Container Classes Versus Control Classes

MORE INFORMATION

Built-in Control Classes

Built-in Container Classes

Differences Between the Two Base Classes

These two base classes are different when it comes to how objects within each class can be referenced and manipulated. Member objects within Control classes can only be manipulated at the design time of the class. These objects cannot be referenced from outside the class at run time. In contrast, objects within Container classes can be manipulated individually at design time or run time, and they can be referenced from outside the class by calling methods or setting properties (this applies to members and methods not defined as PROTECTED).

How Subclassing Works for Container and Control Classes

If a Container is subclassed and child member objects are added to it (Command Buttons, for example), then when an object is instantiated from this container subclass, all methods and properties of any of the child member objects as well those of the parent (the Container subclass) may be accessed as long as they are not protected.

If a Control is subclassed and member objects are added to it, then when this Control is instantiated, only methods and properties of the Control class itself may be accessed, and then only if they are not protected. The methods and properties of child member objects contained in the control cannot be accessed from outside the control. These child members can only be accessed from within the control (a method in the control can set a property or call a method in a child member of the control). All methods and properties of child members of controls are automatically protected.

Member objects must be added to Controls at design time with the ADD OBJECT clause of the DEFINE CLASS command. Member objects may be added to containers either at design time (with the ADD OBJECT clause) or at run time (with the .AddObject() method).

Code Sample

*****************************************************************
* Start of CNTVSCTL.PRG
* This program contrasts Container and Control classes
*****************************************************************

ON ERROR DO errhand ;
   WITH ERROR( ), MESSAGE( )  && Traps unknown member error

oForm1 = CREATEOBJECT( 'frmform1', 'Form with Container', 'cntContainer' )

oForm1.VISIBLE = .T.

oForm2 = CREATEOBJECT( 'frmform1', 'Form with Control', 'ctlControl' )
oForm2.LEFT = 310
oForm2.VISIBLE = .T.
READ EVENTS

ON ERROR

DEFINE CLASS frmForm1 AS FORM

   WIDTH = 300
   SCALEMODE = 3

   PROCEDURE INIT
      PARAMETERS cformcaption cobj2add
      THIS.CAPTION = cformcaption
      THIS.ADDOBJECT( 'oAddedobject', cobj2add )
      THIS.ADDOBJECT( 'oFormbutton', 'cmdbutton2' )
      THIS.oformbutton.VISIBLE = .T.
      THIS.oAddedobject.VISIBLE = .T.
   ENDPROC

   PROCEDURE DESTROY
      CLEAR EVENTS
   ENDPROC

ENDDEFINE

DEFINE CLASS cntContainer AS CONTAINER

   ADD OBJECT oCenter AS center_it
   CAPTION = 'Container'
   HEIGHT = 120


   PROCEDURE INIT
      THIS.ADDOBJECT( 'ocmdButton', 'cmdbutton', 'Container Button' )
      THIS.ocmdButton.VISIBLE = .T.
      THIS.TOP = THIS.oCenter.T_B_center( THISFORM.HEIGHT, THIS.HEIGHT )
      THIS.LEFT = THIS.oCenter.L_R_center( THISFORM.WIDTH, THIS.WIDTH )
   ENDPROC

   PROCEDURE Call_click
      PARAMETERS cClickText
      THIS.ocmdButton.CLICK( cClickText )
   ENDPROC

ENDDEFINE

DEFINE CLASS ctlControl AS CONTROL

   ADD OBJECT oCenter AS center_it
   ADD OBJECT 'ocmdButton' AS 'cmdButton' NOINIT

   * Cannot do ADDOBJECT() in INIT PROCEDURE (at run time) in Control.
   * Must add with ADD OBJECT clause of DEFINE CLASS command.

   CAPTION = 'Control'
   HEIGHT = 120

   PROCEDURE INIT

      * The following command calls the ocmdButton INIT and passes
      * the desired button caption to the INIT
      THIS.ocmdButton.INIT( 'Control Button' )

      THIS.TOP = THIS.oCenter.T_B_center( THISFORM.HEIGHT, THIS.HEIGHT )
      THIS.LEFT = THIS.oCenter.L_R_center( THISFORM.WIDTH, THIS.WIDTH )
   ENDPROC

   PROCEDURE Call_click
      PARAMETERS cClickText
      THIS.ocmdButton.CLICK( cClickText )
   ENDPROC

ENDDEFINE

DEFINE CLASS cmdbutton AS COMMANDBUTTON

   PROCEDURE INIT
      PARAMETERS cButtonCaption
      THIS.WIDTH = 125
      THIS.TOP = THIS.PARENT.oCenter.T_B_center ;
         ( THIS.PARENT.HEIGHT, THIS.HEIGHT )
      THIS.LEFT = THIS.PARENT.oCenter.L_R_center ;
         ( THIS.PARENT.WIDTH, THIS.WIDTH )
      THIS.CAPTION = cButtonCaption
   ENDPROC

   PROCEDURE CLICK
      PARAMETERS cClickText
      WAIT WINDOW IIF(TYPE('cClickText') = 'C', cClickText ;
         ,THIS.PARENT.CAPTION+' Button'+' Click in '+ '"' ;
         +THIS.PARENT.CAPTION+'"') TIMEOUT 5
   ENDPROC

ENDDEFINE

DEFINE CLASS cmdbutton2 AS COMMANDBUTTON

   TOP = 215
   CAPTION = 'Form Button'

   PROCEDURE INIT
      THIS.LEFT =  (THIS.PARENT.WIDTH-THIS.WIDTH)/2
   ENDPROC

   PROCEDURE CLICK   && Passes wait window message to display
      * The following line causes an error when calling Click event
      * of ocmdbutton in Control, but works properly when called
      * in Container.
      THIS.PARENT.oAddedobject.ocmdButton.CLICK('Click Event for ';
         +'"'+THIS.PARENT.oAddedobject.CAPTION+' Button"'+' in ' ;
         +'"'+THIS.PARENT.oAddedobject.CAPTION+'"'+' Called from '+'"' ;
         +THIS.CAPTION+'"'+' Click')
      THIS.PARENT.oAddedobject.Call_click( 'Click Event for ';
         +'"'+THIS.PARENT.oAddedobject.CAPTION+' Button"' ;
         +' Called Through Call_Click Procedure in '+'"' ;
         +THISFORM.oAddedobject.CAPTION+'"')
   ENDPROC

ENDDEFINE

* Define a class to center buttons, containers, & controls:
DEFINE CLASS center_it AS CUSTOM  && s

   FUNCTION T_B_center
      PARAMETERS ncontheight nobjheight
      RETURN (ncontheight-nobjheight)/2
   ENDFUNC

   FUNCTION L_R_center
      PARAMETERS ncontwidth nobjwidth
      RETURN (ncontwidth-nobjwidth)/2
   ENDFUNC

ENDDEFINE

PROCEDURE errhand
   PARAMETERS nError, cMess
   DO CASE
      CASE nerror = 1925
          = MESSAGEBOX('Error # '+ALLTRIM(STR(nError))+ '. '+cMess;
               +CHR(13)+CHR(13) ;
               +'Member ocmdButton is unknown outside of Control!',48)
      OTHERWISE
          = MESSAGEBOX('Error # '+ALLTRIM(STR(nerror))+'. '+cMess)
   ENDCASE
ENDPROC