DOCUMENT:Q134314 13-AUG-1999 [foxpro] TITLE :Container Classes Versus Control Classes PRODUCT :Microsoft FoxPro PROD/VER: OPER/SYS: KEYWORDS: ====================================================================== ------------------------------------------------------------------------------- The information in this article applies to: - Microsoft Visual FoxPro for Windows, version 3.0 ------------------------------------------------------------------------------- SUMMARY ======= There are two main Visual FoxPro base classes, the Control class and the Container class. This article compares and contrasts them. MORE INFORMATION ================ Built-in Control Classes ------------------------ CheckBox ComboBox CommandButton Control Custom EditBox Header Image Label Line ListBox OLEBoundControl OLEControl OptionButton Shape Spinner TextBox Timer Built-in Container Classes -------------------------- Column CommandGroup Container Form Formset Grid OptionGroup Page PageFrame Toolbar 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 Additional query words: VFoxWin ====================================================================== Keywords : Technology : kbVFPsearch kbAudDeveloper kbVFP300 ============================================================================= THE INFORMATION PROVIDED IN THE MICROSOFT KNOWLEDGE BASE IS PROVIDED "AS IS" WITHOUT WARRANTY OF ANY KIND. MICROSOFT DISCLAIMS ALL WARRANTIES, EITHER EXPRESS OR IMPLIED, INCLUDING THE WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE. IN NO EVENT SHALL MICROSOFT CORPORATION OR ITS SUPPLIERS BE LIABLE FOR ANY DAMAGES WHATSOEVER INCLUDING DIRECT, INDIRECT, INCIDENTAL, CONSEQUENTIAL, LOSS OF BUSINESS PROFITS OR SPECIAL DAMAGES, EVEN IF MICROSOFT CORPORATION OR ITS SUPPLIERS HAVE BEEN ADVISED OF THE POSSIBILITY OF SUCH DAMAGES. SOME STATES DO NOT ALLOW THE EXCLUSION OR LIMITATION OF LIABILITY FOR CONSEQUENTIAL OR INCIDENTAL DAMAGES SO THE FOREGOING LIMITATION MAY NOT APPLY. Copyright Microsoft Corporation 1999.