RecyclerSticktHeader.kt

import android.graphics.Canvas
import android.view.View
import android.view.ViewGroup
import androidx.recyclerview.widget.RecyclerView
import androidx.recyclerview.widget.RecyclerView.ItemDecoration

class StickHeaderItemDecoration(private val adapter: StickyAdapter) : ItemDecoration() {
    private var stickyHeaderHeight = 0
    private var currentTopChildPosition: Int? = null
    private var currentTopChildHeaderPosition: Int? = null
    private var currentHeaderViewHolder: RecyclerView.ViewHolder? = null
    override fun onDrawOver(c: Canvas, parent: RecyclerView, state: RecyclerView.State) {
        super.onDrawOver(c, parent, state)
        val topChild = parent.getChildAt(0) ?: return
        val topChildPosition = parent.getChildAdapterPosition(topChild)
        if (topChildPosition == RecyclerView.NO_POSITION) {
            return
        }
        val headerPos = if (currentTopChildPosition == topChildPosition) {
            currentTopChildHeaderPosition
        } else {
            currentHeaderViewHolder = null
            currentTopChildHeaderPosition = getHeaderPositionForItem(topChildPosition)
            currentTopChildHeaderPosition
        }
        currentTopChildPosition = topChildPosition
        if (headerPos != null) {
            val currentHeader = if (currentHeaderViewHolder != null) {
                currentHeaderViewHolder!!.itemView
            } else {
                findHeaderViewForItem(headerPos, parent)
            }
            val contactPoint = currentHeader.bottom
            val childInContact = getChildInContact(parent, contactPoint, headerPos)
            if (childInContact != null && adapter.isStickyHeader(
                    parent.getChildAdapterPosition(
                        childInContact
                    )
                )
            ) {
                moveHeader(c, currentHeader, childInContact)
                return
            }
            drawHeader(c, currentHeader)
        }
    }
    private fun getHeaderPositionForItem(itemPosition: Int): Int? {
        for (i in itemPosition downTo 0) {
            if (adapter.isStickyHeader(i)) {
                return i
            }
        }
        return null
    }
    private fun findHeaderViewForItem(headerPosition: Int, parent: RecyclerView): View {
        val headerType = adapter.getItemViewType(headerPosition)
        val viewHolder = adapter.onCreateViewHolder(parent, headerType)
        val header = viewHolder.itemView
        adapter.onBindViewHolder(viewHolder, headerPosition)
        fixLayoutSize(parent, header)
        currentHeaderViewHolder = viewHolder
        return header
    }
    private fun drawHeader(c: Canvas, header: View) {
        c.save()
        c.translate(0f, 0f)
        header.draw(c)
        c.restore()
    }
    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 getChildInContact(
        parent: RecyclerView,
        contactPoint: Int,
        currentHeaderPos: Int
    ): View? {
        var childInContact: View? = null
        for (i in 0 until parent.childCount) {
            var heightTolerance = 0
            val child = parent.getChildAt(i)
            //measure height tolerance with child if child is another header
            if (currentHeaderPos != i) {
                val isChildHeader = adapter.isStickyHeader(parent.getChildAdapterPosition(child))
                if (isChildHeader) {
                    heightTolerance = stickyHeaderHeight - child.height
                }
            }
            //add heightTolerance if child top be in display area
            var childBottomPosition: Int
            childBottomPosition = if (child.top > 0) {
                child.bottom + heightTolerance
            } else {
                child.bottom
            }
            if (childBottomPosition > contactPoint) {
                if (child.top <= contactPoint) {
                    // This child overlaps the contactPoint
                    childInContact = child
                    break
                }
            }
        }
        return childInContact
    }
    /**
     * Properly measures and layouts the top sticky header.
     * @param parent ViewGroup: RecyclerView in this case.
     */
    private fun fixLayoutSize(parent: ViewGroup, view: View) {
        // Specs for parent (RecyclerView)
        val widthSpec = View.MeasureSpec.makeMeasureSpec(parent.width, View.MeasureSpec.EXACTLY)
        val heightSpec =
            View.MeasureSpec.makeMeasureSpec(parent.height, View.MeasureSpec.UNSPECIFIED)
        // Specs for children (headers)
        val childWidthSpec = ViewGroup.getChildMeasureSpec(
            widthSpec,
            parent.paddingLeft + parent.paddingRight,
            view.layoutParams.width
        )
        val childHeightSpec = ViewGroup.getChildMeasureSpec(
            heightSpec,
            parent.paddingTop + parent.paddingBottom,
            view.layoutParams.height
        )
        view.measure(childWidthSpec, childHeightSpec)
        view.layout(0, 0, view.measuredWidth, view.measuredHeight.also {
            stickyHeaderHeight = it
        })
    }
    fun attach(recyclerView: RecyclerView) {
        recyclerView.addItemDecoration(this)
    }
    fun detach(recyclerView: RecyclerView) {
        recyclerView.removeItemDecoration(this)
    }
    interface StickyAdapter {
        /**
         * It's directly available in an adapter
         * This method gets called by [StickHeaderItemDecoration] to get layout ViewHolder and View
         */
        fun onCreateViewHolder(viewGroup: ViewGroup, viewType: Int): RecyclerView.ViewHolder
        /**
         * This method gets called by [StickHeaderItemDecoration] to setup the header ViewHolder.
         * It's directly available in an adapter
         */
        fun onBindViewHolder(viewHolder: RecyclerView.ViewHolder, position: Int)
        /**
         * This method gets called by [StickHeaderItemDecoration] to verify whether the item represents a header.
         * @param itemPosition int.
         * @return true, if item at the specified adapter's position represents a header.
         */
        fun isStickyHeader(itemPosition: Int): Boolean
        fun getItemViewType(position: Int): Int
    }
}