Transitions

You can have arbitrary visualisations and transitions for any NavModel. For example, all of these are different representations of the same Back stack:

Below you can find the different options how to visualise NavModel state changes.

No transitions

Using the provided Child-related composables you'll see no transitions as a default – UI changes resulting from the NavModel's state update will be rendered instantly.

Jetpack Compose default animations

You can use standard Compose animations for embedded child Nodes in the view, e.g. AnimatedVisibility:

var visibility by remember { mutableStateOf(true) }

Child(navElement) { child, _ ->
    AnimatedVisibility(visible = visibility) {
        child()
    }
}

Appyx transition handlers

All the child composables provided by Appyx accept an optional transitionHandler argument too:

• You can use the provided ones as they're a one-liner to add – you can check the individual NavModels for the ones they come shipped with.
• You can also implement your own.

The benefit of using transition handlers is you can represent any custom state of elements defined by your NavModel with Compose Modifiers.

The example below is taken from custom navigation models. It matches custom transition states to different scaling values, and returns a scale Modifier.

class FooTransitionHandler<T>(
    private val transitionSpec: TransitionSpec<Foo.State, Float> = { spring() }
) : ModifierTransitionHandler<T, Foo.State>() {

    // TODO define a Modifier depending on the state.
    //  Here we'll just mutate scaling: 
    override fun createModifier(
        modifier: Modifier,
        transition: Transition<Foo.State>,
        descriptor: TransitionDescriptor<T, Foo.State>
    ): Modifier = modifier.composed {
        val scale = transition.animateFloat(
            transitionSpec = transitionSpec,
            targetValueByState = {
                when (it) {
                    Foo.State.CREATED -> 0f
                    Foo.State.FOO -> 0.33f
                    Foo.State.BAR -> 0.66f
                    Foo.State.BAZ -> 1.0f
                    Foo.State.DESTROYED -> 0f
                }
            })

        scale(scale.value)
    }
}

// TODO remember to add:
@Composable
fun <T> rememberFooTransitionHandler(
    transitionSpec: TransitionSpec<Foo.State, Float> = { spring() }
): ModifierTransitionHandler<T, Foo.State> = remember {
    FooTransitionHandler(transitionSpec)
}

More info

1. You can find more complex examples in the implementations of other NavModels, such as the Promoter carousel
2. You can find Codelabs tutorials that help you master custom transitions
3. You can find Coding challenges related to custom transitions