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1. dispatch groups

   let group = dispatch_group_create()
   let mainQueue = dispatch_get_main_queue()
   
   for i in 1...10 {
   
       dispatch_group_enter(hgReadGroup)
   
       longSubTask(param:String, completionHandler: { (results) -> () in
           // ...
           dispatch_group_leave(group)
       })
   }
   
   dispatch_group_notify(group, mainQueue) {
       print("-> all subtasks are finished")
       completionHandler(entries:entries)
   }