Swift completion handler in class and function - swift

Question

I have a Class with a function that connect to a firestoreDB and get some data:
import UIKit
import CoreLocation
import Firebase
private let _singletonInstance = GetBottlesFromDB()
class GetBottlesFromDB: NSObject {
class var sharedInstance: GetBottlesFromDB { return _singletonInstance }
var Pins = [LayoutBottlesFromDB]()
// MARK: - init
override init() {
super.init()
populatePinList(completion: { pin in self.Pins } )
//print("GET ALL PINS: \(Pins)")
}
func populatePinList(completion: #escaping ([LayoutBottlesFromDB]) -> ()) {
Pins = []
AppDelegate.ADglobalVar.db.collection("Bottles").whereField("pickupuser", isEqualTo: NSNull()).getDocuments { (querySnapshot, err) in
if let err = err {
print("Error getting documents: \(err)")
} else {
print("start getting documents:")
for document in querySnapshot!.documents {
//print("\(document.documentID) => \(document.data())")
//print("\(document.documentID)")
let bottleID:String = document.documentID
let bottlekind:Int = document.data()["bottle"] as! Int
var bottletitel:String
var bottlesub:String
var bottleurl:String = (document.data()["pic"] as? String)!
let pin = LayoutBottlesFromDB(document.data()["lat"] as! CLLocationDegrees, document.data()["long"] as! CLLocationDegrees, ID: bottleID, title: bottletitel, subtitle: bottlesub, type: bottlekind, url:bottleurl)
//print("GET DAATA from DB: \(pin)")
self.Pins.append(pin)
} //for
completion(self.Pins)
} //else
} //querysnap
}//function
}//class
in my ViewController I call this function.
for pin in GetBottlesFromDB.sharedInstance.Pins{
print("Add Pin : \(pin)")
}
My pProblem is that the function will called but the print is empty.
The function doesn't wait for a completion. What did I do wrong?

Answer 1

You are calling directly GetBottlesFromDB.sharedInstance.Pins and this will not wait for completion of populatePinList method so that's why you are getting blank So You need to wait for completion or you can check if data is not available in pins variable the you need to call completion method like this way:
GetBottlesFromDB.sharedInstance.populatePinList { (pins) in
for pin in pins{
print("Add Pin : \(pin)")
}
}

Answer 2

Nothing in your code waits for the execution of the asynchronous method, so that's no surprise. Also, it would be a terrible design because it would block your app. In addition, your singleton implementaion is overly verbose and doesn't guarantee that it stays a singleton, so I'd recommend to change it to
class GetBottlesFromDB {
private(set) var Pins = [LayoutBottlesFromDB]()
static let shared = GetBottlesFromDB()
private init() {}
// populatePinList as before
}
and in your view controller, e.g. in viewDidLoad do:
override func viewDidLoad() {
GetBottlesFromDB.shared.populatePinList { pins in
pins.forEach { print("Add Pin: \(pin)") }
}
}