Sync.m

@interface Sync()

@property (nonatomic) dispatch_queue_t syncQueue;

@end

@implementation Sync

@synthesize syncQueue;

- (id) init {
    self = [super init];
    if(self){
        self.syncQueue = dispatch_queue_create("com.example.sync", DISPATCH_QUEUE_SERIAL);
    }
    return self;
}

//...

@end