Cannot create an instance of class ViewModel

I am trying to write a sample app using Android architecture components and but even after trying for days I could not get it to work. It gives me the above exception.

Lifecycle owner:-

public class MainActivity extends LifecycleActivity {


public void onCreate(Bundle savedInstanceState)
{
super.onCreate(savedInstanceState);
setContentView(R.layout.activity_main);
TextView textView = findViewById(R.id.tv_user);
PostViewModel viewModel = ViewModelProviders.of(this).get(PostViewModel.class);
viewModel.loadPosts();
viewModel.getPost().observe(this, new Observer<Post>() {
@Override
public void onChanged(@Nullable Post post) {
if(post != null) {
textView.setText(post.toString());
}
}
});
}
}

ViewModel:-

public class PostViewModel extends ViewModel {
private MediatorLiveData<Post> post;
private PostRepository postRepo;


PostViewModel() {
post = new MediatorLiveData<>();
postRepo = new PostRepository();
}


public LiveData<Post> loadPosts() {
post.addSource(postRepo.getPost(),
post -> this.post.setValue(post)
);
return post;
}


@NonNull
public LiveData<Post> getPost() {
return post;
}
}
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最佳答案

Make your constructor public.

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It was not completely obvious to me, but when getting this error I resolved it by creating a public constructor. My constructor was derived from the Android Developer examples and contained the Repository as a parameter. Creating an additional constructor that was empty with no params and having it public solved the issue.

i.e., in your case

public PostViewModel() {}

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Make the class and constructor public it solved my problem .

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Make sure your ViewModel has constructor with only one parameter i.e. Application.

example:

public YourViewModel(Application application) {
super(application);
...
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If you are using Kotlin make sure to replace any annotationProcessor in build.gradle with kapt.

Like:

annotationProcessor "android.arch.persistence.room:compiler:$rootProject.roomVersion"

Will become

kapt "android.arch.persistence.room:compiler:$rootProject.roomVersion"

and add

apply plugin: 'kotlin-kapt' on top of the buidl.gradle file.

Annotation Processing with Kotlin

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There are few reason to throw the exception . I have mention some of them..

  1. Make sure your view Model class is public

  2. Make sure your view model class constructor is public

  3. Make sure you have added the dependency in your gradle file for lifecycle also if you use room and other libraries you have added ..

  4. if you create object any other dependent class in your view model class constructor . Other class can throw error to create the instance of viewModel

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  1. Mostly, Solution is making Class and Constructor Public as the other answers
  2. It may also be a runtime error, check the Logcat Error Logs if there are multiple causes listed.
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I got this after migrating to AndroidX.

There's a bug in androidx.lifecycle:lifecycle-viewmodel:2.0.0-beta01 where Proguard removes the constructor.

https://issuetracker.google.com/issues/112230489

Fix by upgrading to 2.0.0, and remember to update your proguard rules if needed.

My error message looked like this:

java.lang.RuntimeException: Cannot create an instance of class my.custom.viewmodel.CustomViewModel
at androidx.lifecycle.ViewModelProvider$AndroidViewModelFactory.create(ViewModelProvider.java:202)
at androidx.lifecycle.ViewModelProvider.get(ViewModelProvider.java:135)
at androidx.lifecycle.ViewModelProvider.get(ViewModelProvider.java:103)
......
Caused by: java.lang.NoSuchMethodException: <init> [class android.app.Application]
at java.lang.Class.getConstructor0(Class.java:2204)
at java.lang.Class.getConstructor(Class.java:1683)
at androidx.lifecycle.ViewModelProvider$AndroidViewModelFactory.create(ViewModelProvider.java:200)
... 34 more




androidx.test.espresso.PerformException: Error performing 'single click - At Coordinates: 539, 1167 and precision: 16, 16' on view 'with id: my.test:id/button_return_to_main_menu'.
at androidx.test.espresso.PerformException$Builder.build(PerformException.java:82)
at androidx.test.espresso.base.DefaultFailureHandler.getUserFriendlyError(DefaultFailureHandler.java:79)
.....
Caused by: java.lang.RuntimeException: Unable to start activity ComponentInfo{my.custom.domain.MainActivity}: java.lang.RuntimeException: Cannot create an instance of class my.custom.viewmodel.CustomViewModel
at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2646)
at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2707)
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Extend AndroidViewModel from your ViewModel class.

public class YourViewModel extends AndroidViewModel {


public YourViewModel(Application application) {
super(application);


//Todo: ...
}


}
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If you used viewmodel inside your activity check that your activity extends "DaggerAppCompatActivity" or not

For instance

public class UserComments extends AppCompatActivity

change this to

public class UserComments extends DaggerAppCompatActivity
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I'm using this example android-arcuitecture-component BasicSample to make a new project, facing a similar error log, found I did'n change de applicatio name

AndroidManifest.xml

and that was my error, to fix put the aplicacion name to de BasicApp, this class is implement in the example.

...
<application
android:name=".BasicApp"
android:allowBackup="false"
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if your PostViewModel class is an inner class, make sure its public and static

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In my case I needed to use a ListItemViewModelFactory to pass in a parameter to my view model.

