[http://developer.apple.com/documentation/Cocoa/Reference/ApplicationKit/Classes/NSGraphicsContext_Class/index.html]
The NSGraphicsContext class is the programmatic interface to objects that represent graphics contexts.
To avoid the following error in the console:
unlockFocus called too many time
Make sure you’re saving/restoring the graphics context properly. Example:
[NSGraphicsContext saveGraphicsState];
[NSGraphicsContext setCurrentContext:[NSGraphicsContext graphicsContextWithGraphicsPort:context flipped:NO]];
// draw something
[NSGraphicsContext restoreGraphicsState];