How can we paginate a firebase / firestore query on ios swift? - ios

Question

I try to find a solution for paginate a firebase query on ios/swift but I couldn't build algorithm for my state.
My method is like this:
func downloadData(completion: #escaping ([Post]) -> Void) {
// download data with pagination
let firestoreDatabase = Firestore.firestore()
var first = firestoreDatabase.collection("posts").order(by: "date", descending: true).limit(to: 5)
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
}
self.postList.removeAll(keepingCapacity: false)
DispatchQueue.global().async {
for document in snapshot.documents {
// getting data from document stuff ...
self.postList.append(self.post)
}
completion(self.postList)
}
// how can I repeat this query as long as lastSnapshot exist
firestoreDatabase.collection("posts").order(by: "date", descending: true).start(afterDocument: lastSnapshot).addSnapshotListener { querySnapshot, error in
}
}
}
I tried following mindset but it didn't work, and entered an infinite loop. I didn't understand why it is.
func downloadData(completion: #escaping ([Post]) -> Void) {
// download data with pagination
let firestoreDatabase = Firestore.firestore()
var first = firestoreDatabase.collection("posts").order(by: "date", descending: true).limit(to: 5)
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
}
self.postList.removeAll(keepingCapacity: false)
DispatchQueue.global().async {
for document in snapshot.documents {
// geting data from document stuff ...
self.postList.append(self.post)
}
completion(self.postList)
}
repeat {
firestoreDatabase.collection("posts").order(by: "date", descending: true).start(afterDocument: lastSnapshot).addSnapshotListener { querySnapshot, 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
}
self.postList.removeAll(keepingCapacity: false)
DispatchQueue.global().async {
for document in snapshot.documents {
// getting data from document stuff ...
self.postList.append(self.post)
}
completion(self.postList)
}
lastSnapshot = snapshot.documents.last
}
} while(lastSnapshot.exists)
}
}
I think lastSnapshot must be nil after the query loop but it is appear that it is still exist.
how can I fix lastSnapshot problem? Or is there different mindset / easiest way to paginate?
In firebase documents, it says just use this but how can we repeat query that has " .start(afterDocument: lastSnapshot) " stuff?

Answer 1

First and foremost, for plain-vanilla pagination, don't use a snapshot listener when fetching documents. You can paginate documents with a snapshot listener but the process is more complex.
I've embedded my notes into the comments in the code below for clarity.
let pageSize = 5
var cursor: DocumentSnapshot?
func getFirstPage(completion: #escaping (_ posts: [Post]?) -> Void) {
let db = Firestore.firestore()
let firstPage = db.collection("posts").order(by: "date", descending: true).limit(to: pageSize)
firstPage.getDocuments { snapshot, error in
guard let snapshot = snapshot else {
// Don't leave the caller hanging on errors; return nil,
// return a Result, throw an error, do something.
completion(nil)
if let error = error {
print(error)
}
return
}
guard !snapshot.isEmpty else {
// There are no results and so there can be no more
// results to paginate; nil the cursor.
cursor = nil
// And don't leave the caller hanging, even on no
// results; return an empty array.
completion([])
return
}
// Before parsing the snapshot, manage the cursor.
if snapshot.count < pageSize {
// This snapshot is smaller than a page size and so
// there can be no more results to paginate; nil
// the cursor.
cursor = nil
} else {
// This snapshot is a full page size and so there
// could potentially be more results to paginate;
// set the cursor.
cursor = snapshot.documents.last
}
var posts: [Post] = []
for doc in snapshot.documents {
posts.append(newPost) // pseudo code
}
completion(posts)
}
}
func continuePages(completion: #escaping (_ posts: [Post]?) -> Void) {
guard let cursor = cursor else {
return
}
let db = Firestore.firestore()
let nextPage = db.collection("posts").order(by: "date", descending: true).limit(to: pageSize).start(afterDocument: cursor)
nextPage.getDocuments { snapshot, error in
guard let snapshot = snapshot else {
completion(nil)
if let error = error {
print(error)
}
return
}
guard !snapshot.isEmpty else {
// There are no results and so there can be no more
// results to paginate; nil the cursor.
cursor = nil
completion([])
return
}
// Before parsing the snapshot, manage the cursor.
if snapshot.count < pageSize {
// This snapshot is smaller than a page size and so
// there can be no more results to paginate; nil
// the cursor.
cursor = nil
} else {
// This snapshot is a full page size and so there
// could potentially be more results to paginate;
// set the cursor.
cursor = snapshot.documents.last
}
var morePosts: [Post] = []
for doc in snapshot.documents {
morePosts.append(newPost) // pseudo code
}
completion(morePosts)
}
}