loop in a loop not working - ios

Question

I'm working on an app for school project.
After accessing the user's contacts, I want to loop through and show only the contacts who are also users of the app. *Their username is their mobile number.
below are 3 functions.
the first one getAppUsers() works fine.
the third one getDisplayedUser() does not work. and i wonder why
the second one getUserContacts() works. but it is only there to check which part of my loop isn't working. :/
so apparently my loop in a loop has something wrong which i can't figure out (it didn't even get to the "you're HERE"). please help me out. THANKS!
var appUsers = [String]()
var contactStore = CNContactStore()
var userContacts = [CNContact]()
var displayedContacts = [name: phoneNumber]()
func getAppUsers() {
let appUsersQuery = PFUser.query()
appUsersQuery?.findObjectsInBackground { (objects, error) in
if error != nil {
print("WTF")
} else if let users = objects {
for object in users {
print("FYEAH!")
if let user = object as? PFUser {
self.appUsers.append(user.username!)
print(user.username)
}
}
}
}
}
func getUserContacts() {
for b in userContacts {
let c = (b.phoneNumbers[0].value).stringValue
let d = c.replacingOccurrences(of: "\\D", with: "", options: .regularExpression, range: c.startIndex..<c.endIndex)
print("you got here")
print(d)
}
}
func getDisplayedUser() {
for a in appUsers {
for b in userContacts {
let c = (b.phoneNumbers[0].value).stringValue
let d = c.replacingOccurrences(of: "\\D", with: "", options: .regularExpression, range: c.startIndex..<c.endIndex)
print("you're HERE")
print(d)
if a == d {
print("FOUND IT")
print(b.givenName + " " + b.familyName)
}
}
}
}

Answer 1

The getDisplayedUser should be call after the loop finished in in getAppUsers because it is executing in asynchronous mode. I added the row after finished loop below
func getAppUsers() {
let appUsersQuery = PFUser.query()
appUsersQuery?.findObjectsInBackground { (objects, error) in
if error != nil {
print("WTF")
} else if let users = objects {
for object in users {
print("FYEAH!")
if let user = object as? PFUser {
self.appUsers.append(user.username!)
print(user.username)
}
}
// Call it here please ..
self.getDisplayedUser()
}
}
}