While ViewModels are able to hold their state through various configuration changes while the screen is still active, it still has to be decided when this state is loaded and reloaded. Here are two common options you may want to use.
Reload state every time the Fragment attached to the ViewModel becomes visible. The convention for doing this is to define a load method in the ViewModel (this might take parameters, such as the Fragment’s arguments), and call this in the Fragment’s onStart method. This is practical when you want the Fragment to show up-to-date state even when it becomes visible after navigating back to it from another screen, which may have modified data it needs to display.
The need for this type of reloading of data can be avoided by observing a data source in a reactive way, such as a local database as a
Flow(see Room & coroutines).
Load state once and hold onto it for the entire lifetime of the Fragment (of course, updates triggered by user input may still modify the state). This is best done by fetching the state in the ViewModel’s initializer block (init). This approach is useful if fetching the data to display is an expensive operation, or would always fetch the same data anyway.
This logic might need to be moved to a separate method for testing purposes, as code executed in the constructor is difficult to test. In this case, the method can be called in a lifecycle method such as onCreate, and it can perform a check against its current view state to see if it needs to initialize itself. Having a separate Initial state here can be helpful for these checks.
fun load() {
if (viewState !is Initial) return // or viewState as? Initial ?: return
// perform initialization logic
}