Files for Sticky Header Implementation

BaseListAdapter.kt

abstract class BaseListAdapter<T: ListItem, VH : DataBindingBaseViewHolder<T>>
    : ListAdapter<T, VH>(ListItemDiffUtils()) {

}

DataBindingBaseViewHolder.kt

abstract class DataBindingBaseViewHolder<T : ListItem>(
    open val binding: ViewDataBinding
) : RecyclerView.ViewHolder(binding.root) {
    abstract fun onBind(item: T)
}

ListItem.kt

interface ListItem {
    fun getViewType(): Int
    fun areItemsTheSame(newItem: ListItem): Boolean
    fun areContentsTheSame(newItem: ListItem): Boolean
}

StickyHeaderAdapter.kt

abstract class StickyHeaderAdapter<T : StickyHeaderListItem, VH : StickyHeaderViewHolder<T>> :
    BaseListAdapter<T, VH>(), StickyHeaderDecoration.SectionCallback {

    override fun isHeader(position: Int): Boolean = getItem(position).isHeader()

    override fun getHeaderLayoutView(
        parent: RecyclerView,
        itemPosition: Int
    ): View? = getHeaderLayoutView(parent, itemPosition, itemPosition)

    override fun getHeaderLayoutView(
        parent: RecyclerView,
        itemPosition: Int,
        viewHolderPosition: Int
    ): View? {
        val item = getItem(itemPosition)
        if (!item.isHeader()) {
            return null
        }

        val viewHolder = parent
            .findViewHolderForAdapterPosition(viewHolderPosition) as? StickyHeaderViewHolder<StickyHeaderListItem>

        if (viewHolder?.itemViewType != item.getViewType()) {
            return null
        }

        val view = viewHolder.getHeaderView(parent) ?: return null
        return viewHolder.bind(view, getItem(itemPosition))
    }

    override fun updateHeaderData(
        parent: RecyclerView,
        currentHeaderView: View,
        topChildPosition: Int
    ): View? {
        val headerItemPosition = (0..topChildPosition).toList()
            .sortedDescending()
            .firstOrNull { getItem(it).isHeader() }
            ?: return currentHeaderView

        val emptyViewHolder = onCreateViewHolder(parent, getItem(headerItemPosition).getViewType())
        return emptyViewHolder.bind(currentHeaderView, getItem(headerItemPosition))
    }
}

StickyHeaderDecoration.kt

class StickyHeaderDecoration(private val sectionCallback: SectionCallback) :
    RecyclerView.ItemDecoration() {

    var currentHeader: View? = null
    var currentHeaderPosition = RecyclerView.NO_POSITION

    override fun onDrawOver(c: Canvas, parent: RecyclerView, state: RecyclerView.State) {
        super.onDrawOver(c, parent, state)
        updateHeaderView(parent)
        presentHeaderView(c, parent)
    }

    private fun updateHeaderView(parent: RecyclerView) {
        val topChild = parent.getChildAt(0) ?: return

        val topChildPosition = parent.getChildAdapterPosition(topChild)
        if (topChildPosition == RecyclerView.NO_POSITION) {
            return
        }

        if (currentHeader != null && currentHeaderPosition != topChildPosition) {
            sectionCallback.updateHeaderData(parent, currentHeader!!, topChildPosition)
        }

        val view = sectionCallback.getHeaderLayoutView(parent, topChildPosition) ?: return

        updateCurrentHeaderInfo(parent, view, topChildPosition, topChild)
    }

    private fun updateCurrentHeaderInfo(
        parent: RecyclerView,
        newHeaderView: View,
        headerViewPosition: Int,
        headerItemView: View
    ) {
        currentHeader = newHeaderView
        currentHeaderPosition = headerViewPosition
        fixLayoutSize(parent, currentHeader!!, headerItemView.measuredHeight)
    }

    private fun presentHeaderView(c: Canvas, parent: RecyclerView) {
        if (currentHeader == null) {
            return
        }

        val contactPoint = currentHeader!!.bottom

        val childInContact: View = getChildInContact(parent, contactPoint) ?: return

        val childAdapterPosition = parent.getChildAdapterPosition(childInContact)
        if (childAdapterPosition == RecyclerView.NO_POSITION) {
            return
        }

        when {
            sectionCallback.isHeader(childAdapterPosition) ->
                moveHeader(c, currentHeader!!, childInContact)
            else ->
                drawHeader(c, currentHeader!!)
        }
    }

    private fun getChildInContact(parent: RecyclerView, contactPoint: Int): View? {
        var childInContact: View? = null
        for (i in 0 until parent.childCount) {
            val child = parent.getChildAt(i)
            if (child.bottom > contactPoint) {
                if (child.top <= contactPoint) {
                    childInContact = child
                    break
                }
            }
        }
        return childInContact
    }

    private fun moveHeader(c: Canvas, currentHeader: View, nextHeader: View) {
        c.save()
        c.translate(0f, nextHeader.top - currentHeader.height.toFloat())
        currentHeader.draw(c)
        c.restore()
    }

    private fun drawHeader(c: Canvas, header: View) {
        c.save()
        c.translate(0f, 0f)
        header.draw(c)
        c.restore()
    }

    /**
     * Measures the header view to make sure its size is greater than 0 and will be drawn
     * https://yoda.entelect.co.za/view/9627/how-to-android-recyclerview-item-decorations
     */
    private fun fixLayoutSize(parent: ViewGroup, view: View, height: Int) {
        val widthSpec = View.MeasureSpec.makeMeasureSpec(
            parent.width,
            View.MeasureSpec.EXACTLY
        )
        val heightSpec = View.MeasureSpec.makeMeasureSpec(
            parent.height,
            View.MeasureSpec.EXACTLY
        )
        val childWidth: Int = ViewGroup.getChildMeasureSpec(
            widthSpec,
            parent.paddingLeft + parent.paddingRight,
            view.layoutParams.width
        )
        val childHeight: Int = ViewGroup.getChildMeasureSpec(
            heightSpec,
            parent.paddingTop + parent.paddingBottom,
            height
        )
        view.measure(childWidth, childHeight)
        view.layout(0, 0, view.measuredWidth, view.measuredHeight)
    }

    interface SectionCallback {
        fun isHeader(position: Int): Boolean
        fun getHeaderLayoutView(parent: RecyclerView, itemPosition: Int): View?
        fun getHeaderLayoutView(
            parent: RecyclerView,
            itemPosition: Int,
            viewHolderPosition: Int
        ): View?

        fun updateHeaderData(
            parent: RecyclerView,
            currentHeaderView: View,
            topChildPosition: Int
        ): View?
    }
}

StickyHeaderListItem.kt

interface StickyHeaderListItem : ListItem {
    fun isHeader(): Boolean
}

StickyHeaderViewHolder.kt

abstract class StickyHeaderViewHolder<T : StickyHeaderListItem>(binding: ViewDataBinding) :
    DataBindingBaseViewHolder<T>(binding) {

    abstract fun getHeaderView(parent: ViewGroup): View?
    abstract fun bind(view: View, item: T): View?
}

abstract class StickyContentViewHolder<T : StickyHeaderListItem>(binding: ViewDataBinding) :
    StickyHeaderViewHolder<T>(binding) {

    override fun getHeaderView(parent: ViewGroup): View? {
        return null
    }

    override fun bind(view: View, item: T): View? {
        return null
    }
}