筛选框是app的常用功能，如何实现下图红圈功能呢？

首先左侧目录可以通过一个RecyclerView实现，右侧同样可以实现一个RecyclerView，左侧的RecyclerView中我们想实现圆角的，但是该圆角的View已经不在ViewHolder中，我们很难在ViewHolder中画出两个圆角，所以我们通过其他方法实现

ItemDecoration有三个重要方法：

onDraw：在ViewHolder下方绘制

onDrawOver：在ViewHolder上方绘制

getItemOffsets：ViewHolder的偏移量

其中我们可以通过onDrawOver实现，将两个圆角绘制在ViewHolder上方显示出来

首先，我们搭建一个RecyclerView

//MainActivity
private val mRecyclerView by lazy {
        findViewById<RecyclerView>(R.id.recycler)
    }

    private var mAdapter: MainAdapter? = null
    private var mList: ArrayList<Item> = ArrayList()

    override fun onCreate(savedInstanceState: Bundle?) {
        super.onCreate(savedInstanceState)
        setContentView(R.layout.activity_main)
        initData()
        mAdapter = MainAdapter(mList, object : MainAdapter.OnItemClick {
            override fun onItemClick(view: View, position: Int) {
                for (i in mList) {
                    i.apply {
                        isChecked = false
                    }
                }
                mList[position].apply {
                    isChecked = true
                }
                mAdapter?.notifyDataSetChanged()
            }
        })
        mRecyclerView.adapter = mAdapter
        mRecyclerView.layoutManager = LinearLayoutManager(this)
    }

    private fun initData() {
        mList.clear()
        for (i in 0..10) {
            mList.add(Item("第${i}个"))
        }
    }

    data class Item(val text: String, var isChecked: Boolean = false)

    class MainAdapter(private val list: ArrayList<Item>, private val onItemClick: OnItemClick? = null) : RecyclerView.Adapter<RecyclerView.ViewHolder>() {
        private var context: Context? = null

        override fun onCreateViewHolder(parent: ViewGroup, viewType: Int): RecyclerView.ViewHolder {
            val view = LayoutInflater.from(parent.context).inflate(R.layout.vh_item, parent, false)
            context = parent.context
            return MainViewHolder(view, onItemClick)
        }

        override fun onBindViewHolder(holder: RecyclerView.ViewHolder, position: Int) {
            val item = list[position]
            holder as MainViewHolder
            holder.setText(item.text)
            (if (item.isChecked) context?.let { getColor(it, R.color.white) } else Color.parseColor("#F6F6F6"))?.let { holder.setBackgroundColor(it) }
            holder.itemView.tag = if (item.isChecked) "true" else "false"
        }

        override fun getItemCount(): Int = list.size

        class MainViewHolder(itemView: View, onItemClick: OnItemClick?) : RecyclerView.ViewHolder(itemView) {
            init {
                itemView.setOnClickListener {
                    onItemClick?.onItemClick(it, adapterPosition)
                }
            }

            fun setText(text: String?) {
                itemView.findViewById<TextView>(R.id.item_text).text = text ?: ""
            }

            fun setBackgroundColor(color: Int) {
                itemView.setBackgroundColor(color)
            }
        }

        interface OnItemClick {
            fun onItemClick(view: View, position: Int)
        }
    }

//activity_main
<?xml version="1.0" encoding="utf-8"?>
<androidx.constraintlayout.widget.ConstraintLayout
    xmlns:android="http://schemas.android.com/apk/res/android"
    xmlns:app="http://schemas.android.com/apk/res-auto"
    xmlns:tools="http://schemas.android.com/tools"
    android:layout_width="match_parent"
    android:layout_height="match_parent"
    tools:context=".MainActivity"
    >


    <androidx.recyclerview.widget.RecyclerView
        android:id="@+id/recycler"
        android:layout_width="match_parent"
        android:layout_height="match_parent"
        android:background="#F6F6F6"
        />

</androidx.constraintlayout.widget.ConstraintLayout>

//vh_item
<?xml version="1.0" encoding="utf-8"?>
<androidx.constraintlayout.widget.ConstraintLayout
    xmlns:android="http://schemas.android.com/apk/res/android"
    android:layout_width="match_parent"
    android:layout_height="100dp"
    android:background="#F6F6F6"
    >

    <TextView
        android:id="@+id/item_text"
        android:layout_width="match_parent"
        android:layout_height="match_parent"
        android:text="123"
        android:textSize="16sp"
        android:textColor="@color/black"
        android:gravity="center"/>

</androidx.constraintlayout.widget.ConstraintLayout>

当在选中的时候改变VH的颜色，之后我们需要添加一个ItemDecoration，在ItemDecoration中我们需要知道选中的View，获取View宽高后，进行上下圆角的绘制

class BorderItemDecoration() : RecyclerView.ItemDecoration() {

    override fun onDrawOver(c: Canvas, parent: RecyclerView, state: RecyclerView.State) {
        super.onDrawOver(c, parent, state)
        for (view in parent.children) {
            if (view.tag == "true") {
                drawTop(c, view)
                drawBottom(c, view)
            }
        }
    }

    private fun drawTop(c: Canvas, view: View) {
        val paint = Paint()
        val rect = Rect()
        val top = view.top
        val radius = view.resources.displayMetrics.density * 40
        rect.set((view.width - radius).toInt(), (top - radius).toInt(), view.width, view.top)
        paint.color = Color.WHITE
        paint.style = Paint.Style.FILL
        c.drawRect(rect, paint)

        paint.color = Color.parseColor("#F6F6F6")
        c.drawCircle(view.width - radius, top - radius, radius, paint)
    }

    private fun drawBottom(c: Canvas, view: View) {
        val paint = Paint()
        val rect = Rect()
        val bottom = view.bottom
        val radius = view.resources.displayMetrics.density * 40
        c.translate(0f, 0f)
        rect.set((view.width - radius).toInt(), (bottom + radius).toInt(), view.width, view.bottom)
        paint.color = Color.WHITE
        paint.style = Paint.Style.FILL
        c.drawRect(rect, paint)

        paint.color = Color.parseColor("#F6F6F6")
        c.drawCircle(view.width - radius, bottom + radius, radius, paint)
    }
}

绘制的细节如下：

1. 先绘制获取选中view的坐标，计算出圆角半径的矩形位置，使用白色填充
2. 在圆角半径内画圆，填充为#F6F6F6颜色，剩下的就是圆角白色

上下方法相同。

效果如下：