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Appyx Interactions – TransitionModel


A TransitionModel defines the abstract model of the AppyxComponent.


TransitionModel holds an instance of <ModelState>, a type defined by the implementation class.

You can find various examples in the repository. Some possible points of interest:

  • TestDriveModel.State
  • BackStackModel.State
  • SpotlightModel.State
  • CardsModel.State

Model state should have no knowledge of UI-related properties – all of that belongs to the UI layer. You should strive to describe your model in a semantic way that assumes nothing of its visual representation.

For example, TestDriveModel.State describes an element that can be in 4 possible configurations (the example in this documentation's main page).

Notice how the states, even though simplistic, aren't named TOPLEFT, TOPRIGHT, BOTTOMRIGHT, BOTTOMLEFT, as the positioning is only done in the UI. This allows to represent them in various different ways, without any assumptions baked in the model.

    data class State<InteractionTarget>(
        val element: Element<InteractionTarget>,
        val elementState: ElementState
    ) : Parcelable {
        enum class ElementState {
            A, B, C, D;

            fun next(): ElementState =
                when (this) {
                    A -> B
                    B -> C
                    C -> D
                    D -> A

Another example from BackStackModel.State, with purely semantic naming:

    data class State<InteractionTarget>(
         * Elements that have been created, but not yet moved to an active state
        val created: Elements<InteractionTarget> = listOf(),

         * The currently active element.
         * There should be only one such element in the stack.
        val active: Element<InteractionTarget>,

         * Elements stashed in the back stack (history).
        val stashed: Elements<InteractionTarget> = listOf(),

         * Elements that will be destroyed after reaching this state.
        val destroyed: Elements<InteractionTarget> = listOf(),
    ) : Parcelable


This generic type is simply meant for you to identify different child elements in your client code. For example, if you have a tabbed component, and each tab would be identified by a different instance of an enum or a sealed class, then this enum or sealed class would be your <InteractionTarget>.

Then, depending on your <ModelState> and operation implementations, you can affect these targets differently.

Changing the state

A TransitionModel behaves like a state machine. If you want a new ModelState to be created, you must pass it an instance of an Operation. You can read more about them in Operations.


TransitionModel has an output flow:

val output: StateFlow<Output<ModelState>>

In most cases don't need to deal with this output directly. The AppyxComponent takes care of that, and channels this flow to the Visualisation to translate it to a UI representation.