无法创建 class ViewModel 的实例

Question

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;
    }
}

Answer 1

如果您使用的是Hilt ，请确保您的活动/片段具有@AndroidEntryPoint注释

Answer 2

将您的构造函数public 。

Answer 3

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

example:

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

Answer 4

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'

Answer 5

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

Answer 6

DggerHilt也可能是原因，如果您正在使用它，请确保您的activity/fragment上有@AndroidEntryPoint注释。

Answer 7

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.

ie, in your case

public PostViewModel() {}

Answer 8

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 .

Answer 9

公开类和构造器可以解决我的问题。

Answer 10

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 gridle file for lifecycle also if you use room and other libries 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

Answer 11

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

Answer 12

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

Answer 13

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.

Answer 14

Please add below code. It worked for me

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

val viewModel = ViewModelProvider(
            requireActivity(),
            defaultViewModelProviderFactory
            ).get(MainViewModel::class.java)

Answer 15

在 viewModel 之上添加 @HiltViewModel 。

Answer 16

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)

Answer 17

Extend AndroidViewModel from your ViewModel class.

public class YourViewModel extends AndroidViewModel {

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

        //Todo: ...
    }

}

Answer 18

如果您的PostViewModel类是内部类，请确保其公共和静态

Answer 19

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'

Answer 20

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.

Answer 21

如果您在 Kotlin Dagger Hilt 中遇到此问题，即使在 @HiltViewModel 并使用 @Inject 之后，请确保您已更新所有刀柄依赖项。

Answer 22

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"

Answer 23

就我而言，我需要使用ListItemViewModelFactory将参数传递给我的视图模型。

Answer 24

创建一个没有参数且为空的附加构造函数并将其公开解决了该问题。

Answer 25

In my case the reason was that I was trying to get shared instance of the ViewModel in my fragment to soon - before the activity was created. That happens when 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 . Other words: Activity creates the ViewModel and 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 instance of ViewModel; I open Fragment, which get the instance of ViewModel. But when when 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 fragment can't get the ViewModel as Activity had not created it yet.

Solution 1 : Move the code when fragment get the ViewModel from onCreate to onViewCreated. Which is fine as I observe all liveDatas 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 ")
}

Answer 26

我的问题是 IDE 向我的 ViewModel 类添加了一个“抽象”修饰符。

Answer 27

公开ViewModal 类和构造

Answer 28

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)

Answer 29

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.

Answer 30

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"

Answer 31

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!

Answer 32

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)
        }
    }
}

Answer 33

I had a different scenario when creating a view model instance:

Answer 34

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...

Answer 35

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".