Android-practice-8-ViewModel

ViewModel : Android architecture component 1

Overview

View data ownership should be seperated from UI controller and assigned to ViewModel.
UI controllers such as activities and fragments are intended to display UI data and react to user actions like capturing input texts, but nothing more.

We shouldn't give excessive responsibility to UI controllers because we have no controll of it's lifecylces, only the Android framework does. When it's re-created, the data is lost.
ViewModel is a helper class for the UI controller. ViewModel object is retained during configuration changes and the data that it holds is immediately available to the next activity or fragment instance.

If we need to display a list of users, the resposibility for acquiring and keeping the list of users should be of ViewModel.
When activity or fragment is re-created, the ViewModel instance is same as the one that was created by the first activity.
When the activity is finished or fragment is detached, the ViewModel object is destroyed and onCleared() callback is called to clean up the resources.

Add dependency and creat a class that extends ViewModel() class.

ViewModel() is an absract class that contains onCleared() method
Create a reference to the ViewModel inside UI controller through get(Class<T> modelClass) method from ViewModelProvider class.
```
class SomeFragment : Fragment() {
  private lateinit var viewModel: SomeViewModel
 
  override fun onCreateView(inflater: LayoutInflater, container: ViewGroup?, savedInstanceState: Bundle?): 
   View? {
    viewModel = ViewModelProvider(this).get(SomeViewModel::class.java)
    ...
 }                     
}
```
ViewModelProvider(context).get(..) will create new ViewModel object, or return an existing one if it exists. Now this instance should be the one that holds all the UI data!

ViewModel object should not contain references to the associated Activities and Fragments since they don't survive during configuration changes.