如何在NSOperationQueue中暂停NSOperation？

I need to pause a running NSOperation which was inserted in an NSOperationQueue. Currently I am canceling all operations and restarting them. But this would lead to some kind of duplication in terms of process done. I tried with setSuspended flag of NSOperationQueue. But it's not suspending the operation. Is there any way out?I need to pause a running NSOperation which was