[Windows Mobile Software Development] Chainfire:
Well the method I use in my own new UI's is indeed per form (excluding WinAPI controls like edit boxes and such) draw using only one DC.

The problem is that any 'windowed' control, the parent will not draw to the DC if a 'windowed' control overlaps. Due to CLIPCHILDREN all data drawn to that position is simply lost.

Now, handling WM_PAINT you can get the entire update region, which tells you which parts of your form have to be redrawn. You must use this information, because blitting the entire form is very slow!

In essence, to do this right you well end up faking most of the GDI system, including your own 'fake' child windows, invalidating and revalidating portions, calculating the intersections of your 'fake' invalidated regions of the screen with the update region you get in WM_PAINT and redrawing those parts.

There are several different strategies to go about this, one is to redraw on demand, another one is to use double buffering.

I personally mostly use the double buffering technique, as this easily provides every 'fake' control with a bitmap of it's own region. A child control can then alphablend using the parent's buffer as one of the alphablend sources.

You can of course combine this with keeping state information whether a child, grandchild, etc is using alpha / transparency and this with an algorithm deciding which control needs double buffering or can draw on-demand, which can give both speed and memory use advantages. In a lot of situations you can then suffice with only double buffering the 'top' component (form) and a select number of child components.

Of course two drawbacks of per-control double buffering are speed and memory use. You can eliminate the complete-form double buffer with some smart coding and calculating. This will give you a slight speed and memory advantage. Memory use is high because many of your controls will have a copy of their current state.

This can be as complicated, feature-filled, fast and efficient as you are willing to make it. The better you can design the code the better it will work, but it is not a trivial task. There are many ways to go at this, no one way is definitely better than the other ways. It depends on what your applications does with it's display, how simple you are drawing (are you making a simple white background, or a background based on images for example gradient?), which method is more efficient.

The other method is getting the update region and actually perform redrawing of those invalidated sections (instead of copying from buffer). I can tell you from experience that if you are using image backgrounds and alphablend calls, this will be _much_ slower than double buffering (if done right).

I know all of this probaby makes little sense, I'm not a hero with explaining things ;)... you really have to figure this out for yourself, I guess.

Other advantages of building your own UI system are that if you do it smartly and buffering, it is very easy to port to directdraw, and possibly even GL. But I must warn you, on far the most devices actually using directdraw for this stuff is not much faster, it is in fact hardly noticable. If you manage to make a GL port, that can especially on older HTC devices (pre-HD2) be much faster.