Basic CardView

CardView.kt

class CardView(context: Context, attributeSet: AttributeSet) : View(context, attributeSet) {

    private var outerWidth = 0f
    private var outerHeight = 0f
    private var xCenter = 0f
    private var yCenter = 0f
    private var cornerRadius = 10f
    private var shadowSize = 50f
    private val shadowStartColor = 0x20000000
    private val shadowEndColor = 0x00000000

    private val linearGradientPaint = Paint()

    @SuppressLint("DrawAllocation")
    override fun onMeasure(widthMeasureSpec: Int, heightMeasureSpec: Int) {
        outerWidth = View.MeasureSpec.getSize(widthMeasureSpec).toFloat()
        outerHeight = View.MeasureSpec.getSize(heightMeasureSpec).toFloat()
        xCenter = outerWidth / 2f
        yCenter = outerHeight / 2f
        radialGradientPaint.apply {
            isAntiAlias = true
            strokeJoin = Paint.Join.ROUND
            style = Paint.Style.FILL
            val startRatio = (shadowSize / 4 + cornerRadius) / (cornerRadius + shadowSize)
            Log.d(this::class.java.simpleName, "start ration $startRatio")
            shader = RadialGradient(
                shadowSize + cornerRadius,
                shadowSize + cornerRadius,
                shadowSize + cornerRadius,
                intArrayOf(shadowStartColor, shadowStartColor, shadowEndColor),
                floatArrayOf(0f, startRatio, 1f),
                Shader.TileMode.CLAMP
            )
            isDither = true
        }
        linearGradientPaint.apply {
            isAntiAlias = true
            strokeJoin = Paint.Join.ROUND
            style = Paint.Style.FILL
            shader = LinearGradient(
                xCenter,
                shadowSize,
                xCenter,
                0f,
                intArrayOf(shadowStartColor, shadowStartColor, shadowEndColor),
                floatArrayOf(0f, 0.25f, 1f),
                Shader.TileMode.CLAMP
            )
            isDither = true
        }
        super.onMeasure(widthMeasureSpec, heightMeasureSpec)
    }

        override fun onDraw(canvas: Canvas) {
        super.onDraw(canvas)

        val path = Path()
        path.apply {
            moveTo(shadowSize + cornerRadius, shadowSize + cornerRadius)
            arcTo(0f, 0f, shadowSize * 2 + 2 * cornerRadius, shadowSize * 2 + 2 * cornerRadius, 180f, 90f, false)
            moveTo(shadowSize + cornerRadius, shadowSize + cornerRadius)
            arcTo(shadowSize, shadowSize, shadowSize + 2 * cornerRadius, shadowSize + 2 * cornerRadius, 270f, -90f, false)
        }
        canvas.drawPath(path, radialGradientPaint)
        canvas.drawRect(RectF(cornerRadius + shadowSize, 0f, outerWidth - cornerRadius - shadowSize, shadowSize), linearGradientPaint)
        canvas.save()
        canvas.rotate(180f, xCenter, yCenter)
        canvas.drawRect(RectF(cornerRadius + shadowSize, 0f, outerWidth - cornerRadius - shadowSize, shadowSize), linearGradientPaint)
        canvas.drawPath(path, radialGradientPaint)
        canvas.restore()
        canvas.save()
        canvas.rotate(90f, xCenter, yCenter)
        canvas.drawRect(RectF(cornerRadius + shadowSize, 0f, outerHeight - cornerRadius - shadowSize, shadowSize), linearGradientPaint)
        canvas.drawPath(path, radialGradientPaint)
        canvas.restore()
        canvas.rotate(270f, xCenter, yCenter)
        canvas.drawRect(RectF(cornerRadius + shadowSize, 0f, outerHeight - cornerRadius - shadowSize, shadowSize), linearGradientPaint)
        canvas.drawPath(path, radialGradientPaint)

    }

    private val radialGradientPaint: Paint = Paint()


}