Alternative to SCATTER and GATHER in the New VFP Model

• Microsoft Visual FoxPro for Windows, versions 3.0, 5.0
• Microsoft Visual FoxPro for Macintosh, version 3.0b

SUMMARY

In FoxPro versions 2.x, many developers used the SCATTER and GATHER commands to edit data in memory variables. These commands allowed the developer to verify the contents of each field before placing the data in the .DBF file, and resolved some record locking issues.

Visual FoxPro has added a data buffering feature that can be used as an alternative approach to using SCATTER and GATHER in your programs. Data buffering provides the following benefits to you as a developer:

• When you use buffering, you can automatically access the validation rules for fields and records that have been entered into the data base container. When you use memory variables, you cannot automatically access these rules.
• You can determine when a record is locked (prior to editing, or after editing and before saving) to handle contention for record locks in a multiuser environment. In this respect, the buffering features eliminate the need for READ NOLOCK.
• Table buffering allows you to save edits for multiple records in memory at the same time. For example, you can keep all of the line items for an invoice in memory and write them at one time if the user chooses to save the invoice.
• Visual FoxPro provides several new functions to assist you in applying or discarding updates to a table, and in verifying data integrity in a multiuser environment.

MORE INFORMATION

Step-by-Step Example for Implementing Buffering in a Form

You can easily implement buffering in a form as an alternative to scatter and gather. The following example walks you through the process of creating a table with validation on a specific field and designing a form that uses buffering to enforce this validation on a form.

1. In the Command window, issue this command:

   SET DEFAULT TO \VFP\SAMPLES\MAINSAMP\DATA

2. Create a new form.

3. From the View menu, choose Data Environment. From the Data Environment menu, choose Add Table. Add an existing .DBF to the data environment. To take advantage of field or record level validation, you must make the .DBF part of a database container and must have validation rules defined in the .DBC file.

4. Drag one or more fields from the data environment window to the form.

5. Set the Form.BufferMode property to 2 for Optimistic Buffering. This will set optimistic table buffering for any grids bound to data and optimistic row buffering for all other controls bound to a data source.

6. Add a command group to the form. Set the ButtonCount property to 2. Set the caption of the first button to "Save" and the caption of the second button to "Cancel."

7. To allow users to save changes to the table, add the following code to the command1.click method:

   lUpdated = TABLEUPDATE() IF !lUpdated

         =MESSAGEBOX("Update failed", 64)
   

   ENDIF THISFORM.REFRESH

8. To allow users to cancel changes to the table, add the following code to the command2.click button:

   iRecords = TABLEREVERT() =MESSAGEBOX("Changes to " + STR(iRecords, 2) + " were discarded", 64) THISFORM.REFRESH

9. Save and run the form.

Additional Considerations

You probably will want to add the following additional features to your form:

• Navigation buttons to allow movement between records.
• An on error handling routine to handle error 109, "Record in use by another."
• An expansion of step 7 to allow for common scenarios that would cause the tableupdate() function to fail. Possible reasons for failure include an inability to lock the record and the possibility that another user modified the record while the current user was editing it.

You do not need to write code to save the initial values of each field. The CURVAL(), OLDVAL(), and GETFLDSTATE() functions can be useful in handling situations where a different user has modified the current record.

REFERENCES

Refer to the Visual FoxPro "Professional Features Guide" and "Developers Guide" for more information on buffering and developing forms.

For additional information on setting buffering in your application, please see the following article in the Microsoft Knowledge Base:

   ARTICLE-ID: Q128205
   TITLE     : How to Set Buffering Mode in Visual FoxPro