swift - Assign value of a Firestore document to a variable

Question

I am trying to read the value of a Firestore document. I have tried doing it two different ways, but each fails. In the first one, an error is thrown on the return line: Unexpected non-void return value in void function. I found out why this happened, and so, I implemented the second way.

import UIKit
import Firestore

func readAvailableLists(forUser user: String) -> [String] {
    let db = Firestore.firestore()
    db.collection("userslist").document(user).getDocument { (document, err) in
        if let document = document, document.exists {
            return UserInformationDocument(dictionary: document.data()!)?.lists!
        } else {
            print("Document does not exist")
        }
    }
}

In the second method, I assign the UserInformationDocument(dictionary: document.data()!)?.lists! to a variable and return that variable at the end of the function (see code below). However, when I do this, it the function returns an empty array. What surprises me is that the print return the correct value, but after long after the function has executed the return statement. Is it because it is an async demand? And if so, how should I fix this?

import UIKit
import Firestore

func readAvailableLists(forUser user: String) -> [String] {
    let db = Firestore.firestore()
    var firestoreUserDocument: [String] = []
    db.collection("userslist").document(user).getDocument { (document, err) in
        if let document = document, document.exists {
            firestoreUserDocument = (UserInformationDocument(dictionary: document.data()!)?.lists!)!
        print((UserInformationDocument(dictionary: document.data()!)?.lists!)!)
        } else {
            print("Document does not exist")
        }
    }
    return firestoreUserDocument
}

Answer 1

Waitting for answers