Swift Firebase Firestore userlist (too many reads) - swift

Question

I'm trying to find a solution for a little problem I have with my app. It's a chess app and it works with Firebase and is written in swift. I use the database for user authentication, user information and uploading moves to play against each other online. All the userInfo is saved in a document in the collection "allUsers".
Everything is working fine, but I have a user screen where you can press a refresh button to update the current online users, with the .getDocuments() function. The problem is that every time a user refreshes, they query through all of the registered accounts, check if they have the flag "isOnline = true" and then list only those users in a TableView. I believe this counts in firebase as 1 read for every registered account, even though the user is not online.
The app is already live in the AppStore and I have quite a few people registered already. About 300. But, to be honest a lot of people just try it once or a few times and then leave again and never use the app again. But every time someone wants to update the online users, they cycle through 300 users and this gives me 300 reads with firebase. Right now it's not a big problem, but once people really start to use the app, I will reach my quotum quite quickly.
I could try .addSnapshotListener , but this will refresh the user screen everytime something happens in the userlist. It will be too chaotic. I've read about reading data from cache, but I'm not sure how to go about this. I could also get a fresh userlist when the app starts, save it locally and check every now and then if there are new online users, but I want the list to be updated whenever the user wants to.
Is there a way to compare a locally saved list to the online database list and only read/get the documents that are changed / new?
Sorry for the long text. Hopefully anyone can help me out!
Thanks.
Below is my code to load the users. It's a bit messy sorry.. Basically it retrieves all users and sorts them by online and offline. If a user searches for another user, it takes a String "query" as input.
Code :
func loadAllAvailableUsers(query : String) {
availableEmails = []
availableUsers = []
onlineUsers = []
isInGameIndex = []
var av : [String] = []
var ae : [String] = []
var wins : [Int] = []
var losses : [Int] = []
var draw : [Int] = []
var matches : [Int] = []
let collection = db.collection("allUsers")
collection.getDocuments { (snapshot, err) in
if let e = err {
print(e.localizedDescription)
} else {
if let documents = snapshot?.documents {
print("found users")
for doc in documents {
print(doc.documentID)
if doc.documentID != currentEmail && query == "" {
print(doc.data()["isOnline"] as! Bool)
if let name = doc.data()["username"] as? String, let w = doc.data()["wins"] as? Int, let l = doc.data()["losses"] as? Int, let d = doc.data()["draw"] as? Int, let numOfMatches = doc.data()["numberOfMatches"] as? Int, let online = doc.data()["isOnline"] as? Bool {
print("adding user : \(name) to list")
if online {
matches.append(numOfMatches)
av.append(name)
ae.append(doc.documentID)
wins.append(w)
losses.append(l)
draw.append(d)
onlineUsers.append(name)
if doc.data()["isInGame"] as! Bool == true {
print(i)
self.delegate?.addToInGameIndex(name: name)
}
}
}
} else if query != "" {
if let name = doc.data()["username"] as? String, let w = doc.data()["wins"] as? Int, let l = doc.data()["losses"] as? Int, let d = doc.data()["draw"] as? Int, let online = doc.data()["isOnline"] as? Bool, let numOfMatches = doc.data()["numberOfMatches"] as? Int {
print("Searched : adding user : \(name) to list")
if doc.documentID == currentEmail {
continue
}
let lowerName = name.lowercased()
let lowerQuery = query.lowercased()
if lowerName.contains(lowerQuery) && online {
av.append(name)
ae.append(doc.documentID)
wins.append(w)
losses.append(l)
draw.append(d)
matches.append(numOfMatches)
if doc.data()["isInGame"] as! Bool == true {
print(i)
self.delegate?.addToInGameIndex(name: name)
}
onlineUsers.append(name)
} else if lowerName.contains(lowerQuery) && !online {
av.append(name)
ae.append(doc.documentID)
wins.append(w)
losses.append(l)
draw.append(d)
matches.append(numOfMatches)
}
}
}
}
}
if ae.count > 0 {
self.delegate?.reloadTheTable(wins: wins, losses: losses, draw: draw, ae: ae, au: av, matches: matches)
}
print(availableUsers)
}
}
}