Flip animation for any view (Also supports two views) - Kotlin

FlipAnimation.kt

/**
 * Creates a 3D flip animation between two views.
 *
 * @param fromView First view in the transition.
 * @param toView Second view in the transition.
 */
class FlipAnimation(private var fromView: View?, private var toView: View?) : Animation() {
    private var camera: Camera? = null

    private var centerX: Float = 0.toFloat()
    private var centerY: Float = 0.toFloat()
    private var forward = true

    init {
        duration = 500
        fillAfter = false
        interpolator = AccelerateDecelerateInterpolator()
    }

    fun reverse() {
        forward = false
        val switchView = toView
        toView = fromView
        fromView = switchView
    }

    override fun initialize(width: Int, height: Int, parentWidth: Int, parentHeight: Int) {
        super.initialize(width, height, parentWidth, parentHeight)
        centerX = (width / 2).toFloat()
        centerY = (height / 2).toFloat()
        camera = Camera()
    }

    override fun applyTransformation(interpolatedTime: Float, t: Transformation) {
        // Angle around the y-axis of the rotation at the given time
        // calculated both in radians and degrees.
        val radians = Math.PI * interpolatedTime
        var degrees = (180.0 * radians / Math.PI).toFloat()

        // Once we reach the midpoint in the animation, we need to hide the
        // source view and show the destination view. We also need to change
        // the angle by 180 degrees so that the destination does not come in
        // flipped around
        if (interpolatedTime >= 0.5f) {
            degrees -= 180f
            fromView!!.visibility = View.GONE
            toView!!.visibility = View.VISIBLE
        }

        if (forward)
            degrees = -degrees //determines direction of rotation when flip begins

        val matrix = t.matrix
        camera!!.save()
        camera!!.rotateY(degrees)
        camera!!.getMatrix(matrix)
        camera!!.restore()
        matrix.preTranslate(-centerX, -centerY)
        matrix.postTranslate(centerX, centerY)
    }
}

MainActivity.kt

private fun animateFab(v: View, v2: View) { // If it's only one view pass a view for both
    val flipAnimation = FlipAnimation(v, v2)
    if (v.visibility == View.GONE) {
        flipAnimation.reverse()
    } else {
        v.startAnimation(flipAnimation)
    }
}