[Lazarus] TTabControl (d)evolution

[Hans-Peter Diettrich]

The implementation of the TTabControl seems to cause headaches, and 
reveals wrong assumptions of the devlopers about the behaviour of that 
control :-(

When I recently submitted an patch, with a new implementation of the 
TTabControl (named TTabs in the patch), this implementation was tested 
with the Win32 widgetset, and was fully compatible with the Delphi 
TTabControl - in contrast to any preceding implementation.

Felipe acted a bit too fast, and replaced the TTabControl by the new 
implementation, without waiting for the complete implementation with all 
widgetsets. The consequential problems showed up very soon :-(

When consequently problems with other widgetsets occured, he removed the 
intented implementation of TTabs (TNewTabControl), making it unusable at 
all. This step was undone later.

When it turned out that the IDEInspector uses a TTabControl, and stopped 
working (#19803) due to an misplaced override of ChildClassAllowed, this 
problem was not fixed in the suggested and obvious way (by moving that 
method into TPageControl). Instead the wrong assumption was stated, that 
a TTabControl cannot have child controls - what is not true for a Delphi 
compatible control.


So where are we now? We are back to the old TTabControl, that is not 
Delphi compatible, and have a TNewTabControl without an designer. Was 
this worth all the trouble?

IMO we should accept that a Delphi compatible TTabControl doesn't exist 
yet, for widgetsets other than Win32. What can/should we do, when it 
turns out that such a control can not be implemented in all widgetsets?


Why not offer the new implementation (TTabs/TNewTabControl) as 
TTabControl, to make Delphi migrants happy, and restrict its use to only 
the supported widgetsets? At the same time the old implementation can be 
renamed into TLazTabControl, that is not fully Delphi compatible, but 
immediately usable with all widgetsets?

And since the old TTabControl and TNotebook effectively share the same 
code, except for the control that shows the tabs, it would be a good 
idea to merge both into a common base class. The difference between both 
controls is implemented in their Pages classes (TUNBPages, 
TTabControlNoteBookStrings), so that effectivly only their constructors 
have to be different, where this object is created. An even simpler 
solution were to make the tabs switchable ON/OFF.

More things can be simplified, in detail the difference between visible 
and invisible pages, which applies *only* to the TPageControl. By moving 
that part out of the widgetsets into TPageControl, the implementation of 
the widgetsets and all TCustomTabControl descendants could be simplified 
a lot. Then also the restrictions would become more obvious, introduced 
by the various widgetsets.

DoDi