RecyclerView的实现有以下几个步骤
实现Adapter
实现ViewHolder
实现分割线ItemDecoration
本期的吸顶效果主要就是围绕ItemDecoration 做文章
首先先定义一个bean

data class Star(
    val name: String,
    val group: String
)

然后是Adapter的实现

class StickyAdapter(val list: List<Star>): RecyclerView.Adapter<StickyAdapter.StickyViewHolder>() {
    override fun onCreateViewHolder(parent: ViewGroup, viewType: Int): StickyViewHolder {
        val view = LayoutInflater.from(parent.context).inflate(R.layout.item_star, null)
        return StickyViewHolder(view)
    }

    override fun onBindViewHolder(holder: StickyViewHolder, position: Int) {
        holder.tvStar.text = list[position].name
    }

    override fun getItemCount(): Int {
        return list.size
    }

    class StickyViewHolder(itemView: View) : RecyclerView.ViewHolder(itemView) {
        val tvStar: TextView = itemView.findViewById(R.id.tv_star)
    }
}

接下来是定义分割线

class StickyDecoration(val context: Context) : RecyclerView.ItemDecoration() {
    override fun onDraw(c: Canvas, parent: RecyclerView, state: RecyclerView.State) {
        super.onDraw(c, parent, state)
    }
    override fun onDrawOver(c: Canvas, parent: RecyclerView, state: RecyclerView.State) {
        super.onDrawOver(c, parent, state)
    }
    override fun getItemOffsets(
        outRect: Rect,
        view: View,
        parent: RecyclerView,
        state: RecyclerView.State
    ) {
    }
}

通过 RecyclerView 的 addItemDecoration可以添加自定义的分割效果，这里的吸顶效果主要就在 StickyDecoration 中实现
首先我们要理解这3个待实现方法的含义
getItemOffsets 的含义就是设置留白区域，outRect.set(0, 1, 0, 0) 此段代码的含义就是设置每个itemView上方的留白区域为1像素高度。
onDraw 就是在留白区域绘制，如果超出留白区域就会被其他的item 覆盖
onDrawOver就是在任何区域绘制。会覆盖其他区域
不难想到我们要实现吸顶效果需要的三个步骤：

1. 设置留白区域 getItemOffsets

        //获取当前view的位置
        val position = parent.getChildAdapterPosition(view)
        val adapter = parent.adapter
        if (adapter is StickyAdapter) {
            val firstGroup = adapter.isFirstGroup(position)
            // 如果是第一个位置则预留头部高度
            if (firstGroup) {
                outRect.set(0, headerHeight, 0, 0)
            } else {
                outRect.set(0, 1, 0, 0)
            }
        }

2. 在留白区域绘制分割效果 onDraw

        val left: Float = parent.paddingLeft.toFloat()
        val right: Float = (parent.width - parent.paddingRight).toFloat()
        var bottom: Float
        var top: Float

        val adapter = parent.adapter as StickyAdapter
        // 在分割线处画一个底色
        for (i in 0 until parent.childCount) {
            val child = parent.getChildAt(i)
            val pos = parent.getChildLayoutPosition(child)
            val first = adapter.isFirstGroup(pos)
            val star = adapter.list[pos]
            // 对应的留白区域画上颜色
            if (first) {
                bottom = child.top.toFloat()
                top = (child.top - headerHeight).toFloat()
                c.drawRect(left, top, right, bottom, dividerPaint)
                // 找到文字的x, y 坐标，
                val rect = Rect()
                //获取文字的矩形区域
                textPaint.getTextBounds(star.group, 0, star.group.length, rect)
                // 画文字
                c.drawText(
                    star.group,
                    left + 2,
                    top + headerHeight / 2 + rect.height() / 2,
                    textPaint
                )
            } else {
                // 找到上下位置
                bottom = child.top.toFloat()
                top = (child.top - 1).toFloat()
                // 画上分割线
                c.drawRect(left, top, right, bottom, dividerPaint)
            }
        }

3. 吸顶绘制 onDrawOver

        // 画吸顶效果
        val adapter = parent.adapter as StickyAdapter
        // 拿到第一个可见item的位置
        val firstVisiblePos =
            (parent.layoutManager as LinearLayoutManager).findFirstVisibleItemPosition()
        // 找到第一个view
        val view = parent.findViewHolderForAdapterPosition(firstVisiblePos)?.itemView
        val firstViewBottom = view?.bottom?:return
        val star = adapter.list[firstVisiblePos]
        // 吸顶应该展示的位置
        val left = parent.paddingLeft
        val right = parent.width - parent.paddingRight
        val top = parent.paddingTop
        // 是否为顶部
        val isFirstGroup = adapter.isFirstGroup(firstVisiblePos + 1)
        if (isFirstGroup){
            // 底部为 高度和 第一个可见view的最小值
            val bottom = top + Math.min(headerHeight, firstViewBottom)
            c.drawRect(left.toFloat(), top.toFloat(), right.toFloat(), bottom.toFloat(), dividerPaint)
            // 找到文字的x, y 坐标，
            val rect = Rect()
            //获取文字的矩形区域
            textPaint.getTextBounds(star.group, 0, star.group.length, rect)
            // 画文字
 // 设置区域 防止文字超出范围
            c.clipRect(left, top, right, bottom)
            c.drawText(
                star.group,
                left + 2f,
                firstViewBottom - headerHeight / 2f + rect.height() / 2,
                textPaint
            )

        } else {
            // 普通吸顶效果
            val bottom = top + headerHeight
            c.drawRect(left.toFloat(), top.toFloat(), right.toFloat(), bottom.toFloat(), dividerPaint)
            // 找到文字的x, y 坐标，
            val rect = Rect()
            //获取文字的矩形区域
            textPaint.getTextBounds(star.group, 0, star.group.length, rect)
            // 画文字
            c.drawText(
                star.group,
                left + 2f,
                top + headerHeight / 2f + rect.height() / 2,
                textPaint
            )
        }