[Lazarus] new widget implementation - class hierarchy help required

[Marc Weustink]

Graeme Geldenhuys wrote:
> Hi,
> 
> I'm working on the LCL-fpGUI widget set.  I'm mainly working on two
> units. FpGuiWSControls.pas and FpGuiWSStdCtrls.pas.
> 
> All components in FpGuiWSStdCtrls seem to descend from TWSxxx classes
> defined in the generic wsstdctrls.pas unit (not the fpGUI specific one
> FpGUIWSStdCtrls.pas).
> 
> So implementing something like the Invalidate() method, it seems I
> need to implement the same thing in FpGUIWSControls and
> FpGuiWSStdCtrls.  Surely if the standard controls (eg: Button)
> descends from the TFpGuiWSWinControl in FpGuiWSControls.pas, then I
> would only need to implement Invalidate() once.
> 
> How does the class hierarchy tie together in the FpGUIWSControls and
> FpGuiWSStdCtrls units?  All the widget set specific standard controls
> seem to descend from the generic TWSxxx classes, and not the widgetset
> specific TfpGUIWSxxx classes.

Thats where the VClasses come in :)

When you register TFpGUIxxx it gets hooked into the tws hierarchy.

Example:

TWSControl - TWSWincontrol - TWSButton

You register TFpGUIWSWincontrol(TWSWincontrol), then the virtual class 
tree will become

TWSControl - TWSWincontrol - TFpGUIWSWincontrol - TWSButton

So when you derive and register TFpGUIWSButton(TWSButton) it will 
"inherit" from TFpGUIWSWincontrol. The only thing not possible is using 
the "inherited" keyword to call an ancestor method. You can work around 
this by using TWSWincontrolClass(ClassParent).SomeMethod.

Marc