I solved it like this:
//call back
static void displayStatusChanged(CFNotificationCenterRef center, void *observer, CFStringRef name, const void *object, CFDictionaryRef userInfo)
{
// the "com.apple.springboard.lockcomplete" notification will always come after the "com.apple.springboard.lockstate" notification
CFStringRef nameCFString = (CFStringRef)name;
NSString *lockState = (NSString*)nameCFString;
NSLog(@"Darwin notification NAME = %@",name);
if([lockState isEqualToString:@"com.apple.springboard.lockcomplete"])
{
NSLog(@"DEVICE LOCKED");
//Logic to disable the GPS
}
else
{
NSLog(@"LOCK STATUS CHANGED");
//Logic to enable the GPS
}
}
-(void)registerforDeviceLockNotif
{
//Screen lock notifications
CFNotificationCenterAddObserver(CFNotificationCenterGetDarwinNotifyCenter(), //center
NULL, // observer
displayStatusChanged, // callback
CFSTR("com.apple.springboard.lockcomplete"), // event name
NULL, // object
CFNotificationSuspensionBehaviorDeliverImmediately);
CFNotificationCenterAddObserver(CFNotificationCenterGetDarwinNotifyCenter(), //center
NULL, // observer
displayStatusChanged, // callback
CFSTR("com.apple.springboard.lockstate"), // event name
NULL, // object
CFNotificationSuspensionBehaviorDeliverImmediately);
}
Note: the “com.apple.springboard.lockcomplete” notification will always come after the “com.apple.springboard.lockstate” notification
Update
The order of the two notifications can no longer be relied upon, as of recent versions of iOS