Explain NSOPERATION Priority.

Ans : 

All operations may not be equally important. Setting the queuePriority property will promote or defer an operation in an NSOperationQueue according to the following rankings:

public enum NSOperationQueuePriority : Int {

    case VeryLow

    case Low

    case Normal

    case High

    case VeryHigh

}

The operations with high priority will be executed first.

let backgroundOperation = NSOperation()

backgroundOperation.queuePriority = .Low

let operationQueue = NSOperationQueue.mainQueue()

operationQueue.addOperation(backgroundOperation)