RetainStateCrossfade

@Composable
fun <T> RetainStateCrossfade(
    targetState: T,
    modifier: Modifier = Modifier,
    animationSpec: FiniteAnimationSpec<Float> = tween(),
    content: @Composable (T) -> Unit
) {
    val items = remember { mutableStateListOf<RetainStateCrossfadeAnimationItem<T>>() }
    val transitionState = remember { MutableTransitionState(targetState) }
    val targetChanged = (targetState != transitionState.targetState)
    transitionState.targetState = targetState
    val transition = updateTransition(transitionState)
    if (targetChanged || items.isEmpty()) {
        // Only manipulate the list when the state is changed, or in the first run.
        val keys = items.map { it.key }.run {
            if (!contains(targetState)) {
                toMutableList().also { it.add(targetState) }
            } else {
                this
            }
        }
        items.clear()
        keys.mapTo(items) { key ->
            RetainStateCrossfadeAnimationItem(key) {
                val alpha by transition.animateFloat(
                    transitionSpec = { animationSpec }
                ) { if (it == key) 1f else 0f }
                Box(Modifier.graphicsLayer { this.alpha = alpha }) {
                    content(key)
                }
            }
        }
    } else if (transitionState.currentState == transitionState.targetState) {
        // Remove all the intermediate items from the list once the animation is finished.
//        items.removeAll { it.key != transitionState.targetState }
    }

    Box(modifier) {
        items.fastForEach {
            mykey(it.key) {
                it.content()
            }
        }
    }
}

@OptIn(ExperimentalContracts::class)
inline fun <T> List<T>.fastForEach(action: (T) -> Unit) {
    contract { callsInPlace(action) }
    for (index in indices) {
        val item = get(index)
        action(item)
    }
}

@Composable
inline fun <T> mykey(
    @Suppress("UNUSED_PARAMETER")
    vararg keys: Any?,
    block: @Composable () -> T
) = block()

private data class RetainStateCrossfadeAnimationItem<T>(
    val key: T,
    val content: @Composable () -> Unit
)