实现RecyclerView局部背景

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实现以上页面效果，很多开发者会用NestedScrollView去做，然后用recyclerview实现中间菜单区域，在给recyclerview整体做一个带圆角的白色背景图。但是如果整体页面用RecyclerView实现，layoutManager用GribLayoutManager(3),那如何解决以下问题呢？

1.顶部用户信息区域与banner区域做一个整体背景图
2.中间菜单区域使用白色圆角背景图呢
3.菜单区域的最左，最右的菜单按钮，其对应leftmargin与rightmargin不一致

为了解决以上问题，我们得要使用ItemDecoration类，google对于它的解释如下

An ItemDecoration allows the application to add a special drawing and layout offset to specific item views from the adapter's data set. This can be useful for drawing dividers between items, highlights, visual grouping boundaries and more.

解释如下：

ItemDecoration允许应用从适配器的数据向特定的多个Item视图，添加特殊的图形和布局偏移。 这对于在项目，突出显示，可视分组边界等之间绘制分隔线很有用。

public abstract static class ItemDecoration {
    // 在条目绘制之前绘制装饰，即在RecyclerView条目的底层展示。
    public void onDraw(Canvas c, RecyclerView parent, State state) {
    }
    // 在条目绘制之后绘制装饰，即在RecyclerView条目的上层展示。
    public void onDrawOver(Canvas c, RecyclerView parent, State state) {
    }
    // 重新设置条目的边距
    public void getItemOffsets(Rect outRect, View view, RecyclerView parent, State state) {
    }
}

基于ui效果，我们只用在ondraw方法进行操作即可

class BackgroundItemDecoration : RecyclerView.ItemDecoration() {
    private val mTempRect = Rect()
    override fun onDraw(c: Canvas, parent: RecyclerView, state: RecyclerView.State) {
        super.onDraw(c, parent, state)
        val childCount = parent.childCount
        var left = -1
        var top = -1
        var right = -1
        var bottom = -1
        for (i in 0 until childCount) {
            val view = parent.getChildAt(i)
            if (view.visibility != View.GONE && isTargetItem(view, parent)) {
                parent.getDecoratedBoundsWithMargins(view, mTempRect)
                if(left == -1){
                    left = mTempRect.left
                    top = mTempRect.top
                }else if(isRightItem(view,parent)){
                    right = mTempRect.right
                }else{
                    bottom = mTempRect.bottom
                }
            }
        }
        val drawable =  ContextCompat.getDrawable(parent.context,R.drawable.shape_round_10_white_bg)
        drawable?.setBounds(left,top,right,bottom)
        drawable?.draw(c)
    }

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

    }

    private fun isTargetItem(view: View, parent: RecyclerView): Boolean {
        var target = false
        if (view.visibility != View.GONE) {
            val adapter = parent.adapter
            if (adapter != null && adapter.getItemViewType(parent.getChildAdapterPosition(view))
                    != BaseQuickAdapter.HEADER_VIEW && adapter.getItemViewType(parent.getChildAdapterPosition(view))
                    != BaseQuickAdapter.FOOTER_VIEW) {
                target = true
            }
        }
        return target
    }

    private fun isRightItem(view: View, parent: RecyclerView): Boolean {
        val isLeft: Boolean
        val layoutManager = parent.layoutManager
        isLeft = if (layoutManager !is GridLayoutManager) {
            true
        } else {
            val spanIndex: Int = layoutManager.spanSizeLookup
                    .getSpanIndex(parent.getChildAdapterPosition(view),
                            layoutManager.spanCount)
            spanIndex == 2
        }
        return isLeft
    }
}

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不知道大家对以上的ui该如何代码实现布局呢？