Architecture component — ViewModel behind the scenes

In this article, I will dive into the implementation details of ViewModel and explain how it works behind the scenes.

In my previous article, I talked about one of the very important feature ViewModel has to offer is to be able to survive from a configuration change, so how does it achieve this? the short answer is by taking advantage of setRetainInstance(true) from Fragment, ViewModel uses a fragment with setRetainInstance set to true behind the scenes.

The source code

Let's first look at the source code to see how ViewModels are stored/associated with an Activity.

All ViewModels of an AppCompatActivity are stored in a hashMap inside ViewModelStore. AppCompatActivity is a ViewModelStoreOwner who is capable of providing a ViewModelStore instance to access its ViewModels.

ViewModelStoreOwner need to provide a ViewModelStore

ViewModelStore has a hashMap to store all ViewModels

When a configuration change happened on an Activity, its onRetainNonConfigurationInstance() callback method will be called to allow you to return an object that can be later retrieved in the newly created Activity via getLastNonConfigurationInstance(). If we look into the source code of ComponentActivity (who is a super class of AppCompatActivity). We can see it returns an instance of wrapper class that contains viewModelStore. Through this mechanism, ViewModels of an Activity are retained.

Source code snippet of ComponentActivity class

Great! we now know how an Activity keeps its ViewModels, but how does Fragment work? Let’s keep digging into the source code.

Source code snippet from Fragment class

setRetainInstance(bool) will allow fragment instance to be retained during Activity re-creation, from the source code above, we can see that the work is delegated to amFragmentManager instance, below source code shows what happens when you call addRetainedFragment and removeRetainedFragment

FragmentManager class has a private property private FragmentManagerViewModel mNonConfig who keeps track of all the fragments that need to be retained and a hash map that stores ViewModelStores and the fragments they belong in a key-value pair fashion with keys being a unique UUID of a fragment.

When a Fragment needs to retrieve its ViewModels from a ViewStore, it asks its FragmentManager to do the work, FragmentManager is responsible to keep track of retained fragments and their ViewModelStores.