Code snippet for the `Leveraging the Power of Snapshots in Jetpack Compose` Medium article.

undo-manager-incomplete.kt

@Stable
class UndoManager(private val stickers: () -> List<Sticker>) {

    private val undoHistory = mutableStateListOf<BoardAction>()
    private val redoHistory = mutableStateListOf<BoardAction>()

    fun register(action: BoardAction) {
        val invertedAction = action.invert(oldSticker = ???)
        undoHistory.add(invertedAction)
        redoHistory.clear()
    }

    suspend fun undo(): List<Sticker> = applyAndMove(
        from = undoHistory,
        to = redoHistory,
        stickers = stickers()
    )

    suspend fun redo(): List<Sticker> = applyAndMove(
        from = redoHistory,
        to = undoHistory,
        stickers = stickers()
    )
}

private suspend fun applyAndMove(
    from: MutableList<BoardAction>,
    to: MutableList<BoardAction>,
    stickers: List<Sticker>
): List<Sticker> {
    if (from.isEmpty()) return stickers
    return when (val action = from.removeLast()) {
    
        is BoardAction.Add -> {
            to.add(action.invert(action.sticker))
            stickers + action.sticker
        }

        is BoardAction.Remove -> {
            val sticker = stickers.last()
            to.add(action.invert(sticker))
            stickers - sticker
        }

        is BoardAction.Transform -> {
            stickers.onEach { sticker ->
                if (sticker.id == action.id) {
                    to.add(action.invert(sticker))
                    sticker.offset = action.offset
                    sticker.zIndex = action.zIndex
                    sticker.rotationZ = action.rotationZ
                    sticker.scaleX = action.scaleX
                    sticker.scaleY = action.scaleY
                }
            }
        }
    }
}

private fun BoardAction.invert(oldSticker: Sticker): BoardAction = when (this) {
    is BoardAction.Add -> BoardAction.Remove(oldSticker.id)
    is BoardAction.Remove -> BoardAction.Add(oldSticker)
    is BoardAction.Transform -> BoardAction.Transform(oldSticker)
}