Item Decorator for sticky headers in Kotlin

HeaderItemDecoration.kt

package com.filipkowicz.headeritemdecorator

/*
solution based on - based on Sevastyan answer on StackOverflow


changes:
- take to account views offsets
- transformed to Kotlin
- now works on viewHolders
- try to cache viewHolders between draw's
- support for clipToPadding=false

Source:
https://stackoverflow.com/questions/32949971/how-can-i-make-sticky-headers-in-recyclerview-without-external-lib/44327350#44327350
*/

import android.graphics.*
import android.view.MotionEvent
import android.view.View
import android.view.ViewGroup
import androidx.recyclerview.widget.RecyclerView

class HeaderItemDecoration(
    parent: RecyclerView,
    private val shouldFadeOutHeader: Boolean = false,
    private val isHeader: (itemPosition: Int) -> Boolean
) : RecyclerView.ItemDecoration() {

    private var currentHeader: Pair<Int, RecyclerView.ViewHolder>? = null

    init {
        parent.adapter?.registerAdapterDataObserver(object : RecyclerView.AdapterDataObserver() {
            override fun onChanged() {
                // clear saved header as it can be outdated now
                currentHeader = null
            }
        })

        parent.doOnEachNextLayout {
            // clear saved layout as it may need layout update
            currentHeader = null
        }
        // handle click on sticky header
        parent.addOnItemTouchListener(object : RecyclerView.SimpleOnItemTouchListener() {
            override fun onInterceptTouchEvent(
                recyclerView: RecyclerView,
                motionEvent: MotionEvent
            ): Boolean {
                return if (motionEvent.action == MotionEvent.ACTION_DOWN) {
                    motionEvent.y <= currentHeader?.second?.itemView?.bottom ?: 0
                } else false
            }
        })
    }

    override fun onDrawOver(c: Canvas, parent: RecyclerView, state: RecyclerView.State) {
        super.onDrawOver(c, parent, state)
        //val topChild = parent.getChildAt(0) ?: return
        val topChild = parent.findChildViewUnder(
            parent.paddingLeft.toFloat(),
            parent.paddingTop.toFloat() /*+ (currentHeader?.second?.itemView?.height ?: 0 )*/
        ) ?: return
        val topChildPosition = parent.getChildAdapterPosition(topChild)
        if (topChildPosition == RecyclerView.NO_POSITION) {
            return
        }

        val headerView = getHeaderViewForItem(topChildPosition, parent) ?: return

        val contactPoint = headerView.bottom + parent.paddingTop
        val childInContact = getChildInContact(parent, contactPoint) ?: return

        if (isHeader(parent.getChildAdapterPosition(childInContact))) {
            moveHeader(c, headerView, childInContact, parent.paddingTop)
            return
        }

        drawHeader(c, headerView, parent.paddingTop)
    }

    private fun getHeaderViewForItem(itemPosition: Int, parent: RecyclerView): View? {
        if (parent.adapter == null) {
            return null
        }
        val headerPosition = getHeaderPositionForItem(itemPosition)
        if (headerPosition == RecyclerView.NO_POSITION) return null
        val headerType = parent.adapter?.getItemViewType(headerPosition) ?: return null
        // if match reuse viewHolder
        if (currentHeader?.first == headerPosition && currentHeader?.second?.itemViewType == headerType) {
            return currentHeader?.second?.itemView
        }

        val headerHolder = parent.adapter?.createViewHolder(parent, headerType)
        if (headerHolder != null) {
            parent.adapter?.onBindViewHolder(headerHolder, headerPosition)
            fixLayoutSize(parent, headerHolder.itemView)
            // save for next draw
            currentHeader = headerPosition to headerHolder
        }
        return headerHolder?.itemView
    }

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

    private fun moveHeader(c: Canvas, currentHeader: View, nextHeader: View, paddingTop: Int) {
        c.save()
        if (!shouldFadeOutHeader) {
            c.clipRect(0, paddingTop, c.width, paddingTop + currentHeader.height)
        } else {
            if (android.os.Build.VERSION.SDK_INT >= android.os.Build.VERSION_CODES.LOLLIPOP) {
                c.saveLayerAlpha(
                    RectF(0f, 0f, c.width.toFloat(), c.height.toFloat()),
                    (((nextHeader.top - paddingTop) / nextHeader.height.toFloat()) * 255).toInt()
                )
            } else {
                c.saveLayerAlpha(
                    0f, 0f, c.width.toFloat(), c.height.toFloat(),
                    (((nextHeader.top - paddingTop) / nextHeader.height.toFloat()) * 255).toInt(),
                    Canvas.ALL_SAVE_FLAG
                )
            }

        }
        c.translate(0f, (nextHeader.top - currentHeader.height).toFloat() /*+ paddingTop*/)

        currentHeader.draw(c)
        if (shouldFadeOutHeader) {
            c.restore()
        }
        c.restore()
    }

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

    private fun getHeaderPositionForItem(itemPosition: Int): Int {
        var headerPosition = RecyclerView.NO_POSITION
        var currentPosition = itemPosition
        do {
            if (isHeader(currentPosition)) {
                headerPosition = currentPosition
                break
            }
            currentPosition -= 1
        } while (currentPosition >= 0)
        return headerPosition
    }
}

inline fun View.doOnEachNextLayout(crossinline action: (view: View) -> Unit) {
    addOnLayoutChangeListener { view, _, _, _, _, _, _, _, _ ->
        action(
            view
        )
    }
}

Hi, there is an expandable adapter, this adapter is successful with HeaderItemDecoration and sticky structure. When I click on the header, I cannot get the click event in the adapter. It works in non-sticky state, but not on the continuous side.

How can I solve this?

@jahirfiquitiva It's because of padding, I solved this problem by rewriting some parts of the code. At the same time I had to get rid of shouldFadeOutHeader.

    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 headerView = getHeaderViewForItem(topChildPosition, parent) ?: return

        val contactPoint = headerView.bottom
        val childInContact = getChildInContact(parent, contactPoint) ?: return
        if (parent.computeVerticalScrollOffset() > parent.paddingTop.px) {
            if (isHeader(parent.getChildAdapterPosition(childInContact))) {
                moveHeader(c, headerView, childInContact, 0)
                return
            }
            drawHeader(c, headerView, 0)
        }
    }

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

Hi, I wanted to point out to a bug just in case some one else struggles with like me.

There is a bug at addOnItemTouchListener(). It will intercept all touch events on top of the sticky header around same height as the sticky header, even if the event is not on the sticky header.
To replicate the bug, you can scroll the RecyclerView and then try to click on the RecyclerView item on top of the sticky header.
This seems to be due to wrong calculation of the touch event position.

Removing parent.addOnItemTouchListener foxes this issue.