How to store a data from a closure firebase in a local variable? - swift

Question

I'm trying to store the value of the completion block into a dictionary. But I don't know how to access and store it in a local dictionary variable.
var id = String()
var answeredDict = [String:[String]]()
var answeredDictUsers = [String:String]()
override func viewDidLoad() {
super.viewDidLoad()
for user in answeredDict.keys{
let ref = Database.database().reference(fromURL: "URL").child("users/\(user)")
ref.child("name").observeSingleEvent(of: .value) { (snap) in
guard let name = snap.value as? String else { return }
self.answeredDictUsers.updateValue(name, forKey: user)
}
}
print(answeredDictUsers)
}
the print(answeredDictUsers) gives [:] (empty dictionary)

Answer 1

Data is loaded from Firebase asynchronously. By the time your print(answeredDictUsers) runs, the code inside the callback hasn't run yet. And since self.answeredDictUsers.updateValue(name, forKey: user) hasn't run yet, the print sees an empty array. You can easily test this for yourself by placing breakpoints on both those lines and running the code in the debugger.
This means that any code that needs data from the database, must be (called from) inside the callback/completion handler that gets that data from the database.
A very simple example:
for user in answeredDict.keys{
let ref = Database.database().reference(fromURL: "URL").child("users/\(user)")
ref.child("name").observeSingleEvent(of: .value) { (snap) in
guard let name = snap.value as? String else { return }
self.answeredDictUsers.updateValue(name, forKey: user)
print(answeredDictUsers)
}
}
The above will print the dictionary each time it has loaded one of your users' data.
If you only want to print the dictionary once the data for all users has been loaded, you could for example keep a counter:
let count = 0
for user in answeredDict.keys{
let ref = Database.database().reference(fromURL: "URL").child("users/\(user)")
ref.child("name").observeSingleEvent(of: .value) { (snap) in
guard let name = snap.value as? String else { return }
self.answeredDictUsers.updateValue(name, forKey: user)
count = count + 1
if count == dict.count {
print(answeredDictUsers)
}
}
}