OperatonQueue.swift

let queue = OperationQueue()
        queue.maxConcurrentOperationCount = 2
         
        let operation1 = BlockOperation(block: {
          let a = 5+1000
            print(a)
        })
        operation1.qualityOfService = .userInitiated
         
        let operation2 = BlockOperation(block: {
          let b = 5+2000
            print(b)
        })
         
        operation1.completionBlock = {
            
        }
        operation2.completionBlock = {
            
        }
         
        operation2.addDependency(operation1)
         
        queue.addOperation(operation1)
        queue.addOperation(operation2)