Dockable.subscribeVisibleStateChanged Method
        
 Fire the visualStateChanged event if callback is not specified. Subscribe handler if callback is specified.
 
         
        Parameters
        
            - callback
            
 
            - 
                
Type: Function
The event Handler for visualStateChanged. 
            - targetDock
            
 
            - 
                
Type: HTMLElement
The HTMLElement of the targetDock specified for the argument of the event handler. 
        
        Return Value
        
Type: string
        
The event subscription ID.