Doing things in sequence using completion handlers - ios

Question

I have three functions that need to run in sequence as follows:
Gather integers from Firebase, place them into an array, and calculate the sum. Once complete...
Gather a different integer value from Firebase. Once complete...
The third function takes the value from function 2, and subtracts it from function 1.
I know I need to use completion handlers, but need some help with the syntax, etc.
//This first function grabs integers from Firebase and sums them up:
func LoadPointsCompleted(completion: #escaping(_ sumOfPointsCompleted:Int) -> Int){
self.challengeList.removeAll()
databaseReference = Database.database().reference()
let userID = Auth.auth().currentUser?.uid
let refChallenges = Database.database().reference(withPath: "Challenges").child(userID!).queryOrdered(byChild: "Status").queryEqual(toValue: "Complete")
refChallenges.observeSingleEvent(of: .value, with: { (snapshot) in
//if the reference have some values
if snapshot.childrenCount > 0 {
//clearing the list
self.challengeList.removeAll()
//iterating through all the values
for Challenges in snapshot.children.allObjects as! [DataSnapshot] {
//getting values
let challengeObject = Challenges.value as? [String: AnyObject]
let Points = challengeObject?["Points"] as! Int
//creating challenge object with model and fetched values
let challenge = pointsModel(Points: (Points as Int?)!)
//appending it to list
self.challengeList.append(challenge)
let sumOfPointsCompleted = self.challengeList.reduce(0) {$0 + $1.Points}
let sumOfPointsCompletedString = String(sumOfPointsCompleted)
self.Calc_Earned.text = sumOfPointsCompletedString
completion(sumOfPointsCompleted)
}
}
}
)}
// This second function just grabs an integer value from another location in Firebase
func loadPointsRedeemed(completion: #escaping (_ Points_Redeem: Int) -> Int) {
databaseReference = Database.database().reference()
let userID = Auth.auth().currentUser?.uid
databaseReference.child("Users").child(userID!).observe(DataEventType.value, with: { (snapshot) in
let value = snapshot.value as? NSDictionary
// let Points_Earn = value?["Points_Earned"] as? String ?? ""
let Points_Redeem = value?["Points_Redeemed"] as! Int
// self.Points_Earned.text = Points_Earn
let points_redeemedString = String(Points_Redeem)
self.Points_Redeemed.text = points_redeemedString
// let pointsRedeemedAs = Points_Redeem
completion(Points_Redeem)
// Do any additional setup after loading the view.
}
)}
// This third function simply takes the values from the first two functions and subtracts them
func BalanceOfPoints(){
let a = LoadPointsCompleted()
let b = loadPointsRedeemed()
let balance = a - b
let balanceString = String(balance)
self.Points_Balance.text = balanceString
}
However, I get some errors in the third function "BalanceOfPoints" as follows:
Missing argument for parameter 'completion' in call - insert 'completion: <(Int) -> Int>'
I have no idea if my syntax is correct when I first start the functions as per here:
func LoadPointsCompleted(completion: #escaping(_ sumOfPointsCompleted:Int) -> Int){
and...
func loadPointsRedeemed(completion: #escaping (_ Points_Redeem: Int) -> Int) {
Could someone please help? Thanks in advance.

Answer 1

LoadPointsCompleted takes a closure as an argument (a completion handler)
you could either give it a nil completion handler
LoadPointsCompleted(completion: nil)
or give it a handler
LoadPointsCompleted() { sumOfPointsCompleted in
return <#some Int#>
}
but your logic in BalanceOfPoints seems a bit off because LoadPointsCompleted and loadPointsRedeemed are being called sequentially at the same time and also do not actually return an Int, only the completion handler of the function does. eg completion(sumOfPointsCompleted) could be changed to be let a = completion(sumOfPointsCompleted) but im sure thats not what you were intending.
I would change the signature of both those functions to
func LoadPointsCompleted(completion: #escaping(_ sumOfPointsCompleted:Int) -> Void){
and
func loadPointsRedeemed(completion: #escaping (_ Points_Redeem: Int) -> Void) {
then in BalanceOfPoints do this instead
func BalanceOfPoints(){
LoadPointsCompleted() { sumOfPointsCompleted in
loadPointsRedeemed() { Points_Redeem in
let balance = sumOfPointsCompleted - Points_Redeem
let balanceString = String(balance)
self.Points_Balance.text = balanceString
}
}
}
but just note that BalanceOfPoints in now an asynchronous function.
Not sure if step 1 and 2 in your question could be done in parallel, but seems like they could be, but as it stands in this answer they are in sequence