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Q130656: Buffering in the TASTRADE Sample Application

Article: Q130656
Product(s): Microsoft FoxPro
Version(s): WINDOWS:3.0
Operating System(s): 
Keyword(s): 
Last Modified: 11-FEB-2000

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The information in this article applies to:

- Microsoft Visual FoxPro for Windows, version 3.0 
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SUMMARY
=======

The ORDENTRY.SCX form in the C:\VFP\SAMPLES\MAINSAMP\FORMS directory contains
controls as well as a grid to create a one-to-many form from tables in the
TASTRADE.DBC database.

This form uses optimistic buffering, as well as custom routines when records are
appended, saved, or the record pointer is moved. Much of the code is supplied in
the base class for the form, so it is not readily apparent when you modify the
ORDENTRY.SCX form itself.

MORE INFORMATION
================

The BufferMode property in the Ordentry.scx form itself sets optimistic
buffering. This property causes grids to use optimistic table buffering and all
other controls to use optimistic row buffering. The remaining code is located in
the class used to create this form.

The ParentClass property is set to tsbaseform. The ClassLibrary property is set
to C:\VFP\SAMPLES\MAINSAMP\LIBS\TSBASE.VCX. These two pieces of information tell
you what you need to know to look at the code behind the order entry form.

1. Choose Open from the File menu, and select Files of Type: Visual Class
  Library.

2. Select the TSBASE.VCX file from the C:\VFP\SAMPLES\MAINSAMP\LIBS directory.
  After you choose OK, a list of classes contained in the library appears on
  the right side of the dialog box. Choose tsbaseform, and click OK again.

Methods Used with the BufferMode Property
-----------------------------------------

The following methods are used in conjunction with the BufferMode property to
handle updates of data in the tables:

Method               Functions Performed
Init                 Adds navigation buttons and sets up error
                    message array.

QueryUnLoad          Writes out current record if it has data, and
                    cancels out a blank, appended record.

*AddNew              Prepares a form to accept a new record, and
                    appends a blank record to the table.

*AskToSave           Asks the user if changes should be saved, and
                    saves or restores data.

*DataChanged         Checks to see if any data has changed in the
                    current workarea.

*Delete              Reverts a newly appended record, or deletes a
                    record that already exists in the table.

*First               Moves the record pointer to the appropriate record.
*Last
*Next
*Prior

*IsNewAndEmpty       Returns true (.T.) if the user has added a new
                    record but has not yet made any changes.

*Restore             Cancels all user changes.

*Save                Saves changes to the tables.

*WriteBuffer         Saves the current value of the field to a buffer if
                    the user clicks the toolbar without leaving the
                    current field.

NOTE: All methods names preceded with an asterisk (*) are user-defined methods
that were added to the form. The asterisk is to categorize the methods here. The
names are not preceeded with an * in the class library.

In most cases, this code could have been placed in a different method, such as
the Click event of a button. The advantage of using special names for these
methods is that the code is not overwritten in memory if the developer adds
custom code to the default methods included in a form.

The code to save or discard changes is encapsulated in two methods. The Restore
method is the only method that reverts data through a TABLEREVERT() function.
The Save method is the only method that saves data from the form into the table;
it accomplishes this through a TABLEUPDATE() function. Each method sets the
lAllRows parameter to true (.T.) for the TABLEREVERT() or TABLEUPDATE()
function. This causes all updates to be written to disk or discarded and allows
the method to work with row or table buffering.

These two methods are called whenever it is necessary to discard changes to a
record or save changes to a table. Both methods assume that the desired work
area has been selected prior to invoking the method. The Restore method is
called by the AddNew, AskToSave, Delete, First, Last, Next, and Prior methods.
The Save method is called only by the AskToSave method.

Additional query words: VFoxWin

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Keywords          :  
Technology        : kbVFPsearch kbAudDeveloper kbVFP300
Version           : WINDOWS:3.0

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