A simple implementation of FrameLayout with rounded corners.

RoundedCornersFrameLayout.kt

package com.github.kazukinr.samole.android.widget

import android.content.Context
import android.graphics.Canvas
import android.graphics.Outline
import android.graphics.Path
import android.graphics.RectF
import android.os.Build
import android.util.AttributeSet
import android.view.View
import android.view.ViewOutlineProvider
import android.widget.FrameLayout
import androidx.annotation.RequiresApi

class RoundedCornersFrameLayout @JvmOverloads constructor(
        context: Context,
        attrs: AttributeSet? = null,
        defStyleAttr: Int = 0
) : FrameLayout(context, attrs, defStyleAttr) {

    companion object {

        private const val CORNER_RADIUS_DP = 4
    }
    
    // You can set cornerRadius by attrs or BindingAdapter as you like.
    private val cornerRadius = context.resources.displayMetrics.density * CORNER_RADIUS_DP
    private var path: Path? = null

    init {
        if (Build.VERSION.SDK_INT >= Build.VERSION_CODES.LOLLIPOP) {
            // For Lollipop or later, we can use ViewOutlineProvider to clip.
            outlineProvider = OutlineProvider(cornerRadius)
        }
    }

    override fun onSizeChanged(w: Int, h: Int, oldw: Int, oldh: Int) {
        super.onSizeChanged(w, h, oldw, oldh)

        if (Build.VERSION.SDK_INT < Build.VERSION_CODES.LOLLIPOP) {
            Path().also {
                it.addRoundRect(RectF(0f, 0f, w.toFloat(), h.toFloat()), cornerRadius, cornerRadius, Path.Direction.CW)
                path = it
            }
        }
    }

    override fun dispatchDraw(canvas: Canvas) {
        if (Build.VERSION.SDK_INT < Build.VERSION_CODES.LOLLIPOP) {
            path?.also {
                val saved = canvas.save()
                canvas.clipPath(it)
                super.dispatchDraw(canvas)
                canvas.restoreToCount(saved)
                return
            }
        }
        super.dispatchDraw(canvas)
    }

    @RequiresApi(Build.VERSION_CODES.LOLLIPOP)
    private class OutlineProvider(private val radius: Float) : ViewOutlineProvider() {

        override fun getOutline(view: View, outline: Outline) {
            outline.setRoundRect(0, 0, view.width, view.height, radius)
            view.clipToOutline = true
        }
    }
}