Sample code

void LisaActions(CFNotificationCenterRef center, void *observer, CFStringRef name, const void *object, CFDictionaryRef userInfo) {
    // Get notification name
    NSString *notifName = (__bridge NSString*)name;

    if([notifName isEqualToString:@"com.apple.springboard.lockstate"])
        // Track if locked or unlocked
        lock = !lock;

    //do something
}

%ctor {
// CODE
CFNotificationCenterAddObserver(CFNotificationCenterGetDarwinNotifyCenter(),
                                    NULL,
                                    LisaActions,
                                    CFSTR("com.apple.springboard.lockstate"),
                                    NULL,
                                    CFNotificationSuspensionBehaviorDeliverImmediately);

// CODE
}