A Non Visual Component to iterate over the records in a TTable

In a non object orientated language, a standard code pattern is the Process a File loop. The standard code looks a little like this:

FTable1.First;
while not Table1.EOF do
begin
   { Process the record - application specific code}
   FTable1.Next;
   Application.ProcessMessages;
end;

However, as this is Delphi, I decided to develop a component to provide the basic structure. Basically, I like to reuse code wherever possible - repeating this code through-out a program seemed wasteful. Another, more important reason, is that should the above skeleton code need to change, then the changes are restricted to one module rather than spread throughout many different components and programs. For example, what if after reading all the records in the TTable, you wanted to restore the file to its original position. Then you would need to add code to create and free TBookmarks:

FTable1Bookmark := FTable1.GetBookmark;
try
   FTable1.First;
   while not Table1.EOF do
   begin
      { Process the record - application specific code}
      FTable1.Next;
      Application.ProcessMessages;
   end;
   FTable1.GotoBookmark(FTable1Bookmark);
finally
   FTable1.FreeBookmark(FTable1Bookmark);
end;

The you might have to set up Ranges on the dataset based on the MasterSource and MasterFields. Similarly, you find that the display flickers because the controls are Enabled so code must be added to disable them then re-Enable them after the loop. All of a sudden our simple loop structure has become very complicated. Each time you need more functionality, all the code written must be checked and if necessary changed and then retested. The odds of getting the code changes correct the first time are not good. Also, with each set of new changes the original code becomes more cluttered and difficult to understand.

But if the code is encapsulated within a component, the code changes are almost a pleasure to make. Also, the component is located on the Component Palette and more likely to be used than if it was buried in some other project in some other directory.

TProcessTable Component

This component can be used to iterate over all or some of the records in a TTable. I have modified the MASTAPP demo from Borland to illustrate how this component is used. The component addresses many of the issues raised above such as disabling controls and restoring the position within the TTable.

TProcessDetailTable

This component is designed to iterate over the Detail records in a Master-Detail relationship. The program that I developed to keep my Tennis Club records, uses this component to search the Receipts database. Originally, I simply matched the index fields of the master and detail tables without considering the order of the fields in either table. Upon reflection, I decided this was wrong and decided to use the MasterSource and MasterField properties. The tricky part is that the component had to cope with either or both TTables being inactive (closed).

Properties

Events

Register Procedure

The Register component is a very important for any component that is to be installed on the component palette. Enough said.

Notification Method

I can only echo what Ray Konopka said in his book Developing Custom Delphi Components Providing a Notification method is especially important for design-time operation (p117). The Delphi Component Writer's Guide states The Notification method notifies the component that the component specified by A Component is about to be inserted or removed, as specified by Operation. By default, components pass along the notification to their owned components, if any. A component can, if needed, act on the notification that a component is being inserted or removed. In particular, if a component has object fields or properties that contains references to other components, it might check the notifications of component removals and invalidate those references as needed. Simply, if your refer to other components in the properties of your component, the Notification Method must update your references to those components as they change otherwise a GPF occurs.

Using Resource Workshop

The Resource Workshop (mine came with Turbo Pascal for Windows) can be used to open any .DCL file. A DCL is the Delphi Component Library but is actually a DLL. By entering the appropriate file name and setting the file type to DLL you can examine the Windows resources in the DCL. You can copy the component bit maps and paste them into another .RES file and change them to create a bit map for your component.

After creating the DCR it is necessary to copy it renaming the extension to DCR (for Delphi Component Resource)

Restrictions

The component only operates with TTable components. TQuery objects are not supported.

gra...@pcug.org.au