ViewPanel.subscribeVisibleChanged Method
        
 Subscribe an handler for the VisibleChanged event.
 
         
        Parameters
        
            - callback
            
 
            - 
                
Type: Function
A function to use as handler. 
            - once
            
 
            - 
                
Type: Boolean - May be null
(Optional) True to unsubscribe automatically after handler is ran once. 
            - unsubscribe
            
 
            - 
                
Type: Boolean - May be null
(Optional) True to unsubscribe.