StickyHeaderItemDecoration

StickyHeaderItemDecoration.kt

/**
    Solution based on Sevastyan answer on StackOverflow
    https://stackoverflow.com/questions/32949971/how-can-i-make-sticky-headers-in-recyclerview-without-external-lib/44327350#44327350
*/

import android.graphics.Canvas
import android.graphics.Rect
import android.view.MotionEvent
import android.view.View
import android.view.ViewGroup
import androidx.annotation.DimenRes
import androidx.core.graphics.withSave
import androidx.core.graphics.withTranslation
import androidx.core.view.updatePadding
import androidx.recyclerview.widget.RecyclerView

class StickyHeaderItemDecoration(
    parent: RecyclerView,
    @DimenRes horizontalHeaderOffset: Int,
    @DimenRes topHeaderOffset: Int? = null,
    @DimenRes bottomHeaderOffset: Int? = null,
    private val isHeader: (itemPosition: Int) -> Boolean
) : RecyclerView.ItemDecoration() {

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

    private val horizontalOffset = parent.context.resources.getDimensionPixelSize(horizontalHeaderOffset)
    private val topHeaderOffset = topHeaderOffset?.let {
        parent.context.resources.getDimensionPixelOffset(topHeaderOffset)
    } ?: 0
    private val bottomHeaderOffset = bottomHeaderOffset?.let {
        parent.context.resources.getDimensionPixelOffset(bottomHeaderOffset)
    } ?: 0

    init {
        parent.adapter?.onDataChanged {
            // clear saved header as it can be outdated now
            currentHeader = null
        }

        parent.addOnLayoutChangeListener { _, _, _, _, _, _, _, _, _ ->
            currentHeader = null
        }

        // handle click on sticky header
        parent.interceptTouchEvent { _, motionEvent ->
            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 topChildPosition = parent.getChildAt(0)?.let { topChild ->
            parent.getChildAdapterPosition(topChild).safe()
        } ?: return

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

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

        val nextHeaderPosition = parent.getChildAdapterPosition(nextHeader).safe() ?: return
        if (isHeader(nextHeaderPosition)) {
            c.moveHeader(headerView, nextHeader)
        } else {
            c.drawHeader(headerView)
        }
    }

    override fun getItemOffsets(outRect: Rect, view: View, parent: RecyclerView, state: RecyclerView.State) {
        val position = parent.getChildAdapterPosition(view).safe() ?: return
        if(isHeader(position)) {
            view.updatePadding(
                left = horizontalOffset,
                top = topHeaderOffset,
                right = horizontalOffset,
                bottom = bottomHeaderOffset
            )
        }
    }

    private fun RecyclerView.getHeaderViewForItem(itemPosition: Int): View? {
        val adapter = adapter ?: return null

        val headerPosition = getHeaderPositionForItem(itemPosition).safe() ?: return null
        val headerType = adapter.getItemViewType(headerPosition)

        // if match reuse viewHolder
        if (currentHeader?.first == headerPosition && currentHeader?.second?.itemViewType == headerType) {
            return currentHeader?.second?.itemView
        }

        val headerHolder = adapter.createViewHolder(this, headerType)
        adapter.onBindViewHolder(headerHolder, headerPosition)
        headerHolder.itemView.updatePadding(
            left = horizontalOffset,
            top = topHeaderOffset,
            right = horizontalOffset,
            bottom = bottomHeaderOffset
        )
        headerHolder.itemView.measuresAndLayoutsTheTopStickyHeader(this)

        // save for next draw
        currentHeader = headerPosition to headerHolder
        return headerHolder.itemView
    }

    private fun Canvas.drawHeader(header: View) = withSave {
        header.draw(this)
    }

    private fun Canvas.moveHeader(currentHeader: View, nextHeader: View) {
        val verticalDistanceToTranslate = (nextHeader.top - currentHeader.height).toFloat()
        withTranslation(y = verticalDistanceToTranslate) {
            currentHeader.draw(this)
        }
    }

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

    /**
     * [parent] ViewGroup: RecyclerView in this case.
     */
    private fun View.measuresAndLayoutsTheTopStickyHeader(parent: ViewGroup) {

        // 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,
            layoutParams.width
        )
        val childHeightSpec = ViewGroup.getChildMeasureSpec(
            heightSpec,
            parent.paddingTop + parent.paddingBottom,
            layoutParams.height
        )

        measure(childWidthSpec, childHeightSpec)
        layout(0, 0, measuredWidth, 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
    }

    private fun Int.safe(): Int? = takeIf { it != RecyclerView.NO_POSITION }
}

fun RecyclerView.Adapter<*>.onDataChanged(action: () -> Unit) {
    registerAdapterDataObserver(object : RecyclerView.AdapterDataObserver() {
        override fun onChanged() {
            action()
        }
    })
}

fun RecyclerView.interceptTouchEvent(interceptAction: (RecyclerView, MotionEvent) -> Boolean) {
    addOnItemTouchListener(object : RecyclerView.SimpleOnItemTouchListener() {
        override fun onInterceptTouchEvent(
            recyclerView: RecyclerView,
            motionEvent: MotionEvent
        ): Boolean = interceptAction(recyclerView, motionEvent)
    })
}