Delphi Firemonkey - can't add sub-subclass of TTabItem to TTabControl - delphi

Question

I can add a TTabItem to a TTabControl, I can add a subclass of a TTabItem to a TabControl, but I can't add a sub-subclass of a TTabItem to a TabControl.
Example Firemonkey application - a form with a TTabControl:
type
TTabItem_subclass = class (TTabItem);
TTabItem_sub_subclass = class (TTabItem_subclass);
procedure TForm1.FormCreate(Sender: TObject);
procedure add_tab (t: TTabItem);
begin
t.Text := t.ClassName;
t.Parent := TabControl1
end;
begin
add_tab (TTabItem.create (TabControl1)); // <-- works
add_tab (TTabItem_subclass.create (TabControl1)); // <-- works
add_tab (TTabItem_sub_subclass.create (TabControl1)); // <-- fails
end;
When the application is run the TTabItem_sub_subclass doesn't display:
I've tried this in both XE5 and Tokyo with the same results. What am I missing?

Answer 1

Short answer: I don't think you are missing anything. If fact, your code does successfully add the sub-sub-classed item to the TabControl, it just doesn't get displayed. I think this problem is caused by a flaw in the way the FMX code derives the style to be used to paint a class which is a sub-sub-class of TTabItem. I don't know enough about FMX to idemtify the exact cause of the problem, but I have identified what seems to be a functional work-around.
Please see the code below of a sample project which successfully displays both
TabItem subClass and TabItem sub_subClass tabs.
The reason the code is structured as it is is to make it easy to set a changed-memory breakpoint on
the FResourceLink field of the TabItem (the variable Item in the code), while I was
trying to trace how the painting process occurs.
From watching the TabItem.Paint method, it was obvious that the tab would only paint
if its FResourceLink is not nil. The problem with your original code (and mine)
was that when Paint is called on TabItem_subClass, its FResourceLink has been assigned
a value whereas for TabItem_sub_subClass it has not. Evidently the FResourceLink
is where it picks up the name of the style used to paint the TabItem and if
it can't be found the TabItem doesn't get painted.
I'm afraid that as I'm no expert in FMX I find its code something of a labyrinth
at the best of times and its implementation of styles even more so. But it
struck me that if I could ensure that a valid style name is returned for
the TabItem GetParentClassStyleLookupName metod, that should suffice. That's the reason
for the TCustomItem_sub_subclass.GetParentClassStyleLookupName override. I imagine
an FMX expert might see it as a bit of a sledgehammer to crack a walnut, but there
you go.
Code
type
TForm1 = class(TForm)
TabControl1: TTabControl;
StyleObject1: TStyleObject; // ignore this
procedure FormCreate(Sender: TObject);
private
public
Item : TTabItem;
end;
[...]
implementation
[...]
type
TCustomItem_subclass = class (TTabItem)
public
constructor Create(AOwner : TComponent); override;
end;
TCustomItem_sub_subclass = class (TCustomItem_subclass)
public
constructor Create(AOwner : TComponent); override;
function GetParentClassStyleLookupName: string; override;
end;
procedure TForm1.FormCreate(Sender: TObject);
procedure add_tab (t: TTabItem);
begin
t.Text := t.ClassName;
t.Parent := TabControl1
end;
begin
{$define UseSubSub}
{$ifdef UseSubSub}
Item := TCustomItem_sub_subclass.Create(TabControl1);
{$else}
Item := TCustomItem_subclass.Create(TabControl1);
{$endif}
Item.Text := Item.ClassName;
Item.Parent := TabControl1;
Caption := TabControl1.ActiveTab.Text;
Item := TCustomItem_subclass.Create(TabControl1);
Item.Text := Item.ClassName;
Item.Parent := TabControl1;
end;
constructor TCustomItem_subclass.Create(AOwner: TComponent);
begin
inherited;
end;
constructor TCustomItem_sub_subclass.Create(AOwner: TComponent);
begin
inherited;
end;
function TCustomItem_sub_subclass.GetParentClassStyleLookupName: string;
begin
Result := 'tabitemstyle';
end;
Btw, in doing this I noticed what seems to be a lurking bug in the function
TStyledControl.GenerateStyleName(const AClassName: string): string in FMX.Controls.Pas'
If the AClassName argument, stripped of a leading TCustom, starts with a double-TT,
as in TCustomTabItem, the code incorrectly removes the T of TabItem. I didn't have
time or energy to explore this further but it's why my TabItem sub-classes omit
the Tab from their names.