How to change the icon of the active tab in TTabControl?

[Incus J 10]

I'd like to change the icon image on a tab in a TTabControl whenever that tab becomes active.  So the icon acts a bit like a highlight for the currently selected tab.  At the moment I have an ImageList assigned to the TTabControl Images property - and this contains two icons (one for Inactive, one for Active).

 

I can use the OnGetImageIndex event to set the appropriate image for each tab - and this works initially.  The currently active tab displays one image, and all inactive tabs display the other image.  So far so good.  Unfortunately OnGetImageIndex seems to fire once per tab only, when the TTabControl first loads up (in TTabControl.Loaded -> UpdateTabImages) - it never fires again (?) - so this mechanism doesn't allow a tab's image index to be changed later, say in response to a different tab becoming active. 

 

Could I somehow call the protected UpdateTabImages procedure manually, in response to a different tab being clicked and becoming active?

[David Hoyle 68]

What about change the icon indexes in the OnChange event?

[Incus J 10]

Maybe - I'm not sure how though (it's a TTabControl rather than a TPageControl - so there's no icon index property for each tab).

[David Hoyle 68]

How are you setting the icons in the first place?

[Incus J 10]

Well the two icons are in an ImageList assigned to the control.  OnGetImageIndex sets the desired icon like this:

procedure TForm1.TabControl1GetImageIndex(Sender: TObject; TabIndex: Integer;
  var ImageIndex: Integer);
begin
  if TabControl1.TabIndex=TabIndex then ImageIndex:=1 else ImageIndex:=0;
  end;

So if the tab is currently active it gets image 1, otherwise image 0.  Which is great - it works.  But only once at start up.  I can't figure out how to assign a different icon index later.

[Uwe Raabe 2057]

You can use a local class helper to access the protected method:

type
  TTabControlHelper = class helper for TTabControl
    procedure UpdateTabImages;
  end;

procedure TTabControlHelper.UpdateTabImages;
begin
  inherited UpdateTabImages;
end;

 

[Incus J 10]

A local class helper did the trick - perfect - thank you!

 

Now I just call UpdateTabImages in my OnChange event handler.

[Remy Lebeau 1394]

On 4/29/2022 at 5:40 AM, Uwe Raabe said:

You can use a local class helper to access the protected method

I generally prefer to use a local accessor class instead:

type
  TTabControlAccess = class(TTabControl)
  end;

TTabControlAccess(TabControl1).UpdateTabImages;

 

[Renate Schaaf 64]

1 hour ago, Remy Lebeau said:

I generally prefer to use a local accessor class instead:

What's the advantage? When do you use a class helper and when do you use a local accessor class, to me they both seem to serve the same purpose.

[Remy Lebeau 1394]

3 hours ago, Renate Schaaf said:

What's the advantage? When do you use a class helper and when do you use a local accessor class, to me they both seem to serve the same purpose.

Personally, I only ever use class helpers when adding new methods/properties to an existing class whose interface can't otherwise be changed yet. That kind of usage is what class helpers were originally intended for.