Add a handler to be notified when the event occurs.
"activeSelectionEvent_var" is a variable referencing an ActiveSelectionEvent object. | 
        
"activeSelectionEvent_var" is a variable referencing an ActiveSelectionEvent object. | 
        
| Type | Description | 
| boolean | Returns true if the addition of the handler was successful. | 
| Name | Type | Description | 
| handler | ActiveSelectionEventHandler | The handler object to be called when this event is fired. |