Article ID: 131027
Article Last Modified on 7/1/2004
APPLIES TO
- Microsoft Visual FoxPro 3.0 Standard Edition
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
