ViewController.swift

import Foundation
import Firebase
import FirebaseFunctions
import FirebaseFirestoreSwift

struct MyCollection: Codable {
  var _id:String?
  var title:String
  var description:String?
  var keywords:[String]?
}

extension Encodable {
  func asDictionary() throws -> [String: Any] {
    let data = try JSONEncoder().encode(self)
    guard let dictionary = try JSONSerialization.jsonObject(with: data, options: .allowFragments) as? [String: Any] else {
      throw NSError()
    }
    return dictionary
  }
}

static func save(collection:MyCollection, _ completion: @escaping (String) -> Void, error: @escaping (String) -> Void ) {
  let db = Firestore.firestore()
  var ref: DocumentReference? = nil

  var data = try! collection.asDictionary()
  data["timestamp"] = FieldValue.serverTimestamp()
  
  if(collection._id != nil && collection._id != "" ){
    ref = try db.collection("collections").document(collection._id!)

    ref?.setData(data) { err in
      if let err = err {
        print("Error adding document: \(err)")
      } else {
        print("Document added with ID: \(ref!.documentID)")
        completion(ref!.documentID)
      }
    }
  }else{
    ref = try db.collection("collections").addDocument(data: data){ err in
      if let err = err {
        print("Error adding document: \(err)")
      } else {
        print("Document added with ID: \(ref!.documentID)")
        completion(ref!.documentID)
      }
    }
  }  
}