Child Panels overview¶

The Child Panels¶

Child Panels is a navigation model for managing a set of up to three child components (panels): Main (required), Details (optional) and Extra (optional). This navigation model can be compared with Compose List-Details Layout.

Similarly to Child Stack, each component has its own Lifecycle automatically controlled by the navigation model depending on the current ChildPanelsMode.

It is possible to have more than one Child Panels navigation model in a component, nested navigation is also supported.

The Child Panels navigation model consists of the following main entities:

• Panels - represents a state of the Child Panels navigation model. The navigation is performed by creating a new navigation state from the previous one.
  • Panels#main - a configuration of the Main panel.
  • Panels#details - an optional configuration of the Details panel, default value is null.
  • Panels#extra - an optional configuration of the Extra panel, default value is null.
  • Panels#mode - the current ChildPanelsMode, determines how lifecycles of the panels within the Child Panels navigation model are changing, default value is ChildPanelsMode.SINGLE.
• ChildPanels - a simple data class that stores all child components (panels) and their configurations, as well as the current mode.
  • ChildPanels#main - a Main child component.
  • ChildPanels#details - an optional Details child component.
  • ChildPanels#extra - an optional Extra child component.
  • ChildPanels#mode - the current ChildPanelsMode.
• ChildPanelsMode - determines how lifecycles of the child components (panels) within the Child Panels navigation model are changing.
• SINGLE - there is only one RESUMED panel at a time. If the Extra panel exists, then it is RESUMED and all other panels are CREATED. Otherwise, if the Details panel exists, then it is RESUMED and the Main panel is CREATED. Otherwise, the Main panel is RESUMED.
• DUAL - there are at most two panels RESUMED at a time. The Main panel is always RESUMED. If the Extra panel exists, then it is RESUMED and the Details panel (if exists) is CREATED. Otherwise, if the Details panel exists, then it is RESUMED.
• TRIPLE - any existing panel is always RESUMED.
• PanelsNavigation - an interface that accepts navigation commands and forwards them to all subscribed observers.

Component Configurations¶

Similarly to Child Stack, each component created and managed by the Child Panels has a configuration, please read the documentation about child configurations.

Initializing Child Panels¶

There are three steps to initialize Child Panels:

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

Displaying panels with Compose¶

Child Panels state can be observed and displayed in Compose by using the ChildPanels Composable function from the experimental Compose extensions module provided by Decompose. Please see the related documentation for more information.

Example¶

Here is a very basic example of a list-details navigation:

import com.arkivanov.decompose.ComponentContext

interface MainComponent

class DefaultMainComponent(
    componentContext: ComponentContext,
    onItemSelected: (id: Long) -> Unit,
) : MainComponent, ComponentContext by componentContext

interface DetailsComponent

class DefaultDetailsComponent(
    componentContext: ComponentContext,
    itemId: Long,
    onFinished: () -> Unit,
) : DetailsComponent, ComponentContext by componentContext

import com.arkivanov.decompose.ComponentContext
import com.arkivanov.decompose.router.panels.ChildPanels
import com.arkivanov.decompose.router.panels.ChildPanelsMode
import com.arkivanov.decompose.router.panels.Panels
import com.arkivanov.decompose.router.panels.PanelsNavigation
import com.arkivanov.decompose.router.panels.activateDetails
import com.arkivanov.decompose.router.panels.childPanels
import com.arkivanov.decompose.router.panels.dismissDetails
import com.arkivanov.decompose.router.panels.setMode
import com.arkivanov.decompose.value.Value
import kotlinx.serialization.Serializable
import kotlinx.serialization.builtins.serializer

interface PanelsComponent {

    val panels: Value<ChildPanels<*, MainComponent, *, DetailsComponent, Nothing, Nothing>>

    fun setMode(mode: ChildPanelsMode)
}

class DefaultPanelsComponent(
  componentContext: ComponentContext,
) : PanelsComponent, ComponentContext by componentContext {

    private val nav = PanelsNavigation<Unit, DetailsConfig, Nothing>()

    override val panels: Value<ChildPanels<*, MainComponent, *, DetailsComponent, Nothing, Nothing>> =
        childPanels(
            source = nav,
            serializers = Unit.serializer() to DetailsConfig.serializer(),
            initialPanels = { Panels(main = Unit) },
            handleBackButton = true,
            mainFactory = { _, ctx ->
                DefaultMainComponent(
                    componentContext = ctx,
                    onItemSelected = { nav.activateDetails(details = DetailsConfig(itemId = it)) },
                )
            },
            detailsFactory = { cfg, ctx ->
                DefaultDetailsComponent(
                    componentContext = ctx,
                    itemId = cfg.itemId,
                    onFinished = nav::dismissDetails,
                )
            },
        )

    override fun setMode(mode: ChildPanelsMode) {
         nav.setMode(mode)
    }

    @Serializable
    private data class DetailsConfig(val itemId: Long)
}

Screen recreation and process death on (not only) Android¶

Child Panels automatically preserves the state when a configuration change or process death occurs. To disable state preservation completely, pass serializers = null argument. When navigation state saving is disabled, the state is reset to the initial value when recreated.

Components are created in their order. I.e. the Main component is created first, then the Details component is created (if exists), and lastly the Extra component is created (if exists). Components are destroyed in reverse order.

Multiple Child Panels in a component¶

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