async-callback.swift

// In the async side...
func someAsyncFunction(completion: @escaping (Error?) -> Void) {
  // Something that takes some time to complete.
  completion(nil) // Or completion(SomeError.veryBadError)
}

// In the consumer side...
someAsyncFunction { error in
  if let error = error {
    print("Oops! Something went wrong...")
  } else {  
    print("It has finished") 
  }
}