let queue1 = DispatchQueue(label: "com.reejo.test",
                                   qos: .userInitiated,
                                   attributes: .concurrent,
                                   autoreleaseFrequency: .inherit,
                                   target: DispatchQueue.global())

        queue1.async {
            print("welcome!")
        }

        let operationQueue = OperationQueue()

        let blockOperation = BlockOperation {
            print("Welcome on BlockOperation")
        }
        blockOperation.queuePriority = .low
        blockOperation.qualityOfService = .background


        let dependentOperation = BlockOperation {
            print("Pre block operation")
        }
        dependentOperation.queuePriority = .veryLow
        dependentOperation.qualityOfService = .background

        let plainOperation = Operation()
        plainOperation.completionBlock = {
            print("plain operation depending on none")
        }
        plainOperation.queuePriority = .veryHigh

        blockOperation.addDependency(dependentOperation)
        operationQueue.addOperation(blockOperation)
        operationQueue.addOperation(dependentOperation)
        operationQueue.addOperation(plainOperation)

//        welcome!
//        Pre block operation
//        plain operation depending on none
//        Welcome on BlockOperation