Completion handler swift 3 return a variable from function

Question

I am confused surrounding the syntax for a completion handler in swift 3.

In the function below, after parsing an xml file from a web service call, it should return a variable (an array [String:String]).
My attempt is below, but obviously it is incorrect.

  enum HistoryKey {
  case success([String:String])
  case failure(String)
 }

 private func getHistoryKeys(searchterm: String, completion: @escaping () -> HistoryKey) {
    let url = PubmedAPI.createEsearchURL(searchString: searchterm)
    let request = URLRequest.init(url: url as URL)
    let task = session.dataTask(with: request) { (data, response, error) in

        if let theData = data{
            let myParser = XMLParser.init(data: theData)
            myParser.delegate = self
            myParser.parse()
        }
    }
    task.resume()

    if keys.isEmpty {
        return .failure("no historyKeyDictionary")
    }else{
        return .success(keys)
    }

}// End of func

I want to use this function as follows

 let result = self.getHistoryKeys(searchTerm)

Answer 1

Two issues:

• The completion handler passes a HistoryKey instance and has no return value so the signature must be the other way round.
• The call of the completion handler must be inside the completion block of the data task.

---

To be able to parse the received data outside the completion block return the data on success

enum ConnectionResult {
   case success(Data)
   case failure(Error)
}

private func getHistoryKeys(searchterm: String, completion: @escaping (ConnectionResult) -> ()) {
   let url = PubmedAPI.createEsearchURL(searchString: searchterm)
   let task = session.dataTask(with: url) { (data, response, error) in
       if let error = error {
          completion(.failure(error))
       } else {
          completion(.success(data!))
       }
  }
  task.resume()
}

and call it

getHistoryKeys(searchterm: String) { connectionResult in 
    switch connectionResult {
       case .success(let data): 
           let myParser = XMLParser(data: data)
           myParser.delegate = self
           myParser.parse()
           // get the parsed data from the delegate methods

       case .failure(let error): print(error)
    }
}

Answer 2

You are not using completion block.
Use it like:

private func getHistoryKeys(searchterm: String, completion: @escaping (_ keys: Array) -> Void) {
    //do the magic
    completion(keys)
}

Then you can call this function as:

getHistoryKeys(searchterm: "str") { (keys) in 
    print("\(keys)")
}