简单说ItemDecoration就是Item的装饰，在Item的四周，我们可以给它添加上自定义的装饰；
（是用Kotlin实现的，如果有需要java版，可留言回复）

ItemDecoration主要就三个方法 : ） 

getItemOffsets(outRect: Rect, view: View, parent: RecyclerView, state: RecyclerView.State){}onDraw(c: Canvas, parent: RecyclerView, state: RecyclerView.State){}onDrawOver(c: Canvas, parent: RecyclerView, state: RecyclerView.State){}

直接上代码（代码带注释）
1. Activity/Fragment中 : ） 
创建：

private val testRecyclerAdapter by lazy {TestRecyclerAdapter()
}
private val linearLayoutManager by lazy {LinearLayoutManager(context)
}
private val headerDecoration by lazy {StickyHeaderDecoration(requireContext())
}

赋值：

with(rv_view) {layoutManager = linearLayoutManageradapter = testRecyclerAdapteraddItemDecoration(headerDecoration)}

同步更新数据：

val textData = TextDataUtils().getTestData()
textData.sortBy { it.title }//排序
val list = textData.map { bean -> bean.title }//记录每个item分组标题
headerDecoration.setCategoryList(list)//同步分组标题数据Decoration
testRecyclerAdapter.addAllItems(textData)//同步数据至Adapter

2. 接下来就是实现StickyHeaderDecoration
直接上代码 : ） 

class StickyHeaderDecoration(context: Context) : RecyclerView.ItemDecoration() {var hideCategoryHeader: ((isHide: Boolean) -> Unit)? = nullvar updateCategoryHeader: ((categoryName: String) -> Unit)? = nullprivate val paint = Paint(Paint.ANTI_ALIAS_FLAG)private val colorBg = context.resources.getColor(R.color.primary_purple)private val colorText = context.resources.getColor(R.color.primary_white)private val categoryList = mutableListOf<String>()private val categorySet = mutableSetOf<String>()//记录有多少组子标题val categoryHeaderMap = mutableMapOf<String, Int>()//记录每组子标题开始的位置private var categoryName = ""fun setCategoryList(value: List<String>) {categoryList.clear()categoryList.addAll(value)categorySet.clear()categorySet.addAll(value)//如果分组只有一个的情况，即隐藏粘性标题if (categorySet.size > 1) {hideCategoryHeader?.invoke(false)} else {hideCategoryHeader?.invoke(true)}}//设置文字属性private val textPaint = TextPaint(Paint.ANTI_ALIAS_FLAG).apply {color = colorTexttextSize = 18.toSp()}private val headerMarginStart = 36.toDp() //子标题内容与左侧的距离private val headerSpaceHeight = 60.toDp() //为每个子标题对应最后一个item添加空隙高度private val headerBackgroundHeight = 40.toDp()//子标题背景高度private val headerBackgroundRadius = 10.toDp()//为子标题背景设置圆角//简单的理解// 设置item布局间隙（留空间给draw方法绘制）override fun getItemOffsets(outRect: Rect,view: View,parent: RecyclerView,state: RecyclerView.State) {if (isHideInventoryHeader()) returnval adapterPosition = parent.getChildAdapterPosition(view)if (adapterPosition == RecyclerView.NO_POSITION) {return}//Top 头部if (isFirstOfGroup(adapterPosition)) {outRect.top = headerBackgroundHeight.toInt()categoryHeaderMap[categoryList[adapterPosition]] = adapterPosition}//Bottom 底部if (isEndOfGroup(adapterPosition)) {outRect.bottom = headerSpaceHeight.toInt()}}//可在此方法中绘制背景override fun onDraw(canvas: Canvas, parent: RecyclerView, state: RecyclerView.State) {if (isHideInventoryHeader()) returnval count = parent.childCountif (count == 0) {return}for (i in 0 until parent.childCount) {val child = parent.getChildAt(i)val adapterPosition = parent.getChildAdapterPosition(child)if (isFirstOfGroup(adapterPosition)) {val left = child.left.toFloat()val right = child.right.toFloat()val top = child.top.toFloat() - headerBackgroundHeightval bottom = child.top.toFloat()val radius = headerBackgroundRadiuspaint.color = colorBg//绘制背景canvas.drawRoundRect(left, top, right, bottom, radius,radius, paint)}}}//留的空间给draw方法绘制内容/粘性标题也在此设置override fun onDrawOver(canvas: Canvas, parent: RecyclerView, state: RecyclerView.State) {if (isHideInventoryHeader()) returnval count = parent.childCountif (count == 0) {return}//在每个背景上绘制文字drawHeaderTextIndex(canvas, parent)//绘制粘性标题drawStickyTimestampIndex(canvas, parent)}private fun drawHeaderTextIndex(canvas: Canvas, parent: RecyclerView) {for (i in 0 until parent.childCount) {val child = parent.getChildAt(i)val adapterPosition = parent.getChildAdapterPosition(child)if (adapterPosition == RecyclerView.NO_POSITION) {return}if (isFirstOfGroup(adapterPosition)) {val categoryName = categoryList[adapterPosition]val start = child.left + headerMarginStartval fontMetrics = textPaint.fontMetrics//计算文字自身高度val fontHeight = fontMetrics.bottom - fontMetrics.topval baseline =child.top.toFloat() - (headerBackgroundHeight - fontHeight) / 2 - fontMetrics.bottomcanvas.drawText(categoryName.toUpperCase(), start, baseline, textPaint)}}}private fun drawStickyTimestampIndex(canvas: Canvas, parent: RecyclerView) {val layoutManager = parent.layoutManager as LinearLayoutManagerval firstVisiblePosition = layoutManager.findFirstVisibleItemPosition()if (firstVisiblePosition != RecyclerView.NO_POSITION) {val firstVisibleChildView =parent.findViewHolderForAdapterPosition(firstVisiblePosition)?.itemViewfirstVisibleChildView?.let { child ->val firstChild = parent.getChildAt(0)val left = firstChild.left.toFloat()val right = firstChild.right.toFloat()val top = 0.toFloat()val bottom = headerBackgroundHeightval radius = headerBackgroundRadiuspaint.color = colorBgval name = categoryList[firstVisiblePosition]if (categoryName != name) {categoryName = name// 监听当前滚动到的标题categoryName?.let { name ->updateCategoryHeader?.invoke(name)}}val start = child.left + headerMarginStart//计算文字高度val fontMetrics = textPaint.fontMetricsval fontHeight = fontMetrics.bottom - fontMetrics.topval baseline =headerBackgroundHeight - (headerBackgroundHeight - fontHeight) / 2 - fontMetrics.bottomvar upwardBottom = bottomvar upwardBaseline = baseline// 下一个组马上到达顶部if (isFirstOfGroup(firstVisiblePosition + 1)) {upwardBottom = min(child.bottom.toFloat() + headerSpaceHeight, bottom)if (child.bottom.toFloat() + headerSpaceHeight < headerBackgroundHeight) {upwardBaseline = baseline * (child.bottom.toFloat() + headerSpaceHeight)/headerBackgroundHeight}}//绘制粘性标题背景canvas.drawRoundRect(left, top, right, upwardBottom, radius, radius, paint)//绘制粘性标题canvas.drawText(categoryName.toUpperCase(), start, upwardBaseline, textPaint)}}}//判断是不是每组的第一个itemprivate fun isFirstOfGroup(adapterPosition: Int): Boolean {return adapterPosition == 0 || categoryList[adapterPosition] != categoryList[adapterPosition - 1]}//判断是不是每组的最后一个itemprivate fun isEndOfGroup(adapterPosition: Int): Boolean {if (adapterPosition + 1 == categoryList.size) return truereturn categoryList[adapterPosition] != categoryList[adapterPosition + 1]}//如果分组只有一个的情况，即隐藏粘性标题private fun isHideInventoryHeader(): Boolean {return categorySet.size <= 1 || categoryList.isNullOrEmpty()}
}

