How to inject repository into ViewModel using Dagger 2 [duplicate]

Question

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

Answer 1

In my case as I'm using HILT, it was lacking one annotation above the Fragment that has a ViewModel: @AndroidEntryPoint

@AndroidEntryPoint
class BestFragment : Fragment() { 
....

Of course in your ViewModel class you also need to Annotate with what HILT needs: @ViewModelInject

class BestFragmentViewModel @ViewModelInject constructor(var userManager: UserManager) : ViewModel() {
....
}

Answer 2

While initializing subclasses of ViewModel using ViewModelProviders, by default it expects your UserModel class to have a zero argument constructor. In your case your constructor has the 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 implements the ViewModelProvider.Factory interface.

I have not tried this yet, but here's a link to an excellent Google sample for this: github.com/googlesamples/android-architecture-components. Specifically, check out this class GithubViewModelFactory.java for Java code and this class GithubViewModelFactory.kt for the corresponding Kotlin code.

Answer 3

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 {
    //
}

Answer 4

For Hilt:

Simple add @AndroidEntryPoint for main acticity and fragments, and @HiltViewModel for viewModels

Example after:

@HiltViewModel
class SplashViewModel @Inject constructor(

@AndroidEntryPoint
class SplashFragment : Fragment() {
    private lateinit var b: SplashFragmentBinding
    private val vm: SplashViewModel by viewModels()

@AndroidEntryPoint
class MainActivity : AppCompatActivity() {
    private lateinit var binding: ActivityMainBinding

Answer 5

I had some issues with @ViewModelInject since it has been deprecated using HILT. To solve the problem change this code:

class MainViewModel @ViewModelInject constructor(
    val mainRepository: MainRepository
): ViewModel()

with:

@HiltViewModel
class MainViewModel @Inject constructor(
    val mainRepository: MainRepository
): ViewModel()

Of course, remember to add the @AndroidEntryPoint annotation above your fragment or activity (wherever you are instantiating your ViewModel) like this:

@AndroidEntryPoint
class UsageFragment : Fragment(R.layout.fragment_usage) {
   .
   .
   .
}

Ultimate tip:

You can immediately see if HILT is working looking if there are the icons on the left in your ViewModel.

Here it does not work:

Here it does work:

If you don't see them after updating the code click on Build -> Rebuild Project

Answer 6

Early in 2020, Google have deprecated the ViewModelProviders class, in version 2.2.0 of the androidx lifecycle library.

It's no longer necessary to use ViewModelProviders to create an instance of a ViewModel, you can pass your Fragment or Activity instance to the ViewModelProvider constructor instead.

If you use the code like:

val viewModel = ViewModelProviders.of(this).get(CalculatorViewModel::class.java)

you'll get a warning that ViewModelProviders has been deprecated.

You can instead do:

val viewModel = ViewModelProvider(this).get(CalculatorViewModel::class.java)

Or alternatively, to use a delegate, make the following changes.

1. In the build.gradle (Module: app) file, use version 2.2.0 of the lifecycle components: implementation 'androidx.lifecycle:lifecycle-extensions:2.2.0'

   Also add implementation "androidx.activity:activity-ktx:1.1.0"

   If you want to use the ViewModel from a Fragment instead, use

   implementation "androidx.fragment:fragment-ktx:1.2.2"

   fragment-ktx automatically includes activity-ktx, so you don't need to specify both in the dependencies.

2. You need to specify Java 8 in the android section :

android {
  compileSdkVersion 28
  defaultConfig {
    applicationId "com.kgandroid.calculator"
    minSdkVersion 17
    targetSdkVersion 28
    versionCode 1
    versionName "1.0"
    testInstrumentationRunner "androidx.test.runner.AndroidJUnitRunner"
  }
  
  buildTypes {
    release {
      minifyEnabled false
      proguardFiles getDefaultProguardFile('proguard-android.txt'), 'proguard-rules.pro'
    }
  }
         
  kotlinOptions {
    jvmTarget = "1.8"
  }
}

3. In your Fragment or Activity, change the import to:

   import androidx.activity.viewModels

4. The code to create a ViewModel then becomes:

   val viewModel: CalculatorViewModel by viewModels()

   instead of

   val viewModel = ViewModelProviders.of(this).get(CalculatorViewModel::class.java)

   Use the viewModel object as :

   val viewModel: CalculatorViewModel by viewModels()

   viewModel.newNumber.observe(this, Observer { stringResult -> newNumber.setText(stringResult) })

where newNumer is a LiveData object

In a Fragment that you want to share the Activity's ViewModel, you'd use

`val viewModel: CalculatorViewModel by activityViewModels()`

**That's the equivalent of passing the Activity instance in the (deprecated) 
ViewModelProviders.of() function.**

Answer 7

2020-07-29 10:13:25

For lifecycle_version = '2.2.0' ViewProviders.of API is deprecated . It`s my situation :

class MainActivityViewModel(application: Application) : AndroidViewModel(application) {

    private var repository: UserRepository

    val allUsers: LiveData<List<User>>
......


error:
val userViewModel = ViewModelProvider(this).get(MainActivityViewModel::class.java)

success:
val factory = ViewModelProvider.AndroidViewModelFactory.getInstance(application)
userViewModel = ViewModelProvider(this,factory).get(MainActivityViewModel::class.java)

Pass application by api ViewModelProvider.AndroidViewModelFactory.getInstance

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