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Extensions for Jetpack/JetBrains Compose

Extensions and utilities for easier integration of Decompose with Jetpack/JetBrains Compose.

Setup

Since Jetpack and JetBrains Compose are published separately into different repositories, Decompose provides separate modules for each variant. Both modules provide similar functionality, but you need to choose the corresponding module depending on the used Compose variant.

Setup extensions for Jetpack Compose

Extensions for Jetpack Compose are provided by the extensions-compose-jetpack module.

implementation "com.arkivanov.decompose:extensions-compose-jetpack:<version>"

implementation("com.arkivanov.decompose:extensions-compose-jetpack:<version>")

Setup extensions for JetBrains Compose

Extensions for JetBrains Compose are provided by the extensions-compose-jetbrains module.

implementation "com.arkivanov.decompose:extensions-compose-jetbrains:<version>"

implementation("com.arkivanov.decompose:extensions-compose-jetbrains:<version>")

Content

As mentioned above both modules provide similar functionality. Most of the links in this document refer to the Jetpack module, however there usually a mirror in the JetBrains module.

Converting Value to State

To convert Decompose Value to Compose State use Value<T>.subscribeAsState(): State<T> extension function:

interface SomeComponent {
    val model: Value<Model>

    data class Model(/*...*/)
}

@Composable
fun SomeContent(component: SomeComponent) {
    val model: State<Model> by component.model.subscribeAsState()
}

Controlling the Lifecycle on Desktop

When using JetBrains Compose, you can have a LifecycleRegistry react to changes in the window state using the LifecycleController() composable. This will trigger appropriate lifecycle events when the window is minimized, restored or closed.

It is also possible to manually start the lifecycle using LifecycleRegistry.resume() when the instance is created.

fun main() {
    val lifecycle = LifecycleRegistry()
    val root = RootComponent(DefaultComponentContext(lifecycle))

    // Alternative: manually start the lifecycle (no reaction to window state)
    // lifecycle.resume()

    application {
        val windowState = rememberWindowState()

        // Bind the registry to the life cycle of the window
        LifecycleController(lifecycle, windowState)

        Window(state = windowState, ...) {
            // The rest of your content
        }
    }
}

Warning

When using Compose in desktop platforms, make sure to always use one of the methods above, or your components might not receive lifecycle events correctly.

The Child Stack navigation model provides ChildStack as Value<ChildStack> that can be observed in a Composable component. This makes it possible to switch child Composable components following the ChildStack changes.

Both Compose extension modules provide the Children(...) function which has the following features:

  • It listens for the ChildStack changes and displays the corresponding child Composable component using the provided slot lambda.
  • It preserves components' UI state (e.g. scrolling position) in the back stack and over configuration changes and process death.
  • It animates between children if there is an animation spec provided.

Here is an example of switching child components on navigation:

// Root

interface RootComponent {
    val childStack: Value<ChildStack<*, Child>>

    sealed class Child {
        data class MainChild(val component: MainComponent) : Child()
        data class DetailsChild(val component: DetailsComponent) : Child()
    }
}

@Composable
fun RootContent(rootComponent: RootComponent) {
    Children(rootComponent.childStack) {
        when (val child = it.instance) {
            is MainChild -> MainContent(child.component)
            is DetailsChild -> DetailsContent(child.component)
        }
    }
}

// Children

interface MainComponent

interface DetailsComponent

@Composable
fun MainContent(component: MainComponent) {
    // Omitted code
}

@Composable
fun DetailsContent(component: DetailsComponent) {
    // Omitted code
}

Pager-like navigation

Warning

This navigation model is experimental, the API is subject to change.

The Child Pages navigation model provides ChildPages as Value<ChildPages> that can be observed in a Composable component.

Both Compose extension modules provide the Pages(...) function which has the following features:

  • It listens for the ChildPages changes and displays child components using HorizontalPager or VerticalPager (see the related Jetpack Compose documentation).
  • It animates page changes if there is an animation spec provided.
Example
@Composable
fun PagesContent(component: PagesComponent) {
    Pages(
        pages = component.pages,
        onPageSelected = component::selectPage,
        scrollAnimation = PagesScrollAnimation.Default,
    ) { _, page ->
        PageContent(page)
    }
}

@Composable
fun PageContent(component: PageComponent) {
    // Omitted code
}

Animations

Decompose provides Child Animation API for Compose, as well as some predefined animation specs. To enable child animations you need to pass the animation argument to the Children function. There are predefined animators provided by Decompose.

Fade animation

@Composable
fun RootContent(component: RootComponent) {
    Children(
        stack = component.childStack,
        animation = stackAnimation(fade()),
    ) {
        // Omitted code
    }
}

Slide animation

@Composable
fun RootContent(component: RootComponent) {
    Children(
        stack = component.childStack,
        animation = stackAnimation(slide()),
    ) {
        // Omitted code
    }
}

Combining animators

It is also possible to combine animators using the plus operator. Please note that the order matters - the right animator is applied after the left animator.

@Composable
fun RootContent(component: RootComponent) {
    Children(
        stack = component.childStack,
        animation = stackAnimation(fade() + scale())
    ) {
        // Omitted code
    }
}

Separate animations for children

Previous examples demonstrate simple cases, when all children have the same animation. But it is also possible to specify separate animations for children.

@Composable
fun RootContent(component: RootComponent) {
    Children(
        stack = component.childStack,
        animation = stackAnimation { child ->
            when (child.instance) {
                is MainChild -> fade() + scale()
                is DetailsChild -> fade() + slide()
            }
        }
    ) {
        // Omitted code
    }
}

It is also possible to take into account the other child and the animation direction when selecting the animation.

