Não é possível criar uma instância da classe ViewModel no novo projeto

Nov 25 2020

Tenho dois projetos diferentes. No primeiro, uso ViewModel com minha atividade e banco de dados Room. Tudo funciona. Em seguida, acabei de criar um novo projeto para finalidades diferentes e aí instanciei ViewModel absolutamente da mesma maneira, mas meu aplicativo trava com este erro:

java.lang.RuntimeException: Unable to start activity ComponentInfo{com.example/com.example.MainActivity}: java.lang.RuntimeException: Cannot create an instance of class com.example.MainViewModel
    at android.app.ActivityThread.performLaunchActivity(Unknown Source:591)
    at android.app.ActivityThread.handleLaunchActivity(Unknown Source:36)
    at android.app.servertransaction.LaunchActivityItem.execute(Unknown Source:57)
    at android.app.servertransaction.TransactionExecutor.executeCallbacks(Unknown Source:99)
    at android.app.servertransaction.TransactionExecutor.execute(Unknown Source:34)
    at android.app.ActivityThread$H.handleMessage(Unknown Source:36) at android.os.Handler.dispatchMessage(Unknown Source:21) at android.os.Looper.loop(Unknown Source:207) at android.app.ActivityThread.main(Unknown Source:107) at java.lang.reflect.Method.invoke(Native Method) at com.android.internal.os.RuntimeInit$MethodAndArgsCaller.run(Unknown Source:11)
    at com.android.internal.os.ZygoteInit.main(Unknown Source:274)
 Caused by: java.lang.RuntimeException: Cannot create an instance of class com.example.MainViewModel
    at androidx.lifecycle.ViewModelProvider$NewInstanceFactory.create(ViewModelProvider.java:221) at androidx.lifecycle.ViewModelProvider.get(ViewModelProvider.java:187) at androidx.lifecycle.ViewModelProvider.get(ViewModelProvider.java:150) at com.kazguiu.MainActivity.applyAdapterAndViewModel(MainActivity.kt:37) at com.kazguiu.MainActivity.onCreate(MainActivity.kt:25) at android.app.Activity.performCreate(Unknown Source:16) at android.app.Activity.performCreate(Unknown Source:1) at android.app.Instrumentation.callActivityOnCreate(Unknown Source:3) at android.app.ActivityThread.performLaunchActivity(Unknown Source:368) at android.app.ActivityThread.handleLaunchActivity(Unknown Source:36)  at android.app.servertransaction.LaunchActivityItem.execute(Unknown Source:57)  at android.app.servertransaction.TransactionExecutor.executeCallbacks(Unknown Source:99)  at android.app.servertransaction.TransactionExecutor.execute(Unknown Source:34)  at android.app.ActivityThread$H.handleMessage(Unknown Source:36) 
    at android.os.Handler.dispatchMessage(Unknown Source:21) 
    at android.os.Looper.loop(Unknown Source:207) 
    at android.app.ActivityThread.main(Unknown Source:107) 
    at java.lang.reflect.Method.invoke(Native Method) 
    at com.android.internal.os.RuntimeInit$MethodAndArgsCaller.run(Unknown Source:11)  at com.android.internal.os.ZygoteInit.main(Unknown Source:274)  Caused by: java.lang.InstantiationException: java.lang.Class<com.example.MainViewModel> has no zero argument constructor at java.lang.Class.newInstance(Native Method) at androidx.lifecycle.ViewModelProvider$NewInstanceFactory.create(ViewModelProvider.java:219)

Eu instancio ViewModel em atividades como esta:

private lateinit var mainViewModel: MainViewModel

override fun onCreate(savedInstanceState: Bundle?) {
    super.onCreate(savedInstanceState)
    setContentView(R.layout.activity_main)

    applyAdapterAndViewModel()

}

private fun applyAdapterAndViewModel() {
    mainViewModel = ViewModelProvider(this).get(MainViewModel::class.java)
}

Meu ViewModel:

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

    private val exampleDao = Database.getDatabase(application).exampleDao()
    private val repository = Repository(exampleDao)

    fun getSubjects() = repository.getSubjects()

    fun addSubjects(subject: Subjects) = viewModelScope.launch(Dispatchers.IO) {
        repository.addSubject(subject)
    }

}

Esta maneira de usar ViewModel é a mesma do meu projeto anterior. A única diferença é que tenho duas dependências adicionais em meu gradle atual:

implementation 'androidx.lifecycle:lifecycle-livedata-ktx:2.2.0'
implementation 'androidx.lifecycle:lifecycle-viewmodel-ktx:2.2.0'

Se eu excluí-los, o Android Studio não reconhece ViewModelProvider, mas não tenho essas dependências em meus projetos anteriores e uso LiveData e ViewModel absolutamente da mesma maneira (até mesmo as importações são as mesmas) lá. Então, o que devo fazer?

Respostas

1 AndrewChelix Nov 25 2020 at 11:52

Seu ViewModel requer um argumento application: Applicationque só pode ser fornecido usando um ViewModelFactory como este:

ViewModelFactory

class MainViewModelFactory(
    private val application: Application
) : ViewModelProvider.Factory {
    @Suppress("unchecked_cast")
    override fun <T : ViewModel?> create(modelClass: Class<T>): T {
        if (modelClass.isAssignableFrom(MainViewModel::class.java)) {
            return MainViewModel(application) as T
        }
        throw IllegalArgumentException("Unknown ViewModel class")
    }
}

Atividade principal

private fun applyAdapterAndViewModel() {
// Not so kotlinic
mainViewModel = ViewModelProvider(this,MainViewModelFactory(application)).get(MainViewModel::class.java)
}

Além disso, em MainActivity, substitua private lateinit var mainViewModel: MainViewModel por

private val viewModel by viewModels<MainViewModel> { 
MainViewModelFactory(application) }