Creating an About Box Plug-in

In the host form, double click on the "About" menuitem and add the following code:

procedure TForm1.About1Click(Sender: TObject);
begin
packhandle := LoadPackage('Myplugin1.bpl');
 if packhandle <> 0 then
begin
AClass := GetClass('TForm2');
 if AClass <> nil then
 begin
with TComponentClass(AClass).Create(Application)
    as TCustomForm do
  begin
    Show;
    Free;
  end;
  end
  else
  begin
  showmessage('Could not load class');
  end;
end
  else
  begin
  showmessage('Package handle empty');
  end;
end;

Also add the following two variables to the forms "var" section:

  packhandle: HModule;
  AClass: TPersistentClass;

The code should be familiar to you by now. First we load the plug-in by calling the loadpackage() function, and then store the returned value in the "packhandle" variable:

packhandle := LoadPackage('Myplugin1.bpl');

Then we check to see whether the handle is empty. If it is not empty, we retrieve the class that is exposed by the package:

if packhandle <> 0 then
begin
AClass := GetClass('TForm2');
 if AClass <> nil then
 begin
with TComponentClass(AClass).Create(Application)
    as TCustomForm do
  begin
    Show;
  end;

The AClass variable holds the exposed class(TForm2) and is then instantiated and called "TCustomForm." This new object gives us access to all the usual methods of a normal form. One of these is "show," which as the name implies shows a form.

Then we simply use the "show" function to view the about box, and then free the TCustomForm object. I have not done this in this code, but it would be better to first check to see whether the package exists by using the fileexists() function (or any other method), and then based on that condition continue with the rest of the code.

That is how simple and easy it is to create a plug-in. Of course you could always improve the code by including some kind of interface class that will reduce the chance of errors. But this should give you enough information to start creating your own plug-ins.

Conclusion

Delphi packages are ideal for building plug-ins. They provide a great way to make your applications small in size and modular. They also make your applications more useful by letting other developers extend their capabilities.