dispatch_group.swift

let contactsGroup = dispatch_group_create()

dispatch_group_enter(contactsGroup) //Enter group for the first time
loadContactsFromServerAPIWithCompletion({ (contacts:[User]) -> () in
  doStuffWithContacts(contacts)
  dispatch_group_leave(contactsGroup) //Leave group
})

dispatch_group_enter(contactsGroup) //Enter group again
loadContactsAddressBookWithCompletion({ (contacts:[User]) -> () in
  doStuffWithContacts(contacts)
  dispatch_group_leave(contactsGroup) //Leave group
})

//GCD stores this block to be called once the group "contactsGroup" has no more entrants
dispatch_group_notify(contactsGroup, dispatch_get_main_queue(), { [weak self] () -> Void in
  sortContacts()
  tableView.reloadData()
})