GridOffsetItemDecoration.kt

package com.redplanet.tune.ui.decorations

import android.graphics.Rect
import android.support.annotation.Px
import android.support.v7.widget.GridLayoutManager
import android.support.v7.widget.RecyclerView
import android.view.View

class GridOffsetItemDecoration(@param:Px private val spacing: Int,
                               private val shouldIncludeEdge: Boolean) : RecyclerView.ItemDecoration() {

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

        if (parent.layoutManager !is GridLayoutManager) {
            return
        }

        val gridLayoutManager = parent.layoutManager as GridLayoutManager
        gridLayoutManager.spanSizeLookup.isSpanIndexCacheEnabled = true

        val adapterPosition = parent.getChildAdapterPosition(view)
        if (adapterPosition == RecyclerView.NO_POSITION) {
            return
        }

        val spanSizeLookup = gridLayoutManager.spanSizeLookup
        val spanSizeLookUpCount = spanSizeLookup.getSpanSize(adapterPosition)
        val spanCount = gridLayoutManager.spanCount
        val spanIndex = spanSizeLookup.getSpanIndex(adapterPosition, spanCount)
        val spanGroupIndex = spanSizeLookup.getSpanGroupIndex(adapterPosition, spanCount)
        val halfSpacing = (spacing / 2f).toInt()

        val firstItemOfRow = spanIndex == 0
        val firstRow = spanGroupIndex == 0

        if (spanSizeLookUpCount == spanCount) {
            if (shouldIncludeEdge) {
                outRect.left = spacing
                outRect.top = if (firstItemOfRow) spacing else 0
                outRect.right = spacing
                outRect.bottom = spacing
            } else {
                outRect.top = if (firstRow) 0 else halfSpacing
            }
        } else {
            if (shouldIncludeEdge) {
                outRect.left = if (firstItemOfRow) spacing else halfSpacing
                outRect.top = if (firstRow) spacing else 0
                outRect.right = if (firstItemOfRow) halfSpacing else spacing
                outRect.bottom = spacing
            } else {
                val lastItemOfRow = spanIndex == spanCount - 1

                outRect.left = if (firstItemOfRow) 0 else halfSpacing
                outRect.right = if (lastItemOfRow) 0 else halfSpacing
                outRect.top = if (firstRow) 0 else spacing
            }
        }
    }

}