @Inject
    lateinit var userViewModelFactory2: UserViewModelFactory2

    override fun onCreate(savedInstanceState: Bundle?) {
        super.onCreate(savedInstanceState)
        setContentView(R.layout.activity_user)
        appComponent.inject(this)

        val userId = intent.getIntExtra("USER_ID", -1)
        val viewModel = ViewModelProviders.of(this, userViewModelFactory2.create(userId))
            .get(UserViewModel::class.java)

        viewModel.greeting.observe(this, Observer { greetingText ->
            greetingTextView.text = greetingText
        })
    }
}

回答4:

The problem can be resolved by extending UserModel from AndroidViewModel which is application context aware ViewModel and requires Application parameter-only constructor. (documentation)

Ex- (in kotlin)

class MyVm(application: Application) : AndroidViewModel(application)

This works for version 2.0.0-alpha1.

回答5:

If you have parameter in constructor then :

DAGGER 2 public constructor for @inject dependency

@Inject
public UserViewModel(UserFacade userFacade)
{ 
    this.userFacade = userFacade;
}

Otherwise dagger 2 will send you error "can not instantiate viewmodel object"

来源：https://stackoverflow.com/questions/44194579/android-viewmodel-has-no-zero-argument-constructor