Waiting until the task finishes

Question

How could I make my code wait until the task in DispatchQueue finishes  Does it need any CompletionHandler or something   func myFunction         var a  Int       DispatchQueue main async           var b  Int   3         a   b               wait until the task finishes  then print       print a     - this will contain nil  of course  because it                 will execute before the code above      I m using Xcode 8 2 and writing in Swift 3

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Swift 5 version of the solution func myCriticalFunction         var value1  String      var value2  String       let group   DispatchGroup         group enter         async operation 1     DispatchQueue global qos   default  async               Network calls or some other async task         value1     out of async task         group leave               group enter         async operation 2     DispatchQueue global qos   default  async              Network calls or some other async task         value2     out of async task         group leave                   group wait        print  quot Value1   value1    Value2   value2  quot

User · Answer

In Swift 3  there is no need for completion handler when DispatchQueue finishes one task   Furthermore you can achieve your goal in different ways  One way is this       var a  Int       let queue   DispatchQueue label   com app queue       queue sync            for  i in 0   lt 10                print        i              a   i                      print  After Queue   a      It will wait until the loop finishes but in this case your main thread will block   You can also do the same thing like this        let myGroup   DispatchGroup       myGroup enter            Do your task      myGroup leave        When your task completes      myGroup notify queue  DispatchQueue main                    do your remaining work         One last thing  If you want to use completionHandler when your task completes using DispatchQueue  you can use DispatchWorkItem   Here is an example how to use DispatchWorkItem   let workItem   DispatchWorkItem          Do something    let queue   DispatchQueue global   queue async       workItem perform     workItem notify queue  DispatchQueue main           Here you can notify you Main thread

User · Answer

Use dispatch group dispatchGroup enter   FirstOperation completion      in     dispatchGroup leave      dispatchGroup enter   SecondOperation completion      in     dispatchGroup leave      dispatchGroup wait      Waits here on this thread until the two operations complete executing

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Use DispatchGroups to achieve this  You can either get notified when the group s enter   and leave   calls are balanced   func myFunction         var a  Int       let group   DispatchGroup       group enter        DispatchQueue main async           a   1         group leave                 does not wait  But the code in notify   gets run         after enter   and leave   calls are balanced      group notify queue   main            print a            or you can wait   func myFunction         var a  Int       let group   DispatchGroup       group enter           avoid deadlocks by not using  main queue here     DispatchQueue global attributes   qosDefault  async           a   1         group leave                 wait         group wait        print a     you could also  return a  here     Note  group wait   blocks the current queue  probably the main queue in your case   so you have to dispatch async on another queue  like in the above sample code  to avoid a deadlock

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Swift 4  You can use Async Function for these situations  When you use DispatchGroup   Sometimes deadlock may be occures   var a  Int   objc func myFunction completion  escaping  Bool  - gt              DispatchQueue main async           let b  Int   3         a   b         completion true            override func viewDidLoad         super viewDidLoad        myFunction    status  in         if status               print self a

[swift] Waiting until the task finishes

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