Swift: Wait for firestore load before next load - swift

Question

I have a view controller that lists data from a firestore database. Inside a firestore collection, I have a bunch of documents with the information shown in the list, and one document called order which contains one field which is an array of strings in the order I want them displayed. My code grabs this:
self.db.collection("officers").document(school).collection(grade).document("order").getDocument {(document, error) in
if let document = document, document.exists {
self.officerNames = (document.data()!["order"] as! Array<String>)
and then is supposed to use the strings in the array order (officerNames) to query the documents in that same collection (all the documents have a different role so it's only getting one document in the snapshot) and display them in the same order as the one set in order (officerNames).
for item in 1...self.officerNames.count {
self.db.collection("officers").document(school).collection(grade).whereField("role", isEqualTo: self.officerNames[item-1]).getDocuments() { (querySnapshot, err) in
if let err = err {
print("Error getting documents: \(err)")
} else {
for document in querySnapshot!.documents {
let officerMessage = document.data()["agenda"] as! String
let officerInfo = document.data()["short"] as! String
(a bunch of code here using that ^ ^ and due to the color I need item to be an integer)
}
}
}
}
I know that if I try printing item before the self.collection("officers")..... the numbers count by one but if I do that in the for document in querySnapshot..... they're all out of order meaning some documents are loaded faster than others. I have read about Async functions in Swift (although I do use those in JavaScript) but am really confused how to use them and hopefully, there is a simpler way to do this. Any way I can wait to make sure the previous document has been loaded and analyzed before iterating through the loop again?
Here's a screenshot of the database:

Answer 1

Honestly, you may want to examine your data structure and see if you can create one that doesn't require multiple queries like this. I can't quite tell what your data structure is, but if you update your question to include it, I can give some suggestions for how to refactor so you don't have to do 2 different get requests.
That being said, since Swift doesn't have promises like JS, it can be tough to keep data in order. For most cases, closures work well, as I wrote about in this blog. But they still won't preserve order in an array of async calls. Assuming you're using some array to store the officer's data, you can declare the size of the array up front by giving each one a default value. This would look something like this:
var officerArray = [Officer](repeating:Officer(), count: self.officerNames.count)
Of course, it'll be different depending on what kind of objects you're populating it with. I'm using some generic Officer object in this case.
Then, rather than appending the newly created Officer object (or whatever you're calling it) to the end of the array, add its value to its particular location in the array.
for item in 1...self.officerNames.count {
self.db.collection("officers").document(school).collection(grade).whereField("role", isEqualTo: self.officerNames[item-1]).getDocuments() { (querySnapshot, err) in
if let err = err {
print("Error getting documents: \(err)")
} else {
for document in querySnapshot!.documents {
let officerMessage = document.data()["agenda"] as! String
let officerInfo = document.data()["short"] as! String
// etc
officerArray[self.officerNames.count-1] = Officer(officerMessage: officerMessage, officerInfo: officerInfo) // or however you're instantiating your objects
}
}
}
}
This preserves the order.