Firebase items returning multiple times in collection view on database change - swift

Question

When the function to create or delete a list is called inside of my app the remaining lists are duplicated and displayed multiple times within the collectionView until the app is reloaded. I only called the fetchLists function twice, in the viewDidLoad and in the pull to refresh function. On pull to refresh the lists return to normal.
Fetch list:
fileprivate func fetchLists() {
self.collectionView?.refreshControl?.endRefreshing()
guard let currentUid = Auth.auth().currentUser?.uid else { return }
let ref = Database.database().reference().child("list-feed").child(currentUid)
ref.observe(.value) { (listFeedSnapshot) in
guard var allObjects = listFeedSnapshot.children.allObjects as? [DataSnapshot] else { return }
allObjects.forEach({ (allObjectsSnapshot) in
let listId = allObjectsSnapshot.key
let listRef = Database.database().reference().child("lists").child(listId)
listRef.observeSingleEvent(of: .value, with: { (snapshot) in
guard let dict = snapshot.value as? [String: Any] else { return }
guard let uid = dict["uid"] as? String else { return }
Database.fetchUserWithUID(uid: uid, completion: { (user) in
guard let dictionary = snapshot.value as? [String: Any] else { return }
var list = List(user: user, dictionary: dictionary)
let listId = snapshot.key
list.id = snapshot.key
self.list = list
self.lists.append(list)
self.lists.sort(by: { (list1, list2) -> Bool in
return list1.creationDate.compare(list2.creationDate) == .orderedDescending
})
self.collectionView?.reloadData()
ref.keepSynced(true)
listRef.keepSynced(true)
})
})
})
}
}
Create list:
let values = ["uid": uid, "title": listNameText, "creationDate": Date().timeIntervalSince1970] as [String : Any]
ref.updateChildValues(values, withCompletionBlock: { (error, ref) in
if let error = error {
self.navigationItem.rightBarButtonItem?.isEnabled = true
print("failed to save user info into db:", error.localizedDescription)
return
}
let memberValues = [uid : 1]
ref.child("list-members").updateChildValues(memberValues)
self.handleUpdateFeeds(with: ref.key!)
self.handleListFeeds(with: ref.key!)
print("successfully created list in db")
Update feeds:
func handleUpdateFeeds(with listId: String) {
guard let uid = Auth.auth().currentUser?.uid else { return }
let values = [listId: 1]
Database.database().reference().child("list-feed").child(uid).updateChildValues(values)
}
func handleListFeeds(with listId: String) {
guard let uid = Auth.auth().currentUser?.uid else { return }
let values = [listId: 1]
Database.database().reference().child("user-lists").child(uid).updateChildValues(values)
}
Firebase database:
{
"list-feed" : {
"otxFDz0FNbVPpLN27DYBQVP4e403" : {
"-LjeAoHJTrYK7xjwcpJ9" : 1,
"-LjeApq-Mb_d_lAz-ylL" : 1
}
},
"lists" : {
"-LjeAoHJTrYK7xjwcpJ9" : {
"creationDate" : 1.5630020966384912E9,
"title" : "Test 1",
"uid" : "otxFDz0FNbVPpLN27DYBQVP4e403"
},
"-LjeApq-Mb_d_lAz-ylL" : {
"creationDate" : 1.563002101329072E9,
"list-members" : {
"otxFDz0FNbVPpLN27DYBQVP4e403" : 1
},
"title" : "Test 2",
"uid" : "otxFDz0FNbVPpLN27DYBQVP4e403"
}
}
}

Answer 1

Since you're calling ref.observe(, you're attaching a permanent observer to the data. This means that if you call fetchLists a second, you're attaching a second observer and you'll get the same data twice.
If you only want the data to be read once per call to fetchLists, you should use observeSingleEventOfType:
fileprivate func fetchLists() {
self.collectionView?.refreshControl?.endRefreshing()
guard let currentUid = Auth.auth().currentUser?.uid else { return }
let ref = Database.database().reference().child("list-feed").child(currentUid)
ref.observeSingleEvent(of: .value) { (listFeedSnapshot) in
Also see the documentation on reading data once.