How to save data from firebase asynchronous request in a global variable? - ios

Question

I know this has been asked on stack overflow several times but I cannot seem to find the answer I am looking for. I am trying to store data from a firebase database (using the observeSingleEvent(snapshot)) method in a global variable. See below code for details.
I've tried adding a completion handler and followed steps online, but doing so, the observeSingleEvent request stops working.
What I had before:
class ItemsTableViewController: UITableViewController {
let listToUsers = "ListToUsers"
var user: User!
let ref = Database.database().reference()
override func viewDidLoad() {
super.viewDidLoad()
let defaultUser = User()
ref.child("users").child(uid).observeSingleEvent(of: .value, with: { (snapshot) in
let value = snapshot.value as? NSDictionary
defaultUser.uid = uid
defaultUser.email = value?["email"] as? String ?? ""
defaultUser.name = value?["name"] as? String ?? ""
defaultUser.grad = value?["grad"] as? Int ?? 0
defaultUser.number = value?["number"] as? String ?? ""
defaultUser.image = UIImage(named: value?["image"] as? String ?? "")!
completion(defaultUser)
}) { (error) in
print("hello")
print(error.localizedDescription)
}
self.user = defaultUser
}
}
What I tried after and still did not work:
class ItemsTableViewController: UITableViewController {
// MARK: Constants
let listToUsers = "ListToUsers"
// MARK: Properties
var user: User!
let ref = Database.database().reference()
override func viewDidLoad() {
super.viewDidLoad()
let use = Auth.auth().currentUser?.uid
self.getUserData(uid:use!) { (user) -> () in
self.user = user
}
}
func getUserData(uid:String , completion: #escaping (User) -> ()) {
ref.child("users").child(uid).observeSingleEvent(of: .value, with: { (snapshot) in
let value = snapshot.value as? NSDictionary
let defaultUser = User()
defaultUser.uid = uid
defaultUser.email = value?["email"] as? String ?? ""
defaultUser.name = value?["name"] as? String ?? ""
defaultUser.grad = value?["grad"] as? Int ?? 0
defaultUser.number = value?["number"] as? String ?? ""
defaultUser.image = UIImage(named: value?["image"] as? String ?? "")!
completion(defaultUser)
}){ (error) in
print("hello")
print(error.localizedDescription)
}
}

Answer 1

Firebase is asynchronous and the self.user = defaultUser in the first code block is going to be called way before Firebase has a chance to return data.
Data is ONLY valid within the closure following the Firebase call. So here's how to fix it. Oh, and remove that error code as it isn't needed.
Assume a user structure like this
users
uid_0
name: "some name"
email: "some email"
and the code to read in uid_0 and populate a global (i.e. class var) user
class MyUser {
var uid = ""
var name = ""
var email = ""
init(aUid: String, aName: String, aEmail: String) {
self.uid = aUid
self.name = aName
self.email = aEmail
}
}
var user: MyUser!
func readOneUser() {
let uid = "uid_0"
let thisUserRef = self.ref.child("users").child(uid)
thisUserRef.observeSingleEvent(of: .value, with: { snapshot in
//let uid = snapshot.key if you don't know the uid, it will be the key to the node
let name = snapshot.childSnapshot(forPath: "name").value as? String ?? "No Name"
let email = snapshot.childSnapshot(forPath: "email").value as? String ?? "No Email"
self.user = MyUser(aUid: uid, aName: name, aEmail: email)
print(self.user.name) //do something with the user here
})
}
and this line
defaultUser.image = UIImage(named: value?["image"] as? String ?? "")!
needs to be addressed but I don't know what the intention is. It could end up being nil if this happens
defaultUser.image = UIImage(named: "")!
so you may want to populate it with a default image of some kind or provide error handling in case it is nil.