CopyBackgroundDrawable.kt

/**
 * Sets a background drawable on this view that will draw a slice of [ancestor]'s background that sits underneath
 * this view. This is useful when [ancestor] has a gradient background and you want another child to seamlessly
 * continue that gradient while also having an opaque background to allow for elevation.
 */
fun View.setBackgroundFromAncestor(ancestor: ViewGroup) {
    check(ancestor.isAncestorOf(this))
    if (ancestor.background == null) return
    background = CopyBackgroundDrawable(this, ancestor)
}

private class CopyBackgroundDrawable(private val view: View, private val ancestor: ViewGroup) : Drawable() {
    private val outRect = Rect()

    override fun draw(canvas: Canvas) {
        val background = ancestor.background ?: return
        ancestor.offsetDescendantRect(view, outRect)
        val offsetX = outRect.left.toFloat()
        val offsetY = outRect.top.toFloat()
        canvas.withTranslation(-offsetX, -offsetY) { background.draw(canvas) }
    }

    override fun getOutline(outline: Outline) {
        super.getOutline(outline)
        outline.alpha = 1f
    }

    override fun setAlpha(alpha: Int) {}

    override fun getOpacity(): Int = OPAQUE

    override fun setColorFilter(colorFilter: ColorFilter?) {}
}