SwiftUI Firestore query cursors pagination not working - ios

Question

I'm trying to implement pagination in my app. when the app loads I want to load first item lets say. then every time he clicks refresh a new item is loaded.
I have implemented this logic in my viewmodel.
First time homeViewModel.load() is called in my SceneDelegate
let homeViewModel = HomeViewModel()
homeViewModel.refresh()
Then every time users wants to get new dates I call homeViewModel.refresh()
The problem is that when the app loads I do not get any results and when I hit refresh I keep getting the second document in the table over and over again.
What am I doing wrong here?
My HomeViewModel:
class HomeViewModel: ObservableObject, LoadProtocol {
var firestoreService: FirestoreService = FirestoreService()
#Published var items: [Item] = []
let db = Firestore.firestore()
var first: Query = Firestore.firestore().collection("items").limit(to: 1)
load() {
self.first.addSnapshotListener { (snapshot, error) in
guard let snapshot = snapshot else {
print("Error retrieving cities: \(error.debugDescription)")
return
}
guard let lastSnapshot = snapshot.documents.last else {
// The collection is empty.
return
}
// Construct a new query starting after this document,
// retrieving the next 25 cities.
let next = self.db.collection("items")
.start(afterDocument: lastSnapshot).limit(to: 1)
self.first = next
// Use the query for pagination.
}
}
func refresh() {
self.firestoreService.fetchCollection(query: self.first) { (result: Result<[Item], Error>) in
switch result {
case .success(let items):
self.items += items
self.addToCategories()
case .failure(let error):
print(error)
}
}
}
}

Answer 1

Here's your refresh function
func refresh() {
self.firestoreService.fetchCollection(query: self.first) { (result: Result<[Item], Error>) in
switch result {
case .success(let items):
self.items += items
self.addToCategories()
case .failure(let error):
print(error)
}
}
}
There's nothing in that function that advances the cursor further so it will read the same data over and over.
If you want to read the next set of data, the curser need to be moved to the last document after each refresh, like this
func refresh() {
self.first.addSnapshotListener { (snapshot, error) in
if let err = error {
print(err.localizedDescription)
return
}
guard let snapshot = snapshot else { return }
guard let lastSnapshot = snapshot.documents.last else { return }
let next = self.db.collection("items").start(afterDocument: lastSnapshot).limit(to: 2)
self.first = next
for doc in snapshot.documents {
print(doc.documentID)
}
}
}