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Appyx Interactions – Operations


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.


Operation defines a (ModelState) -> ModelState change.

interface Operation<ModelState> {

    val mode: Mode

    fun isApplicable(state: ModelState): Boolean

    operator fun invoke(state: ModelState): StateTransition<ModelState>


A mode maybe specified by client code to define how the TransitionModel should handle the state change with regards to interrupts.

interface Operation<ModelState> {

    enum class Mode {

You can test their effects on the below sample. Try spamming the buttons:

In code, both buttons invoke the same operation, only with different modes:


// vs

{ = IMMEDIATE, animationSpec = spring(
    stiffness = Spring.StiffnessVeryLow,
    dampingRatio = Spring.DampingRatioMediumBouncy
)) }

In KEYFRAME mode the current transition isn’t interrupted, and it will be guaranteed to finish before any additional transitions. New states are enqueued for execution afterwards. The overall execution progress speeds up proportionally to the size of enqueued operations.

In IMMEDIATE mode the current transition is interrupted and will not be finished. New operations overwrite the current target state, resulting in the UI dynamically changing course always towards the latest one.

Applicability check

fun isApplicable(state: ModelState): Boolean

This method is used for checks without actually executing an Operation.

Invoking the Operation

This method should create a StateTransition<ModelState>:

operator fun invoke(state: ModelState): StateTransition<ModelState>

Which is defined as:

data class StateTransition<ModelState>(
    val fromState: ModelState,
    val targetState: ModelState

It is imperative that:

  1. fromState is derived from the baseline state (the one passed to invoke)
  2. fromState adds any new elements to the state that should be animated in and are not contained in the baseline state
  3. targetState is derived from fromState and never directly from the baseline state

To make it easier, it is recommended to extend BaseOperation which helps to enforce the above:

abstract class BaseOperation<ModelState> : Operation<ModelState> {

    final override fun invoke(baseLineState: ModelState): StateTransition<ModelState> {
        val fromState = createFromState(baseLineState)
        val targetState = createTargetState(fromState)

        return StateTransition(
            fromState = fromState,
            targetState = targetState

     * @return ModelState If the operation adds any new elements to the scene,
     *                    it MUST add them to the state here.
     *                    Otherwise, just return baseLineState unchanged.
    abstract fun createFromState(baseLineState: ModelState): ModelState

     * @return ModelState Any elements present in this state
     *                    MUST also be present in the fromState already.
    abstract fun createTargetState(fromState: ModelState): ModelState


You can find examples for implementing operations in the repo. Some possible points of interest:

  • TestDrive / MoveTo
  • BackStack / Push, Pop, Replace
  • Spotlight / Next, Prev, First, Last

Notice how each of them also comes with their convenient extension methods that allow these operations to be invoked directly on their corresponding AppyxComponent, so that client code can use a simple API:

// Without extension methods

// With extension methods