DispatchQueue and DispatchGroup in Swift 3.0 as of Oct 4 2016

gistfile1.txt

   let dispatchGroup = DispatchGroup()
        
        let dq  = DispatchQueue.global(qos: .utility)
        dq.async {
            
            for i in 0...3{
                
                dq.async {
                    
                    print("\n i - \(i)")
                    dispatchGroup.enter()
                    
                    (DispatchQueue.global(qos: .userInitiated)).async(group: dispatchGroup, qos: .userInitiated, flags: .assignCurrentContext) {
                        () -> Void
                        in
                        Thread.sleep(forTimeInterval: 2.0)
                        print("\nBlock 1\n")
                        dispatchGroup.leave()
                    }
                    
                    (DispatchQueue.global(qos: .userInitiated)).async(group: dispatchGroup, qos: .userInitiated, flags: .assignCurrentContext) {
                        () -> Void
                        in
                        Thread.sleep(forTimeInterval: 5.0)
                        print("\nBlock 2\n")
                        dispatchGroup.leave()
                    }
                    
                    (DispatchQueue.global(qos: .userInitiated)).async(group: dispatchGroup, qos: .userInitiated, flags: .assignCurrentContext) {
                        () -> Void
                        in
                        Thread.sleep(forTimeInterval: 1.0)
                        print("\nBlock 3\n")
                        dispatchGroup.leave()
                    }
                    
                    
                    
                    dispatchGroup.notify(queue: DispatchQueue.global(qos: .userInitiated)) {
                        () -> Void
                        in
                        print("\nBlock 4\n")
                    }
                    
                }
            }
            
        }

also shows a neat way to call .async on the main queue without assigning it to a var: (DispatchQueue.global(qos: .userInitiated)).async

this is a translation of earlier code from Nishant Basin:
https://gist.github.com/nbhasin2/735cd80298b5d47852f2/

here, I get rid of the variable:

   let dispatchGroup = DispatchGroup()

    (DispatchQueue.global(qos: .userInitiated)).async {

        for i in 0...3{

            (DispatchQueue.global(qos: .userInitiated)).async {

                print("\n i - \(i)")
                dispatchGroup.enter()

                (DispatchQueue.global(qos: .userInitiated)).async(group: dispatchGroup, qos: .userInitiated, flags: .assignCurrentContext) {
                    () -> Void
                    in
                    Thread.sleep(forTimeInterval: 2.0)
                    print("\nBlock 1\n")
                    dispatchGroup.leave()
                }

                (DispatchQueue.global(qos: .userInitiated)).async(group: dispatchGroup, qos: .userInitiated, flags: .assignCurrentContext) {
                    () -> Void
                    in
                    Thread.sleep(forTimeInterval: 5.0)
                    print("\nBlock 2\n")
                    dispatchGroup.leave()
                }

                (DispatchQueue.global(qos: .userInitiated)).async(group: dispatchGroup, qos: .userInitiated, flags: .assignCurrentContext) {
                    () -> Void
                    in
                    Thread.sleep(forTimeInterval: 1.0)
                    print("\nBlock 3\n")
                    dispatchGroup.leave()
                }



                dispatchGroup.notify(queue: DispatchQueue.global(qos: .userInitiated)) {
                    () -> Void
                    in
                    print("\nBlock 4\n")
                }

            }
        }

    }