represents the current sliding ratio relative to the size of the SlidePanel
if ratio > 0 indicates we are sliding to see the pre actions
if ratio < 0 indicates we are sliding to see the post actions
if ratio == 0 indicates we are not sliding, all actions are hidden, only the main child is visible
Discards any resources used by the object. After this is called, the
object is not in a usable state and should be discarded (calls to
addListener will throw after the object is disposed).