Convert Data to String in Swift 3

This is an example using a completion handler:

class func getFilm(filmID: Int, completion: @escaping (String) -> ()) {
    let url = URL(string: "https://api.kinopoisk.cf/getFilm?filmID=\(filmID)")!
    
    URLSession.shared.dataTask(with:url) { (data, response, error) in
      if error != nil {
        print(error!)
        completion("")
      } else {
        if let returnData = String(data: data!, encoding: .utf8) {
          completion(returnData)
        } else {
          completion("")
        }
      }
    }.resume()
}

And you call it

MyClass.getFilm(filmID:12345) { result in
   print(result)
}

In case of an error the completion handler returns an empty string.

MyClass is the enclosing class of getFilm method. Most likely the web service will return JSON, so you might need to deserialize the JSON to an array or dictionary.

In a more sophisticated version create an enum with two cases and associated values

enum ConnectionResult {
  case success(String), failure(Error)
}

With a little more effort demonstrating the subtle power of Swift you can return either the converted string on success of the error on failure in a single object.

class func getFilm(filmID: Int, completion: @escaping (ConnectionResult) -> ()) {
    let url = URL(string: "https://api.kinopoisk.cf/getFilm?filmID=\(filmID)")!
    
    URLSession.shared.dataTask(with:url) { (data, response, error) in
      if error != nil {
        completion(.failure(error!))
      } else {
        if let returnData = String(data: data!, encoding: .utf8) {
          completion(.success(returnData))
        } else {
          completion(.failure(NSError(domain: "myDomain", code: 9999, userInfo: [NSLocalizedDescriptionKey : "The data is not converible to 'String'"])))
        }
      }
    }.resume()
}

On the caller side a switch statement separates the cases.

MyClass.getFilm(filmID:12345) { result in
    switch result {
    case .success(let string) : print(string)
    case .failure(let error) : print(error)
    }
}

Meanwhile – with async/await in Swift 5.5+ – it has become much more comfortable

class func getFilm(filmID: Int) async throws -> String {
    print("getFilm")
    
    let url = URL(string: "https://api.kinopoisk.cf/getFilm?filmID=\(filmID)")!
    let (data, _) = try await URLSession.shared.data(from: url)
    return String(data: data, encoding: .utf8)!
}