State handling

Note: you should’ve read the overview on the home page before jumping into this material.

The Thoughts about State Handling on Android and Thoughts about Event Handling on Android articles explain the design choices behind RainbowCake, and might provide you insight for how to work with the architecture.

For a video introduction, watch Handling View State and Events with RainbowCake from Kotliners 2020.

Each screen (or view) in the application is a Fragment. While Activity based screens are also supported, most of the documentation will focus on Fragments, as that’s the recommended approach.

The RainbowCakeFragment base class provides integration with dependency injection to grab the appropriate ViewModel for a given Fragment. It also connects to and disconnects from the ViewModel automatically, and receives state changes and events.

View states may be created in one of two ways, either as sealed classes or just data classes, depending on the requirements of your screen. See the View state page for more about these.

The screen template for the architecture can generate both styles for you.

For now, we’ll look at a simple screen that loads a user profile, displaying a username and profile image. Here’s the sealed class representing our view state:

sealed class UserViewState

object Loading : UserViewState()

data class UserLoaded(
        val userName: String,
        val profileImage: Uri? = null
) : UserViewState()

We’ll need a ViewModel for our screen. This UserViewModel class inherits from RainbowCakeViewModel, and it provides the base class with its view state class as a type argument, as well as its initial state (Loading) via the constructor call. When the state of the view has to be updated, we use the viewState property of RainbowCakeViewModel to set the new one:

class UserViewModel : RainbowCakeViewModel<UserViewState>(Loading) {
    fun loadUser() {
        viewState = UserLoaded(userName = "Jane Doe")

Now, let’s see the UserFragment implementation. It will inherit from RainbowCakeFragment, and pass both the view state and ViewModel as type arguments to it. It will also implement three required methods:

  • provideViewModel should either call the getViewModelFromFactory() helper function that’s provided by the Dagger or Koin integrations of RainbowCake, or your own DI solution, to fetch a ViewModel for the current screen (see here for details).
  • getViewResource should return the layout XML to be inflated for the Fragment. If you need to customize layout inflation, simply return 0 here, and override onCreateView without calling super.
  • render is how the Fragment observes the state stored in the ViewModel, and is called every time the view state changes. Its responsibility is to update the state of the UI to reflect the current view state. It must be implemented in a way so that any previous view states do not affect the current state of the UI. In other words, the same view state being set must always result in the same state for the Fragment’s UI.
class UserFragment : RainbowCakeFragment<UserViewState, UserViewModel>() {
    override fun provideViewModel() = getViewModelFromFactory()
    override fun getViewResource() = R.layout.fragment_user

    override fun render(viewState: UserViewState) {
        when (viewState) {
            is Loading -> {
                progressBar.isVisible = true
                profileContainer.isVisible = false
            is UserLoaded -> {
                progressBar.isVisible = false
                profileContainer.isVisible = true
                usernameText.text = viewState.userName

Note the use of the exhaustive extension property. This no-op property forces our when clause to be exhaustive, so that we are forced to handle all possible states of our screen. This is made possible by using a sealed class for our view state.

For some more tips about rendering single view states, take a look at Designing and Working with Single View States on Android.

In order to move from the Loading to the UserLoaded state, our Fragment has to call the ViewModel’s loadUser method at some point. There are multiple good choices for doing this, one of them is in the onStart method:

class UserFragment : RainbowCakeFragment<UserViewState, UserViewModel>() {
    override fun onStart() {

This will trigger a change in the view state, and run the render method, displaying the user. Yay!