Swift, How to Get Completion Handler working as I Wish? - swift

Question

I am fetching user information from Firebase and my goal is to fetch a batch of users and then display them on the screen one at a time in a card stack. To do this i use a completion handler. However my code inside the completion handler runs before the fetch of all users is done.
Thank you for any help.
Here is my code. I want "fetchOneUser()" to run when "fetchAllUsers" is done:
fetchAllUsers(completion: { message in
print(message)
print("FetchOneUser")
self.fetchOneUser()
})
Here is fetchAllUser function:
func fetchAllUsers(completion: #escaping (_ message: String) -> Void){
//User or advertiser?
Database.database().reference(withPath: "Advertiser").child(uid).observeSingleEvent(of: .value, with: { (snapshot) in
if snapshot.exists(){
myAdvertiserVar.advertiser = true
self.currentUserKind = "Advertiser"
self.otherUserKind = "Users"
}
else{
self.currentUserKind = "Users"
self.otherUserKind = "Advertiser"
}
// Fetch
let query = self.ref?.child(self.otherUserKind).queryOrdered(byChild: "email")
query?.observeSingleEvent(of: .value) {
(snapshot) in
for child in snapshot.children.allObjects as! [DataSnapshot] {
let id = child.key
//If Already Accepted, don't fetch
Database.database().reference(withPath: self.currentUserKind).child(self.uid).child("Accepted").child(id).observeSingleEvent(of: .value, with: {(accepted) in
if accepted.exists(){
print("\(id) är redan Accepted")
}
else{
if myAdvertiserVar.advertiser == true{
let value = child.value as? NSDictionary
let username = value?["Username"] as? String
let occupation = value?["Occupation"] as? String
let age = value?["Age"] as? String
let bio = value?["Bio"] as? String
let email = value?["email"] as? String
let user = User(id: id, username: username, occupation: occupation, age: age, bio: bio, email: email)
self.usersArray.append(user)
}
else{
let value = child.value as? NSDictionary
let username = value?["Owner"] as? String
let occupation = value?["Location"] as? String
let age = value?["Rent"] as? String
let bio = value?["About"] as? String
let email = value?["email"] as? String
let user = User(id: id, username: username, occupation: occupation, age: age, bio: bio, email: email)
self.usersArray.append(user)
}
}
})
}
print(self.usersArray.count)
completion("Users list fetched")
}
})
}

Answer 1

You need to use DispatchGroup as the inner calls are asynchronous
func fetchAllUsers(completion: #escaping (_ message: String) -> Void){
//User or advertiser?
Database.database().reference(withPath: "Advertiser").child(uid).observeSingleEvent(of: .value, with: { (snapshot) in
if snapshot.exists(){
myAdvertiserVar.advertiser = true
self.currentUserKind = "Advertiser"
self.otherUserKind = "Users"
}
else{
self.currentUserKind = "Users"
self.otherUserKind = "Advertiser"
}
// Fetch
let query = self.ref?.child(self.otherUserKind).queryOrdered(byChild: "email")
query?.observeSingleEvent(of: .value) {
(snapshot) in
let g = DispatchGroup()
for child in snapshot.children.allObjects as! [DataSnapshot] {
let id = child.key
//If Already Accepted, don't fetch
g.enter()
Database.database().reference(withPath: self.currentUserKind).child(self.uid).child("Accepted").child(id).observeSingleEvent(of: .value, with: {(accepted) in
if accepted.exists(){
print("\(id) är redan Accepted")
}
else{
if myAdvertiserVar.advertiser == true{
let value = child.value as? NSDictionary
let username = value?["Username"] as? String
let occupation = value?["Occupation"] as? String
let age = value?["Age"] as? String
let bio = value?["Bio"] as? String
let email = value?["email"] as? String
let user = User(id: id, username: username, occupation: occupation, age: age, bio: bio, email: email)
self.usersArray.append(user)
}
else{
let value = child.value as? NSDictionary
let username = value?["Owner"] as? String
let occupation = value?["Location"] as? String
let age = value?["Rent"] as? String
let bio = value?["About"] as? String
let email = value?["email"] as? String
let user = User(id: id, username: username, occupation: occupation, age: age, bio: bio, email: email)
self.usersArray.append(user)
}
}
g.leave()
})
}
g.notify(queue: .main, execute: {
print(self.usersArray.count)
completion("Users list fetched")
})
}
})
}

Answer 2

Based on Firebase documentation:
Firebase use refrence() method to get a database refrence for the root of your real time database asynchronous.
this means that result takes more time to fetch than for loop, in this situation your for loop finishes and completion block calls and takes you out of method, then result of your request will return.
your code should look like
var firebaseDatabaseRefrence: DatabaseReference!
override func viewDidLoad() {
super.viewDidLoad()
Database.database().reference(withPath: self.currentUserKind)
}
func someMethod() {
self.firebaseDatabaseRefrence
.child(self.uid)
.child("Accepted")
.child(id).observeSingleEvent(of: .value, with: {(accepted) in
}