Microsoft KB Archive/131027

= How To Encapsulate Data in the Form Builder =

Article ID: 131027

Article Last Modified on 7/1/2004

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APPLIES TO


 * Microsoft Visual FoxPro 3.0 Standard Edition

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This article was previously published under Q131027



SUMMARY
In a true object-oriented programming paradigm, data used in a form is encapsulated from the underlying form. This hides the names of the tables and fields from the form. By using this approach, a table or field name could change before the form is run, and the form should still function properly. As a result, the form does not need to know specific information about the data tables. The tables can change independent of any form.

This article describes one way to encapsulate data from the form.



MORE INFORMATION
The sample code in this article includes a custom class definition (clMyData) and a subclass (clcustomer) built from this custom class. The program is saved in a file named DATAENC.PRG.

To use this approach, add several lines of code to the form's Init procedure. If the form is part of a formset, place the code in the Init of the formset instead. * SET PROCEDURE to file containing class definition

IF !"DATAENC"$ SET("PROCEDURE") SET PROCEDURE TO dataenc ADDITIVE ENDIF

* Declare public variables to hold instantiations of custom classes PUBLIC oMyData, oCustomer

* Instantiate each class oMyData = CREATEOBJECT("clMyData") oCustomer = CREATEOBJECT("clCustomer")

Program Logic
The custom class (clMyData) examines the form being loaded. It uses the form's control array to itemize all of the controls on the form. If a control is a container (for example, a command button group or a page frame), the program itemizes all controls within the container as well.

After identifying all controls on the form, the program replaces the original rowsource property specified when the form was designed. The replacement value for each control is specified in the subclass (clCustomer).

A subclass similar to clCustomer is created for each table used in the form. The subclass method DIFFERS creates a two-column array, with one row for each field contained in the table. The first column of the array contains the design-time rowsource. The second column of the array contains the run-time rowsource, usually the field name from a table.

This array is passed to the parent class and used in the GETDIFFERS method to dynamically change the rowsource properties of all applicable controls at runtime. The form file on disk is never changed.

All method code required to do the conversion is included in the parent class. Only one method in the subclass needs to be edited to add an additional table to the form.

The class listed in this article is a simple example. It could easily be expanded to handle additional functionality, such as building select statements for list boxes.

Sample Code
******* Begin code example

************************************************************************  * Class Name   : clMyData * Parent Class : Custom * Subclasses  : ************************************************************************

Define Class clMyData as Custom

************************************************************************  *   Properties ************************************************************************

lIsFormSet = .t. && logical - is this a formset? nNumberofForms=1   && number of forms flag = .f.

************************************************************************

PROCEDURE OpenTables

Parameters pDBC, pTable * Called from each subclass to open needed dbcs and dbfs * Set buffering SET MULTILOCKS ON

* Open tables IF !DBUSED(pDBC) OPEN DATA (pDBC) ENDIF IF !USED(pTable) USE (pTable) IN 0 ELSE SELECT (pTable) ENDIF =CURSORSETPROP("Buffering",5,(pTable))

* Populate first object THIS.GETINFO(pTable, @pArray)

************************************************************************  * This is called once in each subclass to create a unique array name

PROCEDURE DIFFERS

PARAMETERS pTable aDiffers=SYS(2015) PUBLIC &adiffers(1,2) RETURN aDiffers

* Place control source property for form in first column * Place fields which make this field in second column

*********************************************************************  * The following procedure is called by the subclass one time for each * field in the data table

PROCEDURE POPDIFFERS PARAMETERS pArrayName, pControlSource, pInputExpr

iCurrentRow = ALEN((pArrayName),1) cArrayName = pArrayName + "(iCurrentRow,"     &cArrayName.1) = pControlSource &cArrayName.2) = pInputType

nNewSize = iCurrentRow + 1 DIMENSION &pArrayName.(nNewsize, 2) ************************************************************************

PROCEDURE INIT

* Figure out if part of a formset by trying to generate * an error number 1938 and setting HighestClass appropriately

temp = thisformset.name

* Cycle through all of the controls on the current form and find the * current control source where needed. * Then, the init of each subclass will refer to this cursor later on     * to update the control source properties

CREATE CURSOR curMyProps (cName c(75), source c(75)) CREATE CURSOR curContainers (cName c(75)) IF lIsFormset nNumberofForms = _screen.FORMCOUNT FOR i = 1 to nNumberofForms HighestClass = "_SCREEN.FORMS(" + alltrim(str(i)) +")" this.allforms(highestclass) NEXT i     ELSE HIGHESTClass = "THISFORM" THIS.AllForms(HighestClass) ENDIF

ENDPROC

************************************************************************  * The following procedure is executed 1 time for each form * by the class init method