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In my case, the reason was that I was trying to get a shared instance of the ViewModel in my fragment too soon - before the activity was created. What happens when the application is restoring its state after being killed.

Preconditions:

  1. My ViewModel has a public constructor.
  2. My ViewModel has multiple arguments. But this is absolutely fine as I use ViewModelFactory to construct the ViewModel.
  3. My Fragment and Activity shares the same instance of the ViewModel. In other words: Activity creates the ViewModel and the fragment receives the same instance later.

Code in activity:

override fun onCreate(savedInstanceState: Bundle?) {
super.onCreate(savedInstanceState)


//factory is constructed using Dagger
val factory = App.get().components().appComponent.getMapViewModelFactory()
//activity creates the instance of MapViewModel
viewModel = ViewModelProviders.of(this, factory)[MapViewModel::class.java]
}

Code in fragment:

override fun onCreate(savedInstanceState: Bundle?) {
super.onCreate(savedInstanceState)


//fragment receives the instance of MapViewModel
viewModel = ViewModelProviders.of(activity!!)[MapViewModel::class.java]
...
}

When I open the app for the first time, everything works fine: activity creates an instance of ViewModel; I open Fragment, which gets the instance of ViewModel. But when the application is trying to restore its state after being killed, first it calls the body of onCreate of the Fragment and then the body of onCreate of the Activity. At that point, the fragment can't get the ViewModel as Activity had not created it yet.

Solution 1: Move the code when the fragment gets the ViewModel from onCreate to onViewCreated. This is fine as I observe all live data in onViewCreated as well.

override fun onViewCreated(view: View, savedInstanceState: Bundle?) {
super.onViewCreated(view, savedInstanceState)


viewModel = activity?.run { ViewModelProviders.of(this)[MapViewModel::class.java] } ?: throw Exception("Invalid Activity")


viewModel.getSurveyDateLiveData().observe(viewLifecycleOwner, Observer<String> { dateTextView.text = it })
...
}

Solution 2: Create the instance of ViewModel in Activity.onCreate before super.onCreate is called. In this case, you can get the ViewModel in your fragment's onCreate.

override fun onCreate(savedInstanceState: Bundle?) {
    

val factory = App.get().components().appComponent.getMapViewModelFactory()
viewModel = ViewModelProviders.of(this, factory)[MapViewModel::class.java]
    

super.onCreate(savedInstanceState)
Timber.d("cc: onCreate: $this ")
}

Solution 3:

If you are injecting repository instance in your ViewModel, Check that you are not using @Inject constructor(...): ViewModel() to inject your repository, but rather **@ViewModelInject constructor(...): ViewModel()**

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I had this problem following google's roomdb CodeLab. Solution was changing the following.

Edited

Add the following Build dependencies to Gradle file (as of 2/22/2020)

implementation 'androidx.fragment:fragment:1.2.2'
implementation 'androidx.lifecycle:lifecycle-process:2.2.0'
implementation 'androidx.lifecycle:lifecycle-service:2.2.0'
implementation 'androidx.lifecycle:lifecycle-viewmodel-savedstate:2.2.0'
annotationProcessor 'androidx.lifecycle:lifecycle-compiler:2.2.0'

Imports within the fragment

import androidx.lifecycle.ViewModelProvider;
import androidx.fragment.app.Fragment;
import androidx.lifecycle.Observer;

Creating the viewModel. Add one of the following methods.

Note: I'v seen this done many ways. I believe the correct way is using getDefaultViewModelProviderFactory(). But I have been using requireActivity().

 new ViewModelProvider(requireActivity(),getDefaultViewModelProviderFactory()).get(YourViewModel.class);

|

 new ViewModelProvider(requireActivity()).get(YourViewModel.class);

ViewModelProvider Docs

Deprecated

implementation 'androidx.lifecycle:lifecycle-extensions:2.2.0-rc01'
annotationProcessor 'androidx.lifecycle:lifecycle-compiler:2.2.0-rc01'
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My problem was that the IDE had added a "abstract" modifier to my ViewModel class.

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In my case, it was gradle a dependencies problem.

If you are using Livedata,,

build.gradle(Module.app)

not

implementation 'android.arch.lifecycle:extensions:1.1.1'
kapt 'android.arch.lifecycle:common-java8:1.1.1'

use these

implementation 'androidx.lifecycle:lifecycle-extensions:2.2.0'
kapt 'androidx.lifecycle:lifecycle-common-java8:2.2.0'
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Make ViewModal class and constructure public

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If constructor of your viewmodel is public and accepts only application then make sure you can create your own model without ViewModelProvider. Error message might be much more clear:

val model = YouViewModel(app)
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If you are using Hilt then don't forget to add these four dependencies.

    implementation "com.google.dagger:hilt-android:2.28-alpha"
kapt "com.google.dagger:hilt-android-compiler:2.28-alpha"
implementation 'androidx.hilt:hilt-lifecycle-viewmodel:1.0.0-alpha01'
kapt "androidx.hilt:hilt-compiler:1.0.0-alpha01"

Note:- If any of these dependencies are missing you will get Cannot create an instance of class ViewModel error

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Please add below code. It worked for me

val binding = FragmentLayoutBinding.inflate(inflater, container, false)


val viewModel = ViewModelProvider(
requireActivity(),
defaultViewModelProviderFactory
).get(MainViewModel::class.java)
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if you are using Hilt, ensure your activity/fragment is having @AndroidEntryPoint annotation

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I'm a proficient Android developer and I have used ViewModel 100s of times with no issue. Today I came across this issue. Spent hours and scrolled through various SO posts. Didn't get solved.

