Great article, Dan. I have to admit, I've been one to use TPageControls just as you described.

One useful addition would be a helper method to make sure that the tabsheets are configured correctly.



procedure TSampleForm.LinkTabToFrame(ts:TTabSheet; fc:TBaseFrameClass);

begin

ts.tag := integer(fc);

ts.OnShow := TabSheetShow;

ts.OnHide := TabSheetHide;

end;

Thanks, Marc. Hopefully, this won't be needed in the next version of the code and article since I'll just extend the TPageControl and TTabSheet to handle all of this.

Very timely article! I'm just about to start a project that is browser-esque in the fact that very few secondary forms are ever shown, instead differents views and pages are loaded which the main form as the parent. I was going to just use a lot of TForm's and set their Owner to the mainform but I have a feeling TFrames and a TPageSheet method will work much better.



I'd love to see any improvements\enhancements on this method. Thanks for sharing.

One more thing...



I know this is an example but I noticed you code in a slightly more .NET way in terms of your naming style. You have lowercase local variables, a la C#\.NET, while the 'Delphi' way is first-letter upppercase.



Not being picky, I'm actually struggling with switching back and forth myself. Do I use one style per language or do I try and use the same style for both?

Great, just what I was looking for!



Some doubts about the GetFrame function though:



1. The first if-statemen, "if not Assigned(.." doesn't make much sense if the ATabsheet doesn't have a default value of nil.

2. The second if-statement starts with "if Assigned(.. which seems redundant as we just tested for "not Assigned" and took appropriate action.

3. Furthermore this if-statement will cause an exeption if there are no controls present and the last and-part is executed before the second. (AFAIK Delphi does not guarantee the order of testing for "and" conditions.)



So my version of the function would be:



function TSampleForm.GetFrame(ATabSheet: TTabSheet=nil): TFrame;

begin

if not Assigned(ATabSheet) then

ATabSheet := PageControl.ActivePage;

Result := nil;

if ATabSheet.ControlCount > 0 then

if ATabSheet.Controls[0] is TFrame then

Result := TFrame(ATabSheet.Controls[0]);

end;

Thanks for the feedback, Jan. To address your comments:

1. You can still call GetFrame(nil) to get the frame for the active page. As a matter of fact, I left it this way intentionally so that you always had to explicitly state what tabsheet you were after. I can see some benefit to adding the optional param, though.

2. Redundant, yes. It's also defensive. What happens if PageControl.ActivePage returns nil?

3. The if statement is fine. Due to short-circuit boolean evaluation, if any part of the if statement evaluates to false, the next part of the if statement doesn't execute.



So, in summary, it appears that perhaps an optional parameter could be added, but the rest is fine.

Point 1 is maybe mostly a question of programming standards. Neither the alternative "GetFrame" nor "GetFrame(nil)" make much sense logically when what one wants to say is something like "GetFrame(CurrentPage)". Neither of the two alternatives is intuitively easier to understand when maintaining the code but I think the parameterless form is the more common solution. No big deal anyway!

Point 2: Your solution is of course the right one!

Point 3: Still a bit confused. Can we safely assume left-to-right evaluation in Delphi?.



As an aside, I now have my first conversion of an application to utilize your PageControl/Frames setup up and running. What a relief to get rid of that 2000+ lines unit that now is split into 5 different units. Just doing the conversion allowed me to get rid of a lot of redundancies and duplications that had crept into the code and made it almost unmaintainable.

Perfect article. I want to convert my editor application which craetes dynamicly tabsheets in the pagecontrol. How do I dynamicly create the frames (TBaseFrame) and then assign these to the tabsheets.

Any hints are most welcome,

Robert Linn (Hamburg - Germany)

Both the sample code, and Marc's comment above should show you how to assign the frames to the tabsheet. The article also covers this showing how to assign the class reference to the TabSheet.Tag property.



Or did I not understand what you were asking?

Dan, sorry for my english es very poor.

When I declare TFrameClass = class of TFrame; leaves then following message "Fatal Error: Requiered Package Teeimage not found, as I solve this?. Tanks