BottomNavChangeHandler.kt

class BottomNavChangeHandler : TransitionChangeHandler() {

  override fun getTransition(container: ViewGroup, from: View?, to: View?,
                             isPush: Boolean): Transition {

    if (from == null || to == null) {
      throw IllegalArgumentException("from and to must not be null.")
    }

    val transition = TransitionSet()
        .addTransition(Fade())
        .addTransition(Scale(0.95f))
        .setDuration(170)
        .addTarget(to)

    transition.doOnEnd {
      if (container.childCount > 1) container.removeView(from)
    }

    return transition
  }

  override fun executePropertyChanges(container: ViewGroup, from: View?, to: View?,
                                      transition: Transition?, isPush: Boolean) {

    if (from == null || to == null) {
      throw IllegalArgumentException("from and to must not be null.")
    }

    if (to.parent == null) container.addView(to)
  }
}