Then I saw that the package name in which I have the ViewModel contains new. Like this:

com.myapp.myfeature.new.feature

I changed new to neww for testing like this: com.myapp.myfeature.neww.feature

and it worked! I hope someone find it useful.

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DggerHilt can also be the reason, If you are using it make sure your activity/fragment is having @AndroidEntryPoint annotation on it.

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If you're using Hilt Dependency Injection, You probably have missed @ViewModelInject. Because, Hilt provide its own injection for viewmodel.

In my case, I used and @Inject due to this caught into the error.

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If you are using version 2.33-beta and upper remove these dependencies;

implementation "androidx.hilt:hilt-lifecycle-viewmodel:1.0.0-alpha03"
kapt "androidx.hilt:hilt-compiler:1.0.0-beta01"

Keep only these two dependency

implementation "com.google.dagger:hilt-android:2.33-beta"
kapt "com.google.dagger:hilt-android-compiler:2.33-beta"
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For people using Jetpack Compose, Navigation and Hilt

Make sure to use the hiltNavGraphViewModel instead of viewModel.

This is provided by androidx.hilt:hilt-navigation-compose dependency.

More details in the docs.

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If you face this issue in Kotlin Dagger Hilt even after @HiltViewModel and using @Inject, make sure you have updated all hilt dependencies.

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Add @HiltViewModel on top of your viewModel .

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For ViewModels used with Jetpack Compose

In case you are using Jepack Compose with a ViewModel in your component. The @Preview annotation may cause this error. That was the problem in my case.

Hope it helps someone!

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I fixed the same problem by doing this.

Note:- I am using Dagger hilt, Room database, MVVM, Data binding

Added the annotation.

class AlertViewModel
@Inject
constructor(private val userRepository: AlertRepository) : ViewModel(){
val getList:LiveData<List<Alert>> get() =
userRepository.getList.flowOn(Dispatchers.Main)
.asLiveData(context = viewModelScope.coroutineContext)


fun insert(user:Alert){
viewModelScope.launch {
userRepository.insert(user)
}
}
}

To

@HiltViewModel // Added this annotation
class AlertViewModel
@Inject
constructor(private val userRepository: AlertRepository) : ViewModel(){
val getList:LiveData<List<Alert>> get() =
userRepository.getList.flowOn(Dispatchers.Main)
.asLiveData(context = viewModelScope.coroutineContext)


fun insert(user:Alert){
viewModelScope.launch {
userRepository.insert(user)
}
}
}
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I had a different scenario when creating a view model instance:

  1. I was requesting for the instance in a fragment.
  2. My ViewModel required a parameter to be passed on the constructor.
  3. I was not using Dependency Injection.

Solution In a scenario where your viewmodel requires a parameter to be passed you have to create a ViewModelFactory to define your instances

Solution In Practice

- ViewModel Sample


class SharedViewModel(private val repository: UserRepository) : ViewModel() {
    

init {
viewModelScope.launch {
repository.refreshDataInDb()
}
}
    

}
    

    



- Creating ViewModel Factory


    

class ViewModelFactory(
private val repository: UserRepository
) : ViewModelProvider.NewInstanceFactory(){
override fun <T : ViewModel> create(modelClass: Class<T>): T {
    

return SharedViewModel( repository as UserRepository) as T
          

}
}
    

    



- Creating ViewModel Instannce in a Fragment


private  lateinit var factory: ViewModelFactory
private lateinit var searchViewModel: SharedViewModel
private lateinit var repository: UserRepository
    



repository = UserRepository()
factory = ViewModelFactory(repository)
searchViewModel = ViewModelProvider(requireActivity(), factory)[SharedViewModel::class.java]
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Well this will fix the issue for sure

First of all make sure you have this dependency

//Room DB
implementation "androidx.room:room-runtime:2.2.5"
annotationProcessor 'android.arch.persistence.room:compiler:1.1.1'

Then remove viewmodel constructor, then create init function as your constructor

 public void init(Application application){
instance = new FirebaseDatabaseInstance();
databaseRepository = new DatabaseRepository(application);
allChatMembers = databaseRepository.getAllChatMembers();
}

Then this will solve...

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Not for OP's case, but adding it here for newcomers.

If you inherit from AndroidViewModel and there is some heavy work in the main Thread (like accessing the database) it'll wrongly throw this error.

After I switched to inherit from ViewModel, it showed the correct error and I could move the heavy work to Dispatchers.IO. After moving UI blocking stuff to Dispatchers.IO, I re-tested using AndroidViewModel and everything works fine again.

Conclusion: "UI blocking work on the main thread when inheriting from AndroidViewModel can throw this error".


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