Fetch document only once when Firestore has updated - arrays

Question

I have a tableView that bring all documents from Firestore collection and i want to fetch the last document only once when it is added to the Firestore after user refresh the tableView and after that I want to remove the listener so when the user refresh the tableView it only fetch document once , i used detach listener [from the documentation here][1] but it didn't work
func updatedFireStoredata() {
let listener = Firestore.firestore().collection("collection1").order(by: "date").limit(toLast: 1).addSnapshotListener() { querySnapshot, error in
guard let snapshot = querySnapshot else {
print("Error fetching snapshots: \(error!)")
return
}
snapshot.documentChanges.forEach { diff in
if (diff.type == .added) {
let data = diff.document.data()
self.amount.append(data["amount"] as? String ?? "")
print("New city: \(diff.document.data())")
}
}
}
table2.reloadData()
listener.remove()
}

Answer 1

If your goal is to only call once to retrieve this document, I would consider using the offline persistence that is built in to the SDK. You will need to request the document once with offline persistence enabled, but after that call you can request this data from that local cache.
https://firebase.google.com/docs/firestore/manage-data/enable-offline
https://firebase.google.com/docs/firestore/manage-data/enable-offline#disable_and_enable_network_access
Alternatively you could store the document locally, and not include it in reload.
If it is the case that this data does change, just not as often as every refresh, it may make sense to move this to an eventing model. You can use firestore watch/listen to monitor that query and get update callbacks as needed. https://firebase.google.com/docs/firestore/query-data/listen

Answer 2

There are a couple of things
First, requiring a user to keep clicking refresh to get the current data is not a good experience. The data should be presented to the user automatically without interaction
Second, Firebase is asynchronous - code outside the Firebase closure will execute before the data in the closure. See my answer Firebase is asynchronous <- that's for the Realtime Database but the concept applies to asynchronous functions in general.
To fix both issues here's updated code
func updatedFireStoredata() {
let listener = Firestore.firestore().collection("collection1").order(by: "date").limit(toLast: 1).addSnapshotListener() { querySnapshot, error in
if let err = error {
print("Error fetching snapshots: \(err.localizedDescription)")
return
}
snapshot.documentChanges.forEach { diff in
if (diff.type == .added) {
let data = diff.document.data()
self.amount.append(data["amount"] as? String ?? "")
print("New city: \(diff.document.data())")
}
}
table2.reloadData() //needs to be inside the closure
}
}
I moved the tableView reload inside the closure so it executes when new data arrives and then removed the code to remove the listener so your tableView will update with new data.
I also updated the error handler to handle an actual error code.
One thing you will want to do is to keep track of what the user has selected is is looking at - you don't want a selected item, for example, to un-select if other data is added which is what will happen when the tableView reloads.