Delegated Properties for RecyclerView Adapters

AdapterObservableListItem.kt

class ObservableListField<T, VH : RecyclerView.ViewHolder>(
    private val adapter: RecyclerView.Adapter<VH>,
    private val diffUtilCallbackProducer: ((List<T>, List<T>) -> DiffUtil.Callback)?,
    initialValue: List<T>
) : ObservableProperty<List<T>>(initialValue = initialValue) {

    private var result: DiffUtil.DiffResult? = null

    override fun beforeChange(
        property: KProperty<*>,
        oldValue: List<T>,
        newValue: List<T>
    ): Boolean {
        val diffUtilCallback = diffUtilCallbackProducer?.invoke(oldValue, newValue) ?: SimpleDiffUtilCallback(
            oldItems = oldValue,
            newItems = newValue
        )
        result = DiffUtil.calculateDiff(diffUtilCallback)
        return true
    }

    override fun afterChange(property: KProperty<*>, oldValue: List<T>, newValue: List<T>) {
        result?.dispatchUpdatesTo(adapter)
        super.afterChange(property, oldValue, newValue)
    }

    /**
     * Very simple diff util callback
     */
    private inner class SimpleDiffUtilCallback<T> constructor(
        private val oldItems: List<T>,
        private val newItems: List<T>
    ) :
        DiffUtil.Callback() {
        override fun areItemsTheSame(oldItemPosition: Int, newItemPosition: Int): Boolean =
            oldItems[oldItemPosition] == newItems[newItemPosition]

        override fun getOldListSize(): Int = oldItems.size

        override fun getNewListSize(): Int = newItems.size

        override fun areContentsTheSame(oldItemPosition: Int, newItemPosition: Int): Boolean =
            oldItems[oldItemPosition] == newItems[newItemPosition]
    }
}

fun <T, VH : RecyclerView.ViewHolder> RecyclerView.Adapter<VH>.observableList(
    initialValue: List<T>,
    diffUtilCallbackProducer: ((List<T>, List<T>) -> DiffUtil.Callback)? = null
): ObservableProperty<List<T>> {
    return ObservableListField(
        adapter = this,
        diffUtilCallbackProducer = diffUtilCallbackProducer,
        initialValue = initialValue
    )
}