ViewModelFactory in Android

Overview

ViewModelFactory is used to create objects of ViewModel. When you want to use initialize a parameter your ViewModel at the time of its object creation then you must use ViewModelFactory.

Steps to Follow

• Add the required dependency in build.gradle (Module) file

 // ViewModel
 def viewModel_version = "2.4.0"
 implementation "androidx.lifecycle:lifecycle-viewmodel-ktx:$viewModel_version"

• Create a class and extend it with ViewModel() class. Write the logic in this class.

 class MainViewModel(private val initialValue: Int) : ViewModel() {
    /* Do not make the variable private because you have to use this variable in MainActivity */
    var counter: Int = initialValue;

    /* Do not make the function private because you have to use this function in MainActivity */
    fun increment() {
        counter++
    }
 }

• Create a class and implement it with ViewModelProvider.Factory interface

 class MainViewModelFactory(private val counter: Int) : ViewModelProvider.Factory {
    override fun <T : ViewModel> create(modelClass: Class<T>): T {
        return MainViewModel(counter) as T
    }
 }

• Create an instance of ViewModel in MainActvity (or the Activity where you want to display the data) and use its instance to run the method defined in ViewModel class and / or use its instance to display the properties defined in ViewModel

 class MainActivity : AppCompatActivity() {
    private var counter: Int = 0;
    private lateinit var mainViewModel: MainViewModel
    override fun onCreate(savedInstanceState: Bundle?) {
        super.onCreate(savedInstanceState)
        setContentView(R.layout.activity_main)
        mainViewModel =
            ViewModelProvider(this, MainViewModelFactory(10))[MainViewModel::class.java]
        setContent()
    }

    /* Do not declare the fun as private because this function is declared in the xml file of the activity*/
    fun increment(view: View?) {
        mainViewModel.increment()
        setContent()
    }

    private fun setContent() {
        tvCounter.text = mainViewModel.counter.toString()
    }

 }

Reference

1. https://github.com/iamraajkanchan/ViewModel_Android/tree/viewModelFactoryImplementation