PROCEDURE AllForms PARAMETERS HighestClass iNumControls = &highestclass..controlcount this.enumerate(HighestClass, iNumControls) DO WHILE .T.        SELECT * from curContainers into CURSOR TEMP IF _tally = 0 EXIT ENDIF SET SAFETY OFF CREATE CURSOR curContainers (cName c(75)) SELECT TEMP SCAN HighestClass = alltrim(cName) iNumControls = &HighestClass..ControlCount this.enumerate(HighestClass, iNumControls) ENDSCAN ENDDO

ENDPROC

************************************************************************  * The following procedure figures out which controls exist on the * container

PROCEDURE enumerate Parameters HighestClass, iNumControls

FOR i = 1 TO iNumControls IF &highestclass..baseclass != "Commandgroup" cCurControlBase = &highestclass..controls(i).baseclass cCurControl = UPPER(highestclass + "." + ;               &highestclass..controls(i).name) cCurControlSource = ; UPPER(&highestclass..controls(i).controlsource) ELSE cCurControlBase = &highestclass..buttons(i).baseclass cCurControl = UPPER(highestclass + "." + ;               &highestclass..buttons(i).name) cCurControlSource = ; UPPER(&highestclass..buttons(i).controlsource) ENDIF IF cCurControlBase = "Textbox" ; OR cCurControlBase = "Check box" ; OR cCurControlBase = "Commandbutton" ; OR cCurControlBase = "Optionbutton" ; OR cCurControlBase = "Spinner" ; OR cCurControlBase = "Column" ; OR cCurControlBase = "EditBox" SELECT CurMyProps APPEND BLANK REPLACE cName WITH cCurControl REPLACE source WITH cCurControlSource ELSE IF cCurControlBase = "Page" ; OR cCurControlBase = "PageFrame"; OR cCurControlBase = "Form" ; OR cCurControlBase = "Grid" ; OR cCurControlBase = "Toolbar"

SELECT curContainers APPEND BLANK REPLACE cName with cCurControl ENDIF ENDIF NEXT i

ENDPROC

************************************************************************  * This procedure changes the rowsource in the cursor

PROCEDURE UpdateCursor Parameters pArray, cTableName

SELECT curMyProps =cursorsetprop("Buffering",1,(cTableName)) FOR i = 1 TO ALEN((pArray),1) cNewSource = &pArray.(i,2) cOldSource = &pArray.(i,1) REPLACE source WITH cNewSource ; FOR ALLTRIM(source) = UPPER(cOldSource) ENDFOR SCAN cNewName = alltrim(cName) &cNewName..controlsource = alltrim(source) ENDSCAN

************************************************************************

PROCEDURE error Parameters pErrorNum, p3, p4     IF pErrorNum = 1938 lIsFormSet = .F.     ENDIF

************************************************************************

PROCEDURE destroy PARAMETERS pArrayName RELEASE (pArrayName)

************************************************************************

ENDDEFINE && End Definition of clMyData

************************************************************************ * END CLASS DEFINITION ************************************************************************

************************************************************************  * Class Name   : clCUSTOMER * Parent Class : clMyData * Subclasses  : ************************************************************************

Define Class clCUSTOMER as clMyData

************************************************************************  *   Properties ************************************************************************

cTableName = "CUSTOMER" &&Name of the .dbf file cDBCName  = "POES"      &&Name of the .DBC file cMyArray  = ""

************************************************************************

PROCEDURE INIT clMyData::OpenTables(THIS.cDBCName, THIS.cTableName) cMyArray = clMyData::Differs clMyData::PopDifferS(cMyArray,"FullName","customer.c_Name") clMyData::PopDifferS(cMyArray,"SSN","customer.c_SSN") clMyData::PopDifferS(cMyArray,"STREET","customer.c_STREET") clMyData::PopDifferS(cMyArray,"CITY","customer.c_CITY") clMyData::PopDifferS(cMyArray,"STATE","customer.C_STATE") clMyData::PopDifferS(cMyArray,"ZIP","customer.c_ZIP") clMyData::PopDifferS(cMyArray,"PHONE","customer.c_PHONE") clMyData::PopDifferS(cMyArray,"CCN","customer.c_CCN") this.parentPopDifferS(cMyArray,"CCEXP","customer.c_CCEXP")

clMyData::UpdateCursor(cMyArray, this.cTableName) PUBLIC aCustomer(1,2) SELECT c_ssn, c_name FROM CUSTOMER INTO ARRAY aCustomer

************************************************************************

ENDDEFINE && End Definition of clCustomer

************************************************************************  * END CLASS DEFINITION ************************************************************************

Keywords: kboop kbcode KB131027

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