Q:
Why do I have problems when I use DeviceLoop for non-drawing-related
tasks like finding out which screens contain a given rectangle?
A:
Because DeviceLoop should only be used for drawing, not for
other tasks. Among other things, DeviceLoop assumes a valid
current port which cannot always be guaranteed when called
at other times. Also, DeviceLoop has a substantial overhead
and can adversely affect performance because it needs to
save and restore state information in the port.
/* To determine which devices intersect a given rectangle
in global coordinates, you can use this function from
CGDirectDisplay.h See Technote 2007 for more information */
CGDisplayErr CGGetDisplaysWithRect(
CGRect rect,
CGDisplayCount maxDisplays,
CGDirectDisplayID * dspys,
CGDisplayCount * dspyCnt);
/* Or use the Display Manager in a loop */
for (GDHandle gdh = DMGetFirstScreenDevice(TRUE);
gdh != 0;
gdh = DMGetNextScreenDevice(gdh, TRUE))
{
/* compare (**gdh).gdRect with your window's bounds */
}
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Listing 1. Finding Displays Intersecting a Given Rectangle
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If you need to find out which monitors intersect a given
rectangle (a common misuse of DeviceLoop), see the code in
Listing 1 above.
[Apr 11 2001]
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