View.spring() in #io19 motional intelligence session

ViewSpringAnimation.kt

fun animateVisibility(view: View, visible: Boolean) {
  val targetAlpha = if (visible) 1f else 0f
  if (view.alpha == targetAlpha) return
  view.visibility = View.VISIBLE
  val spring = view.spring(SpringAnimation.ALPHA)
  
  (view.getTag(R.id.tag_pending_end_listener) as? DynamicAnimation.OnAnimationEndListener)?.let {
    spring.removeEndListener(it)
  }
  
  if (!visible) {
    val listener = {
      override fun onAnimationEnd() {
        spring.removeEndListener(this)
        view.visibility = View.GONE
      }
    }
    spring.addEndListener(listener)
    view.setTag(R.id.tag_pending_end_listener, listener)
  }
  spring.animateToFinalPosition(targetAlpha)
}

fun View.spring(
  property: ViewProperty,
  damping: Float = SpringForce.DAMPING_RATIO_NO_BOUNCY,
  stiffness: Float = 500f
): SpringAnimation {
  val key = getKey(property)
  var springAnim = getTag(key) as? SpringAnimation?
  if (springAnim == null) {
    springAnim = SpringAnimation(this, property).apply {
      spring = SpringForce().apply {
        this.dampingRatio = damping
        this.stiffness = stiffness
      }
      setTag(key, springAnim)
    }
  }
  return springAnim
}