DC madness..

[Borje Hagsten]

Tip for those of you who like to write custom controls: I had some
minor problems in my editor - sometimes scrolling slowed down and
became "jerky". No matter how I tried, I couldn't find a solution.
Control could work fine for hours and then suddenly this problem came up.

The editor uses a memDC which is refreshed in WM_SIZE, to keep all
things down to a minimum but still handle eventual resize, etc. Funny
thing was that when this problem occurred, simple resize and thereby
refresh of DC made everything ok again. Led me to suspect some sort of
memory fragmentation, whatever, occurred in the control's DC after a while.

Looked around at MSDN and found interesting page about Window Classes at http://msdn.microsoft.com/library/te...n_classy32.htm

So, started experimenting with the Window class I use for it and found
that using CS_CLASSDC instead of CS_PARENTDC in WNDCLASS style solved
the problem, plus actually made repaint/scrolling a tiny little bit faster.

Control has been in use for +5 hours now, no problem at all. May not
apply to all types of controls, but can be worth experimenting with..


------------------

[Paul Noble]

Hi Borje,

Sorry, not on-topic, but is your algonet e-mail working at the
moment ?

Thanks -

[Chris Boss]

Borje;

I think what you want is CS_OWNDC !

Using CS_CLASSDC makes all the controls of your class use the
same DC. CS_OWNDC allows each instance of your control to have its
own DC.

If you only need one control of your custom class, CS_OWNDC should
work well. If you need many instances of the custom control, then
CS_CLASSDC should be used to save GDI memory (each DC uses 800 bytes).

------------------


[This message has been edited by Chris Boss (edited May 14, 2001).]

[Borje Hagsten]

Paul, seems to be something wrong with it, yes. Have to look into that..

Chris, the weird thing is, with CS_OWNDC, PARENTDC or none declared at all,
problem still is there. CS_CLASSDC is the only one that provides a fix for
this particular problem. It's a tricky one, since the problem only occurs
after a while and disappears directly when the DC is refreshed.

Seems like the problem is within the DC itself. BTW, the article at MSDN
says something about "use CS_OWNDC sparsely in Win95", because it means
using up +800 bytes per Window of the 64KB GDI resource heap. Same thing
probably applies to Win98.

I tested with some other of my controls, like the Virtual listbox, and
using CS_CLASSDC actually gives a tiny bit better performance at redraw
and scroll there too. Interesting..


------------------