Android ViewModel has no zero argument constructor
AndroidMvvmAndroid Architecture-ComponentsAndroid Problem Overview
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
Android Solutions
Solution 1 - Android
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() {
....
}
Solution 2 - Android
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.
Solution 3 - Android
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 {
//
}
Solution 4 - Android
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
Solution 5 - Android
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
Solution 6 - Android
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.
-
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.
-
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"
}
}
-
In your Fragment or Activity, change the import to:
import androidx.activity.viewModels
-
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
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.**
Solution 7 - Android
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
Solution 8 - Android
I had the same error. I'm using Hilt, and in my case it was a missing second hilt compiler dependency
now i have both:
kapt com.google.dagger:hilt-android-compiler:#version
and
kapt androidx.hilt:hilt-compiler:#version
in my app level build.gradle file and it works.
com.google.dagger:hilt-android-compiler is needed when your using the Hilt Android Gradle plugin (see docs) and androidx.hilt:hilt-compiler:#version is apparently needed when you want Hilt and Jetpack integration, like injecting Android Jetpack ViewModel (see docs)
Solution 9 - Android
if you're using hilt, you probably might forgot to annotate your activity or fragment with @AndroidEntryPoint
Solution 10 - Android
The most common reason for this failure is Missing @AndroidEntryPoint at the start of your Fragment/Activity as shown below:
@AndroidEntryPoint
class MyFragment : Fragment {
val viewModel by viewModels<MyViewModel>()
}
Similarly, you ViewModel should be annotated by HiltViewModel as shown following:
@HiltViewModel
class MyViewModel@Inject constructor(
private val var1: Type1
) : ViewModel()
Solution 11 - Android
For everyone who has this problem, I encountered it this way:
1- In Your ViewModel, don't create a constructor, just create a function that takes a Context and your other parameters
public class UserModel extends ViewModel {
private Context context;
private ..........;
public void init(Context context, ......) {
this.context = context;
this..... = ....;
}
// Your stuff
}
2- In your activity:
UserViewModel viewModel = ViewModelProviders.of(this).get(UserViewModel.class);
viewModel.init(this, .....);
// You can use viewModel as you want
3- In your fragment
UserViewModel viewModel = ViewModelProviders.of(getActivity()).get(UserViewModel.class);
viewModel.init(getContext(), .....);
// You can use viewModel as you want
Solution 12 - Android
For Koin:
I had this issue, turns out I just imported viewModels() from AndroidX instead of viewModel() from Koin
Solution 13 - Android
If you are using navigation-compose and calling your screen inside the NavHost block, the hilt can't inject the view model. For this, you can use this way;
NavHost(navHostController, startDestination = "HomeScreen") {
composable("HomeScreen") {
HomeScreen(homeScreenViewModel = hiltViewModel())
}
}
Don't forget to add this dependency for hiltViewModel()
-> implementation("androidx.hilt:hilt-navigation-compose:1.0.0-alpha02")
Solution 14 - Android
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
})
}
}
Solution 15 - Android
i had the same issue, fixed it by adding navigation ui library to my project:
implementation 'androidx.navigation:navigation-ui-ktx:2.2.2'
Solution 16 - Android
Create a constructor with out any arguments i.e.
Default/ No-arg constructor
in the viewmodel class .
In my case, I forgot to generate this constructor and wasted 30 minutes when I'm learning - after that it worked for me.
Solution 17 - Android
You can pass data in viewmodel through viewmodel factory. You can also check out this example for reference.
class UserViewModelFactory(private val context: Context) : ViewModelProvider.NewInstanceFactory() {
override fun <T : ViewModel?> create(modelClass: Class<T>): T {
return UserViewModel(context) as T
}
}
class UserViewModel(private val context: Context) : ViewModel() {
private var listData = MutableLiveData<ArrayList<User>>()
init{
val userRepository : UserRepository by lazy {
UserRepository
}
if(context.isInternetAvailable()) {
listData = userRepository.getMutableLiveData(context)
}
}
fun getData() : MutableLiveData<ArrayList<User>>{
return listData
}
}
You can call viewmodel in activity as below.
val userViewModel = ViewModelProviders.of(this,UserViewModelFactory(this)).get(UserViewModel::class.java)
Solution 18 - Android
With regards to the accepted answer, if you are using Hilt and you just added your ViewModel, don't forget to rebuild your project. Simply running the project does not create the needed factory classes (which are supposed to be automatically generated), as discovered the hard way.
The classes below did not exist before the rebuild:
Solution 19 - Android
If you are using dagger hilt and version 2.31 or higher then don't use "ViewModelInject" in view model class. Dagger is providing new way to use viewmodel so please follow below instruction.
1: Add @HiltViewModel on top of class 2: Use Inject intead of ViewModelInject
@HiltViewModel
class AuthViewModel @Inject constructor(
private val authRepository: AuthRepository,
...
) : ViewModel()
{...}
Solution 20 - Android
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
.
Solution 21 - Android
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"