The 
gtk:spinner widget displays an icon size spinning animation.    It is often used as an alternative to a 
gtk:progress-bar widget for
  displaying indefinite activity, instead of actual progress.
  To start the animation, use the 
gtk:spinner-start function, to stop it  use the 
gtk:spinner-stop function.  
CSS nodes
    The 
gtk:spinner implementation has a single CSS node with the name    
spinner. When the animation is active, the 
:checked
    pseudoclass is added to this node.