SectionItemDecoration.kt

import android.graphics.Canvas
import android.graphics.Rect
import android.support.v7.widget.RecyclerView
import android.view.LayoutInflater
import android.view.View
import android.view.ViewGroup

/**
 * Created with Android Studio
 * User: Sergey Petrov [email protected]
 * Date: 18/06/2018
 * Time: 11:13
 *
 */
class SectionItemDecoration(
        parent: RecyclerView,
        private val sectionCallback: Callback,
        layoutResId: Int,
        private val sticky: Boolean
) : RecyclerView.ItemDecoration() {

    private val sectionView: View by lazy {
        val view = LayoutInflater.from(parent.context).inflate(layoutResId, parent, false)
        fixLayoutSize(view, parent)
        view
    }

    override fun getItemOffsets(outRect: Rect, view: View, parent: RecyclerView, state: RecyclerView.State) {
        super.getItemOffsets(outRect, view, parent, state)

        val position = parent.getChildAdapterPosition(view)
        // we need add space above item if section will draw here
        if (sectionCallback.isSection(position)) {
            outRect.top = sectionView.height
        }
    }

    override fun onDrawOver(canvas: Canvas, parent: RecyclerView, state: RecyclerView.State) {
        super.onDrawOver(canvas, parent, state)
        var previousTitle = ""
        for (i in 0 until parent.childCount) {
            val child = parent.getChildAt(i)
            val position = parent.getChildAdapterPosition(child)

            sectionCallback.bindData(sectionView, position)
            val title = sectionCallback.getTitle(position)
            if (!previousTitle.equals(title, ignoreCase = true) || sectionCallback.isSection(position)) {
                drawSection(canvas, child, sectionView)
                previousTitle = title
            }
        }
    }

    private fun drawSection(canvas: Canvas, child: View, sectionView: View) {
        canvas.save()
        if (sticky) {
            canvas.translate(0f, Math.max(0, child.top - sectionView.height).toFloat())
        } else {
            canvas.translate(0f, (child.top - sectionView.height).toFloat())
        }
        sectionView.draw(canvas)
        canvas.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(view: View, parent: ViewGroup) {
        val widthSpec = View.MeasureSpec.makeMeasureSpec(parent.width, View.MeasureSpec.EXACTLY)
        val heightSpec = View.MeasureSpec.makeMeasureSpec(parent.height, View.MeasureSpec.UNSPECIFIED)

        val childWidth = ViewGroup.getChildMeasureSpec(widthSpec, parent.paddingLeft + parent.paddingRight, view.layoutParams.width)
        val childHeight = ViewGroup.getChildMeasureSpec(heightSpec, parent.paddingTop + parent.paddingBottom, view.layoutParams.height)

        view.measure(childWidth, childHeight)

        view.layout(0, 0, view.measuredWidth, view.measuredHeight)
    }

    interface Callback {

        fun isSection(position: Int): Boolean

        fun bindData(sectionView: View, position: Int)

        fun getTitle(position: Int): String
    }
}