RecyclerView 四周带分割线的ItemDecoration

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这是效果图：

这是代码(用kotlin写的有点装)：

class GridLayoutDivider(width:Int,color:Int) : RecyclerView.ItemDecoration() {
    private var mWidth = width
    private var mColor = color
    private var mPaint = Paint(Paint.ANTI_ALIAS_FLAG)
    init {
        mPaint.color = mColor
    }

    override fun onDraw(c: Canvas?, parent: RecyclerView?, state: RecyclerView.State?) {
        super.onDraw(c, parent, state)
        draw(c,parent)
    }

    private fun draw(canvas: Canvas?, parent: RecyclerView?) {
        val childSize = parent?.childCount
        for (i in 0 until childSize!!) {
            val child = parent?.getChildAt(i)
            val layoutParams = child?.layoutParams as RecyclerView.LayoutParams
            //画水平分隔线
            var left = child.left - layoutParams.leftMargin - mWidth
            var right = child.right + mWidth
            var top = child.bottom + layoutParams.bottomMargin
            var bottom = top + mWidth
            if (mPaint != null) {
                canvas?.drawRect(left.toFloat(), top.toFloat(), right.toFloat(), bottom.toFloat(), mPaint)
                if(isFirstRow(i,getSpanCount(parent),childSize,parent) ) {
                    top = if(isFirsSpan(i,getSpanCount(parent),childSize)) {
                        child.top + layoutParams.bottomMargin - mWidth
                    }else{
                        child.top + layoutParams.bottomMargin
                    }
                    bottom = top + mWidth
                    canvas?.drawRect(left.toFloat(), top.toFloat(), right.toFloat(), bottom.toFloat(), mPaint)
                }
            }

            //画垂直分割线
            top = child.top - layoutParams.topMargin - mWidth
            bottom = child.bottom + layoutParams.bottomMargin + mWidth

            if (mPaint != null) {
                left = child.right + layoutParams.rightMargin
                right = left + mWidth
                canvas?.drawRect(left.toFloat(), top.toFloat(), right.toFloat(), bottom.toFloat(), mPaint)
                if(isFirsSpan(i,getSpanCount(parent),childSize)) {
                    left = if(isFirstRow(i,getSpanCount(parent),childSize,parent)){
                        child.left + layoutParams.rightMargin - mWidth
                    }else {
                        child.left + layoutParams.rightMargin
                    }
                    right = left + mWidth
                    canvas?.drawRect(left.toFloat(), top.toFloat(), right.toFloat(), bottom.toFloat(), mPaint)
                }
            }
        }
    }


    override fun getItemOffsets(outRect: Rect?, view: View?, parent: RecyclerView?, state: RecyclerView.State?) {
        super.getItemOffsets(outRect, view, parent, state)
        val itemPosition = (view?.layoutParams as RecyclerView.LayoutParams).viewLayoutPosition
        val spanCount = getSpanCount(parent)
        val childCount = parent?.adapter?.itemCount

        val isFirstRow = isFirstRow(itemPosition,spanCount,childCount,parent)
        val left: Int
        val top: Int
        val bottom : Int = mWidth
        val right = mWidth
        left = if(isFirsSpan(itemPosition,spanCount,childCount)) mWidth else 0
        top = if (isFirstRow) mWidth else 0
        outRect?.set(top, left, right, bottom)
    }
    private fun isFirsSpan(position: Int,spanCount: Int,childCount: Int?):Boolean{
        return position % spanCount == 0
    }
    private fun isFirstRow(position: Int, spanCount: Int, childCount: Int?,parent:RecyclerView?): Boolean {
        val layoutManager = parent?.layoutManager
        if(layoutManager is GridLayoutManager){
            return position / spanCount == 0
        }else if( layoutManager is StaggeredGridLayoutManager){
            if(layoutManager.orientation == StaggeredGridLayoutManager.VERTICAL) {
                if (childCount != null) {
                    val i = childCount - childCount!! % spanCount
                    // 如果是最后一行，则不需要绘制底部
                    if (position >= i)
                        return true
                }
            }else {
                // 如果是最后一行，则不需要绘制底部
                if ((position + 1) % spanCount == 0) {
                    return true
                }
            }
        }
        return false
    }

    private fun getSpanCount(parent: RecyclerView?): Int {
        if (parent != null) {
            val layoutManager = parent.layoutManager
            if(layoutManager is GridLayoutManager)
                return layoutManager.spanCount
            else if(layoutManager is StaggeredGridLayoutManager)
                return layoutManager.spanCount
        }
        return 0
    }
}