A custom Android view to render a circular shadow. The radius of the gradient intersects both edges of the view and the startColor stop is the first visible point of the gradient. Customisable depth and gravity via xml.

attrs.xml

<?xml version="1.0" encoding="utf-8"?>
<resources>
	<declare-styleable name="CircularShadowView">
		<attr name="gravity">
			<flag name="bottom" value="80" />
			<flag name="left" value="3" />
			<flag name="right" value="5" />
			<flag name="top" value="48" />
		</attr>
		<attr name="depth" format="dimension" />
		<attr name="startColor" format="color" />
		<attr name="endColor" format="color" />
	</declare-styleable>
</resources>

CircularShadowView.kt

import android.content.Context
import android.graphics.Color
import android.graphics.RadialGradient
import android.graphics.Shader
import android.graphics.drawable.PaintDrawable
import android.graphics.drawable.ShapeDrawable
import android.graphics.drawable.shapes.RectShape
import android.util.AttributeSet
import android.view.Gravity.*
import android.widget.FrameLayout

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

    private var customDepth: Float? = null
    private var endColor = Color.TRANSPARENT
    private var startColor = Color.BLACK
    private var gravity = BOTTOM

    private val shaderFactory = object : ShapeDrawable.ShaderFactory() {
        override fun resize(width: Int, height: Int): Shader {
            val x: Double
            val y: Double
            val radius: Double
            val depth: Float
            when (gravity) {
                BOTTOM, TOP -> {
                    depth = customDepth ?: height.toFloat()
                    x = width.toDouble() / 2
                    radius = ((Math.pow(depth.toDouble(), 2.0) + Math.pow(x, 2.0)) / (2 * depth))
                    y = if (gravity == BOTTOM) height + radius - depth else -radius + depth
                }
                else -> {
                    depth = customDepth ?: width.toFloat()
                    y = height.toDouble() / 2
                    radius = ((Math.pow(depth.toDouble(), 2.0) + Math.pow(y, 2.0)) / (2 * depth))
                    x = if (gravity == LEFT) -radius + depth else width + radius - depth
                }
            }
            val start = (radius - depth) / radius
            return RadialGradient(x.toFloat(), y.toFloat(), radius.toFloat(),
                    intArrayOf(startColor, startColor, endColor),
                    floatArrayOf(0f, start.toFloat(), 1f),
                    Shader.TileMode.CLAMP)
        }
    }

    init {
        if (attrs != null) {
            val a = context.obtainStyledAttributes(attrs, R.styleable.CircularShadowView)
            if (a.hasValue(R.styleable.CircularShadowView_depth)) {
                customDepth = a.getDimension(R.styleable.CircularShadowView_depth, 0f)
            }
            endColor = a.getColor(R.styleable.CircularShadowView_endColor, endColor)
            startColor = a.getColor(R.styleable.CircularShadowView_startColor, startColor)
            gravity = a.getInt(R.styleable.CircularShadowView_gravity, gravity)
            a.recycle()
        }
        val drawable = PaintDrawable()
        drawable.shape = RectShape()
        drawable.shaderFactory = shaderFactory
        background = drawable
    }
}