Swift Firebase Multithreading Issue - ios

Question

I'm trying to run a couple of for loops inside of a function that should return an array of strings.
Where I'm having trouble is with getting the correct results BEFORE the next for loop is run...and then again returning that results BEFORE I need to return the array of strings to complete the function.
In the first case, I have a for loop that's getting data from Firebase. I was able to use a dispatch group to get the value to print out - but then with the other loop after this - I was having issues from using the dispatch group in the prior task.
The code all works perfectly if executed with the correct values but I'm not sure how to go about this with regards to threading. Would really appreciate any help.
func findTopSpots() -> [String] {
var topFive = [String]()
var locationRatingDictionary = [String:Double]()
let myGroup = DispatchGroup()
let locationsArray = ["wyoming", "kansas", "arkansas", "florida", "california"]
// Use the days to find the most common month
let calendar = NSCalendar.current
var monthArray = [String]()
var date = self.departureDate!
let endDate = self.returnDate!
// Formatter for printing the month name
let fmt = DateFormatter()
fmt.dateFormat = "MMMM"
// Add each days month to an array
while date <= endDate {
date = calendar.date(byAdding: .day, value: 1, to: date)!
monthArray.append(fmt.string(from: date))
}
// Return the primary month from function
let primaryMonth = findMostCommonMonthInArray(array: monthArray).lowercased()
// Create a dictionary of location:rating for the primary month
for doc in locationsArray {
self.db.collection("locations").document(doc).collection("historic").document(primaryMonth).getDocument { (document, err) in
if let document = document, document.exists {
let rating = document["rating"] as? Double
locationRatingDictionary[doc] = rating
} else {
print("Document does not exist")
}
}
}
//---- THE CODE BELOW WILL NOT PRINT WITH ANY VALUES ----//
print(locationRatingDictionary)
// Sort the tuple array by rating
let locationRatingTupleArray = locationRatingDictionary.sorted{ $0.value > $1.value }
// Return 5 results
for (location,rating) in locationRatingTupleArray.prefix(5) {
print(location,rating)
topFive.append(location)
}
print("top five are \(topFive)")
return topFive
}

Answer 1

The issue here is that the firebase returns with query results asynchronously and you are not waiting for it to return.
I can see that you have instantiate DispatchGroup but have not used it. Lets try to use it to solve your issue. Also, you would need to change the method signature to take a closure. This avoids blocking thread to return function output.
func findTopSpots(completionHandler:([String])->Void) {
var topFive = [String]()
var locationRatingDictionary = [String:Double]()
let myGroup = DispatchGroup()
let locationsArray = ["wyoming", "kansas", "arkansas", "florida", "california"]
// Use the days to find the most common month
let calendar = NSCalendar.current
var monthArray = [String]()
var date = self.departureDate!
let endDate = self.returnDate!
// Formatter for printing the month name
let fmt = DateFormatter()
fmt.dateFormat = "MMMM"
// Add each days month to an array
while date <= endDate {
date = calendar.date(byAdding: .day, value: 1, to: date)!
monthArray.append(fmt.string(from: date))
}
// Return the primary month from function
let primaryMonth = findMostCommonMonthInArray(array: monthArray).lowercased()
// Create a dictionary of location:rating for the primary month
for doc in locationsArray {
myGroup.enter() self.db.collection("locations").document(doc).collection("historic").document(primaryMonth).getDocument { (document, err) in
if let document = document, document.exists {
let rating = document["rating"] as? Double
locationRatingDictionary[doc] = rating
} else {
print("Document does not exist")
}
myGroup.leave()
}
}
myGroup.notify(queue:.main) {
//---- THE CODE BELOW WILL NOT PRINT WITH ANY VALUES ----//
print(locationRatingDictionary)
// Sort the tuple array by rating
let locationRatingTupleArray = locationRatingDictionary.sorted{ $0.value > $1.value }
// Return 5 results
for (location,rating) in locationRatingTupleArray.prefix(5) {
print(location,rating)
topFive.append(location)
}
print("top five are \(topFive)")
completionHandler(topFive)
}
}

Answer 2

Your code is asynchronous fastest way is dispatchGroup with completion
//
func findTopSpots(completion:#escaping(_ arr:[string])->void){
let dispatchGroup = DispatchGroup()
var topFive = [String]()
var locationRatingDictionary = [String:Double]()
let locationsArray = ["wyoming", "kansas", "arkansas", "florida", "california"]
// Use the days to find the most common month
let calendar = NSCalendar.current
var monthArray = [String]()
var date = self.departureDate!
let endDate = self.returnDate!
// Formatter for printing the month name
let fmt = DateFormatter()
fmt.dateFormat = "MMMM"
// Add each days month to an array
while date <= endDate {
date = calendar.date(byAdding: .day, value: 1, to: date)!
monthArray.append(fmt.string(from: date))
}
// Return the primary month from function
let primaryMonth = findMostCommonMonthInArray(array: monthArray).lowercased()
// Create a dictionary of location:rating for the primary month
for doc in locationsArray {
dispatchGroup.enter()
self.db.collection("locations").document(doc).collection("historic").document(primaryMonth).getDocument { (document, err) in
if let document = document, document.exists {
let rating = document["rating"] as? Double
locationRatingDictionary[doc] = rating
} else {
print("Document does not exist")
}
dispatchGroup.leave()
}
}
dispatchGroup.notify(queue: .main) {
//---- THE CODE BELOW WILL NOT PRINT WITH ANY VALUES ----//
print(locationRatingDictionary)
// Sort the tuple array by rating
let locationRatingTupleArray = locationRatingDictionary.sorted{ $0.value > $1.value }
// Return 5 results
for (location,rating) in locationRatingTupleArray.prefix(5) {
print(location,rating)
topFive.append(location)
}
print("top five are \(topFive)")
completion(topFive)
}
}