@Composable
fun RootContent(component: RootComponent) {
    Children(
        stack = component.childStack,
        animation = stackAnimation { child, otherChild, direction ->
            // Select and return an animator here
        }
    ) {
        // Omitted code
    }
}

Custom animations

It is also possible to define custom animations.

Implementing StackAnimation manually. This is the most flexible low-level API. The animation block receives the current ChildStack and animates children using the provided content slot.

@Composable
fun RootContent(component: RootComponent) {
    Children(
        stack = rootComponent.childStack,
        animation = someAnimation(),
    ) {
        // Omitted code
    }
}

fun <C : Any, T : Any> someAnimation(): StackAnimation<C, T> =
    StackAnimation { stack: ChildStack<C, T>,
                     modifier: Modifier,
                     content: @Composable (Child.Created<C, T>) -> Unit ->
        // Render each frame here
    }

Using the stackAnimation helper function and implementing StackAnimator. The stackAnimation function takes care of tracking the ChildStack changes. StackAnimator is only responsible for manipulating the Modifier in the given direction, and calling onFinished at the end.

@Composable
fun RootContent(component: RootComponent) {
    Children(
        stack = component.childStack,
        animation = stackAnimation(someAnimator()),
    ) {
        // Omitted code
    }
}

fun someAnimator(): StackAnimator =
    StackAnimator { direction: Direction,
                    isInitial: Boolean,
                    onFinished: () -> Unit,
                    content: @Composable (Modifier) -> Unit ->
        // Manipulate the Modifier in the given direction and call onFinished at the end
    }

Using stackAnimation and stackAnimator helper functions. This is the simplest, but less powerful way. The stackAnimator function takes care of running the animation. Its block has a very limited responsibility - to render the current frame using the provided factor and direction.

@Composable
fun RootContent(component: RootComponent) {
    Children(
        stack = component.childStack,
        animation = stackAnimation(someAnimator()),
    ) {
        // Omitted code
    }
}

fun someAnimator(): StackAnimator =
    stackAnimator { factor: Float,
                    direction: Direction,
                    content: (Modifier) -> Unit ->
        // Render the current frame
    }

Please refer to the predefined animators (fade, slide, etc.) for implementation examples.

Predictive Back Gesture

Warning

Predictive Back Gesture support is experimental, the API is subject to change. For now, please use version 2.1.x.

Child Stack supports the new Android Predictive Back Gesture on all platforms. To enable the gesture, first implement BackHandlerOwner interface in your component with Child Stack, then just pass predictiveBackAnimation to the Children function.

RootComponent
interface RootComponent : BackHandlerOwner {
    val stack: Value<ChildStack<...>>

    fun onBackClicked()
}

class DefaultRootComponent(
    componentContext: ComponentContext,
) : ComponentContext by componentContext, BackHandlerOwner {
    // ComponentContext already implements BackHandlerOwner, no need to implement it separately

    // Omitted body

    override fun onBackClicked() {
        navigation.pop()
    }
}
RootContent
@Composable
fun RootContent(component: RootComponent) {
    Children(
        stack = rootComponent.childStack,
        animation = predictiveBackAnimation(
            backHandler = component.backHandler,
            animation = stackAnimation(fade() + scale()), // Your usual animation here
            onBack = component::onBackClicked,
        ),
    ) {
        // Omitted code
    }
}

Predictive Back Gesture on Android

On Android, the predictive back gesture only works starting with Android T. On Android T, it works only between Activities, if enabled in the system settings. Starting with Android U, the predictive back gesture can be enabled between Child Stack screens inside a single Activity.

Predictive Back Gesture on other platforms

On all other platforms, the predictive back gesture can be enabled by showing a special overlay that automatically handles the gesture and manipulates BackDispatcher as needed.

Initialising the root component
val lifecycle = LifecycleRegistry()
val backDispatcher = BackDispatcher()

val componentContext = 
    DefaultComponentContext(
        lifecycle = lifecycle,
        backHandler = backHandler, // Pass BackDispatcher here
    )

val root = DefaultRootComponent(componentContext = componentContext)
Using Composable PredictiveBackGestureOverlay
PredictiveBackGestureOverlay(
    backDispatcher = backDispatcher, // Use the same BackDispatcher as above
    backIcon = { progress, _ ->
        PredictiveBackGestureIcon(
            imageVector = Icons.Default.ArrowBack,
            progress = progress,
        )
    },
    modifier = Modifier.fillMaxSize(),
) {
    RootContent(
        component = root,
        modifier = Modifier.fillMaxSize(),
    )
}

Compose for iOS, macOS and Web (Canvas)

Compose for iOS, macOS and Web (Canvas) is still work in progress and was not officially announced. However, Decompose already supports it. The support is also experimental and is not part of the main branch - see #74 for more information.

If you want to use Decompose with Compose for iOS/macOS/Web, you have to use special versions of both decompose and extensions-compose-jetbrains modules.

implementation "com.arkivanov.decompose:decompose:<version>-compose-experimental"
implementation "com.arkivanov.decompose:extensions-compose-jetbrains:<version>-compose-experimental"

implementation("com.arkivanov.decompose:decompose:<version>-compose-experimental")
implementation("com.arkivanov.decompose:extensions-compose-jetbrains:<version>-compose-experimental")

Samples

You can find samples in a separate branch - compose-darwin/sample/app-darwin-compose.