Android simple shrinking circle animation

ShrinkCircleAnimation.kt

import android.content.Context
import android.graphics.Canvas
import android.graphics.Paint
import android.view.View
import android.view.animation.Animation

/* How to use
Call as follows in an Activity

        val circleView = CircleView(this)
        setContentView(circleView)

        val circleAnimation = ShrinkCircleAnimation(circleView, 10, 300)
        circleAnimation.setDuration(5000)
        circleView.startAnimation(circleAnimation)

 */
class CircleView(context: Context?) : View(context) {
    var radius = 0f

    override fun onDraw(canvas: Canvas) {
        val centerX = (canvas.width /2).toFloat()
        val centerY = (canvas.height /2).toFloat()
        val paint: Paint = Paint().apply { setARGB(255, 0, 255, 255) }
            .apply{setStyle(Paint.Style.STROKE)}.apply { setStrokeWidth(2.0F) }
        canvas.drawCircle(centerX, centerY, radius, paint)
    }
}

class ShrinkCircleAnimation internal constructor(val circleView: CircleView, private val start: Int, private val end: Int) : Animation() {
    protected override fun applyTransformation(
        interpolatedTime: Float, transformation: android.view.animation.Transformation
    ) {
        // interpolatedTime: 0.f -> 1.0f
        val radius = (end -((end - start) * interpolatedTime))

        circleView.radius = radius
        circleView.requestLayout()
    }
}