Dealing with Firebase observers when userID changes? - ios

Question

I use this function inside an helper class to fetch the trips of a user. It keeps listening for new changes:
import Foundation
import Firebase
class APICardService: NSObject {
class func fetchTrips(forID userID: String, completion: #escaping ([Trip]) -> Void) {
let path = "users/\(userID)/trips"
Database.database().reference().child(path).queryOrdered(byChild: "timestamp").observe(.value) { (snapshot) in
var trips = [Trip]()
for child in snapshot.children {
if let child = child as? DataSnapshot {
if let dict = child.value as? [String: Any] {
let trip = Trip(key: child.key, dbValues: dict)
trips.append(trip)
}
}
}
completion(trips)
}
}
}
class MyViewController: UIViewController {
// but what if the userID changes? I have no way to stop the current observer and start a new one
internal func fetchCards() {
guard let userID = APIAuthService.getUserID() else { return }
APICardService.fetchTrips(forID: userID) { trips in
self.trips = trips.reversed() // show most recent first
self.addAnnotations(for: trips)
self.collectionView.reloadData()
}
}
}
The problem is that whenever the user signs out and logs into a new account this observer will still be listening for new value changes in the data but it won't work because the userID changed. How do I deal with this? I could remove the observer somehow but this is complicated because we're inside a separate class and I wouldn't know how to handle that.

Answer 1

In your class APICardService create these global variables
static var refHandle:DatabaseHandle? = nil
static var ref:DatabaseReference? = nil
in your Method func fetchTrips(forID userID: String, completion: #escapin
assign global variables like this
let path = "users/\(userID)/trips"
ref = Database.database().reference().child(path)
refHandle = ref?.queryOrdered(byChild: "timestamp").observe(.value) { (snapshot) in
Now you have the reference of your observer
just add following method in your class
class func removeObserverForFetchTrip() {
if let refHandleValue = refHandle {
ref?.removeObserver(withHandle: refHandleValue)
}
}
When user logout just call this method
Hope it is helpful

Answer 2

As #PrashantTukadiya says.
Try making something like this:
struct User {
var id: String
...
}
class UserHolder {
static var shared: UserHolder = UserHolder() // Singleton
var currentUser: User? {
willSet {
// Remove all the registered firebase handlers associated with the user
firebaseHandles.forEach({ $0.0.removeObserver(withHandle: $0.1) })
}
}
var firebaseHandles: [(DatabaseReference, UInt)] = []
// Note: Try to add all the firebase handles you register to "firebaseHandles"
// array. The more adequate solution is to make this class register
// the listeners, not all other classes.
}