Class definition files can define static visual handle-based objects (widgets) as PRIVATE data members. These built-in widgets expose a well-defined set of events that an application can respond to, by using the ON statement. The ON statement is supported in the main block of a class definition file. As noted previously, the main block cannot contain executable code. Therefore a class definition file cannot specify an ON statement within executable conditional logic, nor can it contain a WAIT-FOR statement in the main block. However, the main block can contain ON statements that respond to a specific event that occurs on a static visual object.

The following contrived code demonstrates this capability:

CLASS Disp:

  /* These data members are PRIVATE by default. */
  DEFINE BUTTON msg.
  DEFINE BUTTON done.
  DEFINE FRAME f msg done.

  /* Unconditional ON block valid in the main block. */
  ON 'choose':U OF msg
    MESSAGE "click" VIEW-AS ALERT-BOX.

  CONSTRUCTOR PUBLIC Disp ( ):
    modal_display ( ).
  END CONSTRUCTOR.

  /* WAIT-FOR is done in a method. */
  METHOD PRIVATE VOID modal_display ( ):
    ENABLE ALL WITH FRAME f.
    WAIT-FOR CHOOSE OF done.
  END METHOD.

END CLASS.