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1. Queue with concurrent tasks and synchronization at the end

   dispatch_group_t group = dispatch_group_create();
   
   dispatch_group_enter(group);
   dispatch_async(dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_HIGH, 0), ^{
       NSLog(@"-> 1");
       [NSThread sleepForTimeInterval:2];
       NSLog(@"<- 1");
       dispatch_group_leave(group);
   });
   
   dispatch_group_enter(group);
   dispatch_async(dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_HIGH, 0), ^{
       NSLog(@"-> 2");
       [NSThread sleepForTimeInterval:4];
       NSLog(@"<- 2");
       dispatch_group_leave(group);
   });
   
   dispatch_group_enter(group);
   dispatch_async(dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_HIGH, 0), ^{
       NSLog(@"-> 3");
       [NSThread sleepForTimeInterval:3];
       NSLog(@"<- 3");
       dispatch_group_leave(group);
   });
   
   dispatch_group_notify(group, dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_HIGH, 0), ^{
       NSLog(@"finished");
   });
   
   [[NSRunLoop currentRunLoop] run];
   

   Logs:

   -> 1
   -> 2
   -> 3
   <- 1
   <- 3
   <- 2
   finished