An item decoration for recyclerview which adds margin between items.

MarginItemDecoration.kt

import android.content.Context
import android.graphics.Rect
import android.util.LayoutDirection
import android.view.View
import androidx.annotation.DimenRes
import androidx.recyclerview.widget.GridLayoutManager
import androidx.recyclerview.widget.RecyclerView

class MarginItemDecoration(
    private val marginLeft: Int,
    private val marginTop: Int,
    private val marginRight: Int,
    private val marginBottom: Int
) : RecyclerView.ItemDecoration() {

    companion object {
        fun all(context: Context, @DimenRes resId: Int): MarginItemDecoration {
            val margin = context.resources.getDimensionPixelSize(resId)
            return MarginItemDecoration(margin, margin, margin, margin)
        }

        fun bottom(context: Context, @DimenRes resId: Int): MarginItemDecoration {
            val margin = context.resources.getDimensionPixelSize(resId)
            return MarginItemDecoration(0, 0, 0, margin)
        }
    }

    override fun getItemOffsets(
        outRect: Rect, view: View,
        parent: RecyclerView, state: RecyclerView.State
    ) {
        with(outRect) {
            left = marginLeft
            top = marginTop
            right = marginRight
            bottom = marginBottom
        }
    }
}

class MarginGridItemDecoration(private val margin: Int) : RecyclerView.ItemDecoration() {

    private val halfMargin = margin / 2

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

        val colCount = (parent.layoutManager as GridLayoutManager).spanCount
        val (colIndex, colSize) = (view.layoutParams as GridLayoutManager.LayoutParams).run { spanIndex to spanSize }

        val isRtl = parent.layoutDirection == LayoutDirection.RTL

        if (colIndex == 0)    //First column
            outRect.setStart(isRtl, margin)
        else    //colIndex > 0    //Middle
            outRect.setStart(isRtl, halfMargin)

        if (colIndex + colSize == colCount) //Last column
            outRect.setEnd(isRtl, margin)
        else    //colIndex + colSize < colCount     //Middle
            outRect.setEnd(isRtl, halfMargin)
    }
}

fun Rect.setStart(isRtl: Boolean, start: Int) =
    if (!isRtl) left = start
    else right = start

fun Rect.setEnd(isRtl: Boolean, end: Int) =
    if (!isRtl) right = end
    else left = end