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Asked: 2026-06-08T09:58:04+00:00

In my application I need to distinguish between different displays, which I do by

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In my application I need to distinguish between different displays, which I do by using the NSScreenNumber key of the deviceDescription dictionary provided by NSScreen. So far everything worked flawlessly, but now all of a sudden I sometimes get a different screen ID for my main screen (it’s a laptop and I haven’t attached a second screen in months, its always the same hardware). The id used to be 69676672 but now most of the time I get 2077806975.

At first I thought I might be misinterpreting the NSNumber somehow, but that doesn’t seem to be the case, I also checked by using the CGMainDisplayID() function and I get the same value. What is even weirder is that some of the Apple applications still seem to get the old ID: Eg. the desktop image is referenced in its config file using the screen ID and when updating the desktop image the desktop image app by Apple uses the “correct” (=old) ID.

I am starting to wonder if there might have been a change in a recent update (10.7.1 or 10.7.2) that led to the change, has anybody else noticed something similar or had this issue before?

Here is the code that I use:

 // This is in an NSScreen category
 - (NSNumber *) uniqueScreenID {
     return [[self deviceDescription] objectForKey:@"NSScreenNumber"];
 }

And for getting an int:

 // Assuming screen points to an instance of NSScreen
 NSLog(@"Screen ID: %i", [[screen uniqueScreenID] intValue]);

This is starting to get frustrating, appreciate any help/ideas, thanks!

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1 Answer

  1. Editorial Team

    For Mac’s that have built-in graphics and discrete graphics cards (such as MacBook Pro models with on-board Intel graphics and a separate graphics card), the display ID can change when the system automatically switches between the two. You can disable “Automatic graphics switching” in the Energy Saver prefs panel to test whether this is the cause of your screen number changes (when disabled, will always use the discrete graphics card).

    On such systems, the choice of which graphics is in use at a particular time is tied to the applications that are currently running and their needs. I believe any use of OpenGL by an application would cause a switch to the discrete graphics card, for instance.

    If you need to notice when such a switch occurs while your application is running, you can register a callback (CGDisplayRegisterReconfigurationCallback) and examine the changes that occur (kCGDisplayAddFlag, kCGDisplayRemoveFlag, etc). If you’re trying to match a display to one previously used/encountered, you would need to go beyond just comparing display id’s.

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