How ViewModel is persisted on configuration change

Question

Looking at the ViewModel documentation, it says:

In other words, this means that a ViewModel will not be destroyed if its owner is destroyed for a configuration change (e.g. rotation). The new instance of the owner will just re-connected to the existing ViewModel.

How is the ViewModel not destroyed if the activity referencing it is destroyed? and how is it reconnected once we create a new activity?

Answer 1

The answer is if you use ViewModelProviders.of(this).get(YourViewModel.class) to create ViewModel, the library will cache the ViewModel for you. If you create your ViewModel use "new YourViewModel()", the ViewModel will recreate each time when the activity configuration change. In the ViewModelProviders, it will crate a HolderFragment to add to your activity or your fragment, it's invisible, when the configuration changed, activity destroyed, but the cache is still alive, so next time activity create, the ViewModel will reconnect to it.

Answer 2

Behind the scenes a retained fragment is used. A retained fragment has its state retained across Activity re-creation (such as from a configuration change).

See "Architecture Components Introduction" talk from Google IO 2017, where Yigit Boyar talks about ViewModel.