Make code with Firebase asynchronous

Question

I am trying to create an algorithm that creates 12 struct objects(question) and puts them in an array(questions). However, it doesn't seem to work as the objects are created before the data from Firebase has been able to modify them. I was trying to make them asynchronous but nothing that I found online worked. Thanks in advance.

let databaseRef = FIRDatabase.database().reference()
    databaseRef.child("NumberOfQuestions").observeSingleEvent(of: .value, with: { snapshot in
    while self.questions.count < 12{
    var question = questionMGR() //questionMGR is the name of my struct
    let questionNumber = String(Int(arc4random_uniform(snapshot.value as! UInt32) + 1))

    databaseRef.child("questions").child("questionNumber: " + questionNum).child("title").observeSingleEvent(of: .value, with: { snapshot in
        print(snapshot.value ?? "")
        question.title = (snapshot.value as? String)!

        databaseRef.child("questions").child("questionNumber: " + questionNum).child("description").observeSingleEvent(of: .value, with: { snapshot in
            print(snapshot.value ?? "")
            question.desc = (snapshot.value as? String)!
        })
    })
        self.questions.append(question)
   }
 })

Answer 1

When this happens, the solution is usually to move the operation inside the callback:

databaseRef.child("questions").child("questionNumber: " + questionNum).child("title").observeSingleEvent(of: .value, with: { snapshot in
    print(snapshot.value ?? "")
    question.title = (snapshot.value as? String)!

    databaseRef.child("questions").childSnapshot(forPath: "questionNumber: " + questionNum).childSnapshot(forPath: "description").observeSingleEvent(of: .value, with: { snapshot in
        print(snapshot.value ?? "")
        question.desc = (snapshot.value as? String)!
        self.questions.append(question)
    })
})

But in this case I wonder why you don't simply load the entire question in one go:

FIRDatabaseReference qref = databaseRef.child("questions").child("questionNumber: " + questionNum)
qref.observeSingleEvent(of: .value, with: { snapshot in
    var question = questionMGR()
    question.title = (snapshot.childSnapshot(forPath: "title").value as? String)!
    question.desc = (snapshot.childSnapshot(forPath: "description").value as? String)!
    self.questions.append(question)
})

The only reason I'd ever consider two separate calls for title and description is if your questions contain a lot more data that you don't need here. But if that's the case, I'd remodel your JSON to separate the title and description into a separate top-level list.

In NoSQL it's often a Good Thing (tm) to model your data to be very similar to what you show on the screen. So if you have a list if the title and description of each question, you should consider storing precisely that: a list of the the title and description for each question.