Multi Threading in Swift (iOS) using Dispatch Group

Thread_management_iOS.swift

           //Group 4 will be executed last after 1-3 are done executing
           //Multi Threadin in Swift Group Dispatch Example 
           //Global Dispatch Queue
           
            let dispatchGroup = dispatch_group_create()
            
            
             dispatch_async(dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_HIGH, 0), {
                
            
            for i in 0...3{
                
            dispatch_async(dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_HIGH, 0), {
                
            println("\n i - \(i)")
            dispatch_group_enter(dispatchGroup)
            dispatch_group_async(dispatchGroup, dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_HIGH, 0), {
                NSThread.sleepForTimeInterval(2.0)
                println("\nBlock 1\n")
                dispatch_group_leave(dispatchGroup)
                })
            
                  dispatch_group_enter(dispatchGroup)
            dispatch_group_async(dispatchGroup, dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_HIGH, 0), {
                NSThread.sleepForTimeInterval(5.0)
                println("\nBlock 2\n")
                dispatch_group_leave(dispatchGroup)
            })
            
                  dispatch_group_enter(dispatchGroup)
            dispatch_group_async(dispatchGroup, dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_HIGH, 0), {
                NSThread.sleepForTimeInterval(1.0)
                println("\nBlock 3\n")
                dispatch_group_leave(dispatchGroup)
            })

                
                dispatch_group_notify(dispatchGroup, dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_HIGH, 0), {
                
                 println("\nBlock 4\n")
                })
                
            });
                
            }
            });

    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")
                }

            }
        }

    }

for swift 3.0 as of oct 4 2016

Swift code doesn't work as intended. Crashed.

when added dispatchGroup.enter() before each
(DisplatchQueue.global(....)) - block
then ok ...
( and in main process need to wait for those to complete - example add sleep if testing in Mac there ...)