For-loop isn't returning the items in an array in the correct order - arrays

Question

I have an array of Integers that I need to loop through that then make a network request for further information and with the returned information populate an array of a new object I have created.
I want the returned data to come back in the same order in which the array provides it in, however it is coming back in a different order and am assuming it might have something to do with the network request.
I'm new(ish) to development, so the answer may be very obvious, but I'm really at a dead end with what to do next.
I've tried adding delays to the network request on each loop, i've tried calling .sort() on the array to ensure the array stays in the correct order
var tacticalCoverIdArray = [Int]()
var savedTacticalCoverData = [Covers]()
for coverID in tacticalCoverIdArray {
performGetRequestForSpecificCovers(coverID: coverID, targetURL: targetURL, completion: { (data, HTTPSatusCode, error) -> Void in
if HTTPSatusCode == 200 && error == nil {
do {
if coverID != 0 {
let decodedJSON = try JSONDecoder().decode([Covers].self, from: data)
savedTacticalCoverData.append(decodedJSON[0])
} else {
let data = Covers(id: 0, game: 0, image_id: "")
savedTacticalCoverData.append(data)
}
DispatchQueue.main.asyncAfter(deadline: .now() + .milliseconds(1000), execute: {
saveTacticalCoverData()
})
} catch let error {
print("Error decoding JSON: \(error)")
}
} else {
print("HTTP status code error: \(HTTPSatusCode)")
print("Error loading data: \(String(describing: error))")
}
})
}
When putting a print statement under the very first declaration of the for-loop (i.e: print(coverID) the return is what I would expect, in which it loops through each integer and then returns them in order.
However, as soon as I put the same print statement under the 'performGetRequestForSpecificCovers' method, the coverID array is not in the order that it should be in, and therefore I get my returned values in an incorrect order when I append them to my 'savedTacticalCoverData' array.

Answer 1

Your hunch about the network requests having an impact on the ordering seems correct.
What I'm guessing is happening here is that when you are looping over tacticalCoverIdArray and calling performGetRequestForSpecificCovers(), that loop doesn't wait for that network request to complete and for the completion block to get called. It continues with the next iteration. Effectively, you are sending tacticalCoverIdArray.count network requests in parallel. Those completion blocks get called much later, long after the outer loop is complete, and most likely even on a different thread.
The most basic, and worst, option is to use DispatchSemaphore to hold up the outer loop until the completion block is called. You'd create a semaphore, call semaphore.signal() inside the completion handler, and call semaphore.wait() at the end of every loop iteration. The problem with this approach is that you will wait for each network request to complete before proceeding to the next one. Also, you will tie up the thread that is executing the first outer loop, and threads are a finite resource, so it's not a good idea to waste them.
A better option is to dispatch all requests at once, and handle the out-of-order responses. This will complete much faster than serially dispatching them, unless you encounter some kind of limitations with dispatching so many network requests in parallel. Instead of savedTacticalCoverData being an array, perhaps it could be a dictionary, where the key is the index of the outer loop, and the value is what you're trying to save? Each time the completion handler gets called, you could check whether or not the dictionary is full and you've accumulated all of the responses you want, and only then proceed with the final "everything is done" action, presumably saveTacticalCoverData().
You'll have to be careful to get your multithreading right. Unless performGetRequestForSpecificCovers() uses only one callback queue, and it's the same queue that this function is running on, you might get called on different threads. If that's the case, I would recommend making a new DispatchQueue and always operating on your dictionary only from that queue, to ensure consistency when those completion blocks come in on random threads. Something like this:
class MyClass {
var tacticalCoverIdArray = [Int]()
var savedTacticalCoverData = [Int: Covers]()
var queue = DispatchQueue(label: "Class Internal Queue")
func myFunc() {
// ... fill in the blanks here
for (index, coverID) in tacticalCoverIdArray.enumerated() {
performGetRequestForSpecificCovers(coverID: coverID, targetURL: targetURL, completion: { (data, HTTPSatusCode, error) -> Void in
if HTTPSatusCode == 200 && error == nil {
do {
queue.async {
if coverID != 0 {
let decodedJSON = try JSONDecoder().decode([Covers].self, from: data)
self.savedTacticalCoverData[index] = decodedJSON[0]
} else {
let data = Covers(id: 0, game: 0, image_id: "")
self.savedTacticalCoverData[index] = data
}
if self.savedTacticalCoverData.count == self.tacticalCoverIdArray.count {
DispatchQueue.main.asyncAfter(deadline: .now() + .milliseconds(1000), execute: {
self.saveTacticalCoverData()
})
}
}
} catch let error {
print("Error decoding JSON: \(error)")
}
} else {
print("HTTP status code error: \(HTTPSatusCode)")
print("Error loading data: \(String(describing: error))")
}
})
}
}
}