ViewModel

Objective

1. The starter app doesn’t save and restore the app state during configuration changes, such as when the device orientation changes, or when the app shuts down and restarts.
   • onSaveInstanceState() requires us to add extra codes to save and retrieve the state from a bundle
   • We should keep the size of data in a bundle as minimal as possible. (Limitation on the size of a bundle, 500KB to 1MB)
2. The game screen does not navigate to the score screen when the user taps the End Game button.

How to solve?

1. We can make it by using the recommended App architecture. This diagram shows how all the modules should interact with one another after designing the app.

   

2. This codelab follows the separation of concern design principle, dividing classes into sections addressing distinctive concerns.

   • UI with Activity / Fragment
   • ViewModel and Repository <- current scope
   • Model and Data sources

Overview of ViewModel, ViewModelProvider, and ViewModelFactory

1. Definition of ViewModel

   The ViewModel class is designed to store and manage UI-related data in a lifecycle conscious way. The ViewModel class allows data to survive configuration changes such as screen rotations.

   A ViewModel can do simple calculations and transformations on data to prepare the data to be displayed by the UI controller. In this architecture, the ViewModel performs the decision-making, and UI component works according to the data transformed in the ViewModel.

   

   

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

   class GameViewModel : ViewModel() {
      init {
          Log.i("GameViewModel", "GameViewModel created!")
      }
        
     override fun onCleared() {
        super.onCleared()
        Log.i("GameViewModel", "GameViewModel destroyed!")
     }
   }
   

2. Definition of ViewModelProvider

   During configuration changes such as screen rotations, UI controllers such as fragments are re-created. However, ViewModel instances survive. If you create the ViewModel instance using the ViewModel class, a new object is created every time the fragment is re-created. Instead, create the ViewModel instance using a ViewModelProvider.

   Important: Always useViewModelProvider to create ViewModel objects rather than directly instantiating an instance of ViewModel.

   private lateinit var viewModel: GameViewModel
   viewModel = ViewModelProvider(this).get(GameViewModel::class.java)
   

   • ViewModelProvider returns an existing ViewModel, otherwise creates a new one.

   • ViewModelProvider creates a ViewModel instance in the given scope (Activity or Fragment).

3. Definition of ViewModelFactory

   A ViewModelFactory instantiates ViewModel objects, with or without constructor parameters.

   In this example, you want a ViewModel to hold the score, and you will pas in the score value during the ViewModel initialization.

   Factory method pattern that uses factory methods to create objects.

   class ScoreViewModel(finalScore: Int) : ViewModel() {
      // The final score
      var score = finalScore
      init {
          Log.i("ScoreViewModel", "Final score is $finalScore")
      }
   }
   

   class ScoreViewModelFactory(private val finalScore: Int) : ViewModelProvider.Factory {
     override fun <T : ViewModel?> create(modelClass: Class<T>): T {
        if (modelClass.isAssignableFrom(ScoreViewModel::class.java)) {
            return ScoreViewModel(finalScore) as T
        }
        throw IllegalArgumentException("Unknown ViewModel class")
     }
   }
   

   // code for sending score, which is put into ScoreViewModel as a parameter
   val action = GameFragmentDirections.actionGameToScore()
      action.score = viewModel.score
      NavHostFragment.findNavController(this).navigate(action)
   }
   

   private lateinit var viewModel: ScoreViewModel
   private lateinit var viewModelFactory: ScoreViewModelFactory
      
   viewModelFactory = ScoreViewModelFactory(ScoreFragmentArgs.fromBundle(arguments!!).score)
   viewModel = ViewModelProvider(this, viewModelFactory).get(ScoreViewModel::class.java)
   

Homework

1. To avoid losing data during a device-configuration change, you should save app data in which class?
   • ViewModel
   • LiveData
   • Fragment
   • Activity
2. A ViewModel should never contain any references to fragments, activities, or views. True or false?
   • True
   • False
3. When is a ViewModel destroyed?
   • When the associated UI controller is destroyed and recreated during a device-orientation change.
   • In an orientation change.
   • When the associated UI controller is finished (if it’s an activity) or detached (if it’s a fragment).
   • When the user presses the Back button.
4. What is the ViewModelFactory interface for?
   • Instantiating a ViewModel object.
   • Retaining data during orientation changes.
   • Refreshing the data being displayed on the screen.
   • Receiving notifications when the app data is changed.