[lazarus] Window refresh and painting architecture

[Peter Dyson]

I have been studying the message paths and calls made by the
InvalidateRect Update Paint et al. There seems to be an awful 
lot of message passing which can be got rid of if we have to 
encapsulate the windows behaviour.

For Example
control calls invalidate 
invalidate calls API invalidate
windows updates the invalid rect for that window.
windows then sends a paint when msg q is empty
control asks windows for the invalid rects accumulated
then paints it

Now if TWinControl handles its own invalid regions
thats one less msg to spew out, we will still have to 
get an invalid rect from the window manager as only
it knows about other windows affecting this one.
Child controls should get handled in much the same
way as now, this is only required for the few instances
where large drawing areas are used.

Also when the control calls class method update to
force a refresh, the API call  UpdateWindow is called
along with windows posting a wm paint message back.
No gain if you want to force another window but one
nested API call and one deep msg q in less if the 
TWinControl can call its own paint method without
going through all that.

Can anyone see any disadvantages in TWinControl
maintaining its own invalid rect and internal painting.
It would not affect the public interface of the class
and would encapuslate a part of the WinAPI within
the class heirachy, which certainly makes it different 
to Delphi while still compatible.

Peter