如何使用Alamofire和SwiftyJSON从Json保存数据？

Question

I am trying to save "author" data to global variable named "authors" from json(Link:" https://learnappmaking.com/ex/books.json ") with these two libraries. But it only works at the trailing closure of func Alamofire.request(url).responseJSON . When I access the global variable named "authors" from somewhere except the trailing closure, what I get is an empty array of string.

Can someone explain the reason behind this werid situation？ Thanks a lot.

class ViewController: UIViewController {

    var authors = [String]()
    let url = "https://learnappmaking.com/ex/books.json"


    func getAuthorsCount() {
        print("the number of authors : \(authors.count)") // I hope that here, the number of authors should be 3 too! actually, it is 0. Why? 

        // this for loop doesn't get excuted
        for author in authors {
            print(author)
        }
    }

    override func viewDidLoad() {
        super.viewDidLoad()
        // Do any additional setup after loading the view.

        Alamofire.request(url).responseJSON { response in
            if let data = response.data {
                if let json = try? JSON(data: data) {
                    for item in json["books"].arrayValue {
                        var outputString: String
                        print(item["author"])
                        outputString = item["author"].stringValue
                        //urlOfProjectAsset.append(outputString)
                        self.authors.append(outputString)
                        print("authors.count: \(self.authors.count)")
                    }
                }
            }
    }

        getAuthorsCount()
        print("-------------")
    }
}

the actual output is:

---

Update ： I adjusted my code:

class ViewController: UIViewController {

    var authors = [String]()
    let url = "https://learnappmaking.com/ex/books.json"


    func getAuthorsCount() {
        print("the number of authors : \(authors.count)")

        // this for loop doesn't get excuted
        for author in authors {
            print(author)
        }
    }

    override func viewDidLoad() {
        super.viewDidLoad()
        // Do any additional setup after loading the view.

        Alamofire.request(url).responseJSON { response in
            if let data = response.data {
                if let json = try? JSON(data: data) {
                    for item in json["books"].arrayValue {
                        var outputString: String
                        //print(item["author"])
                        outputString = item["author"].stringValue
                        //urlOfProjectAsset.append(outputString)
                        self.authors.append(outputString)
                        //print("authors.count: \(self.authors.count)")
                    }
                self.getAuthorsCount() // I added this line of code.
                }
            }
    }
        getAuthorsCount()
        print("-------------")
    }
}

But why does the func getAuthorsCount() (not self. version) still print an empty array of strings ? I think the result should be the same as the result which func self.getAuthorsCount() printed. I am so confused now... Again, I want to use the data kept in the variable named "authors", but what I only got is an empty array of strings.

Answer 1

I'll try to answer all your questions :

• The data is persistant

• You are doing the following : Alamo.request (Network call) -> getAuthors(print result - empty) -> response (receive response) -> self.authors.append(save response) -> self.authors (print result)

• You need to do : Alamo.request (Network call) -> response (receive response) -> self.authors.append(save response) -> self.getAuthors or getAuthors(same) (inside the response {})

You need to call getAuthors once you have your result, inside the response callback :

override func viewDidLoad() {
        super.viewDidLoad()
        // Do any additional setup after loading the view.

        Alamofire.request(url).responseJSON { response in
            if let data = response.data {
                if let json = try? JSON(data: data) {
                    for item in json["books"].arrayValue {
                        var outputString: String
                        print(item["author"])
                        outputString = item["author"].stringValue
                        //urlOfProjectAsset.append(outputString)
                        self.authors.append(outputString)
                        print("authors.count: \(self.authors.count)")
                    }

                    self.getAuthorsCount()
                    print("-------------")
                   //Do whatever you want from here : present/push 


                }
            }
        }

Then you can use the saved data :

• To send the data to another ViewController you can use various methods (present/push, closure/callback, ...)
• Usually you will have a loading spinner to wait for the network to answer then you will show your next controller

Answer 2

As requested via direct message: a Swift-only approach. Just paste this in a blank Playground:

import Foundation

final class NetworkService {

  enum ServiceError: LocalizedError {
    case invalidUrl
    case networkingError(error: Error)
    case parsingError

    var localizedDescription: String? { return String(describing: self) }
  }


  func request(completion: @escaping (Result<[UserObject], Error>) -> Void ) {

    guard let url = URL(string: "https://jsonplaceholder.typicode.com/users") else {
      completion(.failure(ServiceError.invalidUrl))
      return
    }

    let dataTask = URLSession.shared.dataTask(with: url) { (jsonData, response, error) in
      if let jsonData = jsonData {
        let jsonDecoder = JSONDecoder()

        do {
          let users = try jsonDecoder.decode([UserObject].self, from: jsonData)
          completion(.success(users))
        } catch {
          completion(.failure(ServiceError.parsingError))
        }

      } else if let error = error {
        completion(.failure(ServiceError.networkingError(error: error)))
      }
    }
    dataTask.resume()

  }
}

struct UserObject: Codable {
  let id: Int
  let name: String
  let username: String
  let email: String?
  let website: String?
}


let networkService = NetworkService()
networkService.request { result in

  switch result {
  case .success(let users):
    debugPrint("Received \(users.count) users from REST API")
    debugPrint(users)
  case .failure(let error):
    debugPrint(error.localizedDescription)
  }

}