Is there a way to check if the iOS device is locked/unlocked?

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

Here's a better solution

#import <notify.h>

#define kNotificationNameDidChangeDisplayStatus                 @"com.apple.iokit.hid.displayStatus"

@interface YourClass ()
{    
    int _notifyTokenForDidChangeDisplayStatus;
}

@property (nonatomic, assign, getter = isDisplayOn) BOOL displayOn;
@property (nonatomic, assign, getter = isRegisteredForDarwinNotifications) BOOL registeredForDarwinNotifications;

@end

- (void)registerForSomeNotifications
{
    //
    // Display notifications
    //

    __weak YourClass *weakSelf = self;

    uint32_t result = notify_register_dispatch(kNotificationNameDidChangeDisplayStatus.UTF8String,
                                               &_notifyTokenForDidChangeDisplayStatus,
                                               dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_BACKGROUND, 0l),
                                               ^(int info) {
                                                   __strong YourClass *strongSelf = weakSelf;

                                                   if (strongSelf)
                                                   {
                                                       uint64_t state;
                                                       notify_get_state(_notifyTokenForDidChangeDisplayStatus, &state);

                                                       strongSelf.displayOn = (BOOL)state;
                                                   }
                                               });
    if (result != NOTIFY_STATUS_OK)
    {
        self.registeredForDarwinNotifications = NO;
        return;
    }

    self.registeredForDarwinNotifications = YES;
}

- (void)unregisterFromSomeNotifications
{
    //
    // Display notifications
    //

    uint32_t result = notify_cancel(_notifyTokenForDidChangeDisplayStatus);
    if (result == NOTIFY_STATUS_OK)
    {
        self.registeredForDarwinNotifications = NO;
    }
}

/* Register app for detecting lock state */

 -(void)registerAppforDetectLockState {

     int notify_token;
     notify_register_dispatch("com.apple.springboard.lockstate",     &notify_token,dispatch_get_main_queue(), ^(int token) {
     uint64_t state = UINT64_MAX;
     notify_get_state(token, &state);
     if(state == 0) {
        NSLog(@"unlock device");
     } else {
        NSLog(@"lock device");
     }

     NSLog(@"com.apple.springboard.lockstate = %llu", state);
     UILocalNotification *notification = [[UILocalNotification alloc]init];
     notification.repeatInterval = NSDayCalendarUnit;
     [notification setAlertBody:@"Hello world!! I come becoz you lock/unlock your device :)"];
     notification.alertAction = @"View";
     notification.alertAction = @"Yes";
     [notification setFireDate:[NSDate dateWithTimeIntervalSinceNow:1]];
     notification.soundName = UILocalNotificationDefaultSoundName;
     [notification setTimeZone:[NSTimeZone  defaultTimeZone]];

     [[UIApplication sharedApplication] presentLocalNotificationNow:notification];

  });

 }

Apps are not allowed to listen to device lock notifications now!.

I had receive this:

Dear developer,

We have discovered one or more issues with your recent submission for "xxxx". To process your submission, the following issues must be corrected:

Unsupported operation - Apps are not allowed to listen to device lock notifications.

Once these issues have been corrected, use Xcode or Application Loader to upload a new binary to iTunes Connect. Choose the new binary on the app’s Details page in My Apps on iTunes Connect, and click Submit for Review.

Regards,

The App Store team
April 26 2017 at 10:56