The most efficient way to retrieve data from Firebase

Question

I get the current friends of the users and save the users in an array that appears as a collection. The friends collection view is on my home controller, which means it always stays on the user stack .

My problem is that if the user adds or removes friends through the user profile page and then goes to the home view, I have to update the data and update the collection view in viewWillAppear. This makes the application expensive for the data, and I definitely think that it is not the most effective way.

I know that Firebase allows me to configure observers, but I'm not too sure about this without getting massive data.

This is how I currently get user friends

func fetchFriends() {

    let userID = Auth.auth().currentUser?.uid
    var tempFriend = [UserClass]()

    self.usersArray.removeAll()
    collectionView.reloadData()

    let friendRef = self.databaseRef.child("users").child(userID!).child("Friends")
    friendRef.queryOrderedByKey().observe(.childAdded, with: { (snapshot) in

        let friendID = "\(snapshot.value!)"
        let usersRef = self.databaseRef.child("users")

        usersRef.observeSingleEvent(of: .value, with: { (users) in

            for user in users.children {
                let friend = UserClass(snapshot: user as! DataSnapshot)

                if friendID == friend.uid {
                    tempFav.append(friend)
                }
            }
            self.usersArray = tempFav
            self.collectionView.reloadData()

            if self.usersArray.count == 0 {
                self.noDataLabel.text = "You have no friends!"
                self.noDataLabel.isHidden = false
            } else {
                self.noDataLabel.text = "Loading..."
                self.noDataLabel.isHidden = true
            }
        })
    })
}

This is triggered by the user. viewWillAppear

To make this question as clear as possible, what is the best way to update a user's friends without having to re-dial the entire list whenever a view appears.

Answer 1

It looks like you are removing your observers when the view disappears. At this point, Firebase will delete user data from its memory cache. Therefore, when you reconnect the listener to viewWillAppear, the data must be reloaded from the Firebase servers.

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