3.  RecyclerAdapter 我还是贴一下代码，就正常写：）

class TestRecyclerAdapter : RecyclerView.Adapter<TextViewHolder>() {private val textBeans: MutableList<TextBean> = mutableListOf()override fun onCreateViewHolder(parent: ViewGroup, viewType: Int): TextViewHolder {return TextViewHolder(parent.inflate(R.layout.rv_test_item))}override fun getItemCount()= textBeans.sizeoverride fun onBindViewHolder(holder: TextViewHolder, position: Int) {holder.bind(textBeans[position])}fun addAllItems(items: List<TextBean>) {textBeans.clear()textBeans.addAll(items)notifyDataSetChanged()}
}

 ViewHolder：）

class TextViewHolder(view: View): RecyclerView.ViewHolder(view){open fun bind(testText: TextBean) {with(itemView) {item_text.text = testText.desc}itemView.setOnClickListener {//TODO}}
}

cc: 因为是用 Kotlin实现，里面带有Kotlin的扩展方法，我再补上：）

fun ViewGroup.inflate(@LayoutRes id: Int): View {return LayoutInflater.from(this.context).inflate(id, this, false)
}fun Int.toDp(): Float = (this * Resources.getSystem().displayMetrics.density)fun Int.toSp(): Float = (this * Resources.getSystem().displayMetrics.scaledDensity)

Git地址：（稍后贴上）
感谢你的时间！有什么好的建议一起探讨～