CustomView.kt

typealias ColorFunction = (currentTimeMillis: Long) -> Int

class CustomView : View {
    
    val paint = Paint()
    var colorFunction: ColorFunction? = null
    
    override fun onDraw(canvas: Canvas) {
        super.onDraw(canvas)
        
        val cx = width / 2f
        val cy = height / 2f
        val radius = Math.min(cx, cy)
        paint.color = colorFunction?.invoke(System.currentTimeMillis())
        canvas.drawCircle(cx, cy, radius, paint)
    }
}