Buffering in the TASTRADE Sample Application

• Microsoft Visual FoxPro for Windows, version 3.0

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.

*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.

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.