无法创建 class ViewModel class 的实例

Question

• I am trying to follow MVVM pattern in my Android app but getting error while creating an instance of ViewModel.
• Error: Cannot create an instance of class DemoViewModel class .

Here is my code:

DemoFragment.kt :

class DemoFragment : Fragment(R.layout.fragment_demo) {

    lateinit var mViewModel: DemoViewModel

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

        mViewModel=ViewModelProvider(this).get(DemoViewModel::class.java)
        mViewModel.getSomeData()
    }
}

DemoViewModel.kt :

class DemoViewModel(val demoRepository: DemoRepository) : ViewModel() {

    fun getSomeData() {
        Log.d("DemoViewModel", "${demoRepository.getData()}")
    }
}

DemoRepository.kt :

interface DemoRepository {
    fun getData(): Boolean
}

class DemoImpl : DemoRepository {
    override fun getData() = false
}

Answer 1

You need to use ViewModelFactory. Because there is "demoRepository" in your primary builder.

    class DemoViewModelFactory constructor(private val repository:DemoImpl): ViewModelProvider.Factory {

     override fun <T : ViewModel> create(modelClass: Class<T>): T {
        return if (modelClass.isAssignableFrom(DemoViewModel::class.java!!)) {
            DemoViewModel(this.repository) as T
        } else {
            throw IllegalArgumentException("ViewModel Not Found")
        }
    }
}

Usage

viewModel = ViewModelProvider(this, DemoViewModelFactory(repositoryObject)).get(DemoViewModel::class.java)

Answer 2

I would encourage you to use "by viewModels()" extension function to create viewModel instance easily. Note that you should add following dependency to use it:

implementation 'androidx.fragment:fragment-ktx:1.2.5'

Sample Fragment Implementation:

class DemoFragment : Fragment() {

   // Use the 'by ViewModels()' Kotlin property delegate
   // from the fragment-ktx artifact
   private val model: DemoViewModel by viewModels()

   override fun onViewCreated(view: View, savedInstanceState: Bundle?) {
      super.onViewCreated(view, savedInstanceState)
      model.selected.observe(viewLifecycleOwner, Observer<Item> { item ->
        // Update the UI
      })
   }
}

Then you can inject an instance of your repository via constructor injection by Dagger or Hilt etc.