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Child Overlay overview

The Child Overlay

Child Overlay is a navigation model with either one active component at a time, or none. In other words, each Child Overlay allows to activate a child component, replace with another child component, or dismiss when not needed. It is possible to have more than one Child Overlay in a component, nested overlays are also supported.

The Child Overlay navigation consists of two main entities:

  • ChildOverlay - a simple data class that holds the currently active child, if any.
  • OverlayNavigation - an interface that accepts navigation commands and forwards them to all subscribed observers.

Component Configurations

Each component created and managed by the Child Overlay has a configuration, please read the documentation about child configurations.

Initializing the Child Overlay

There are three steps to initialize the Child Overlay:

  • Create a new instance of OverlayNavigation and assign it to a variable or a property.
  • Initialize the Child Overlay using the ComponentContext#childOverlay extension function and pass OverlayNavigation into it along with other arguments.
  • The childOverlay function returns Value<ChildOverlay> that can be observed in the UI. Assign the returned Value to another property or a variable.


Here is a very basic example of a child overlay:

Dialog component
interface DialogComponent {

    fun onDismissClicked()

class DefaultDialogComponent(
    private val componentContext: ComponentContext,
    private val message: String,
    private val onDismissed: () -> Unit,
) : DialogComponent, ComponentContext by componentContext {

    override fun onDismissClicked() {
Root component
interface RootComponent {

    val dialog: Value<ChildOverlay<*, DialogComponent>>

class DefaultRootComponent(
    componentContext: ComponentContext,
) : RootComponent, ComponentContext by componentContext {

    private val dialogNavigation = OverlayNavigation<DialogConfig>()

    private val _dialog =
            source = dialogNavigation,
            // persistent = false, // Disable navigation state saving, if needed
            handleBackButton = true, // Close the dialog on back button press
        ) { config, componentContext ->
                componentContext = componentContext,
                message = config.message,
                onDismissed = dialogNavigation::dismiss,

    override val dialog: Value<ChildOverlay<*, DialogComponent>> = _dialog

    private fun showDialog(message: String) {
        dialogNavigation.activate(DialogConfig(message = message))

    private data class DialogConfig(
        val message: String,
    ) : Parcelable

Multiple Child Overlays in a component

When multiple Child Overlays are used in one component, each such Child Overlay must have a unique key associated. The keys are required to be unique only within the parent (hosting) component, so it is ok for different components to have Child Overlays with same keys. An exception will be thrown if multiple Child Overlays with the same key are detected in a component.

Two Child Overlays in one component
class Root(
    componentContext: ComponentContext
) : ComponentContext by componentContext {

    private val topNavigation = OverlayNavigation<TopConfig>()

    private val topOverlay =
        childOverlay<TopConfig, TopChild>(
            source = topNavigation,
            key = "TopOverlay",
            // Omitted code

    private val bottomNavigation = OverlayNavigation<BottomConfig>()

    private val bottomOverlay =
        childOverlay<BottomConfig, BottomChild>(
            source = bottomNavigation,
            key = "BottomOverlay",
            // Omitted code