Does Firebase know when data is no longer a part of query? - swift

Question

In my Firebase database, I have children containing Unix times, and I'm querying by times that occur after the current time. I'm putting the data of each child which matches that criteria into a UICollectionView. When the current time surpasses the time of one of the children, I want for the child to expire, and to get removed from the UICollectionView. Currently, it isn't getting removed until I restart the app. Here is some of the code:
// in viewDidLoad
self.events_query = Database.database().reference().child("events").queryOrdered(byChild: "end-time").queryStarting(atValue: Date().timeIntervalSince1970)
// in viewWillAppear
func observeAllEvents() {
self.events_query.observe(.value, with: { (snapshot) in
guard let eids_dict = snapshot.value as? [String : AnyObject] else { return }
let eids = Array(eids_dict.keys)
for eid in eids {
print(eid)
}
Event.getAllEvents(with: eids, ref: self.events_query.ref, completion: { (events) in
if let feed_datasource = self.datasource as? EventFeedDatasource {
feed_datasource.events = events
}
DispatchQueue.main.async {
self.collectionView?.reloadData()
}
})
})
}
// in viewDidDisappear
self.events_query.removeAllObservers()
Here's the function getAllEvents:
static func getAllEvents(with eids: [String], ref: DatabaseReference, completion: #escaping (_ events: [Event]) -> Void) {
var events = [Event]()
let dispatch_groups = [DispatchGroup(), DispatchGroup()]
for eid in eids {
dispatch_groups[0].enter()
ref.child(eid).observeSingleEvent(of: .value, with: { (snapshot) in
guard let dictionary = snapshot.value as? [String : AnyObject] else { return }
dispatch_groups[1].enter()
// I'm not including `load` because all it does is parse the snapshot
Event.load(with: dictionary, completion: { (event) in
events.append(event)
dispatch_groups[1].leave()
})
dispatch_groups[0].leave()
})
}
dispatch_groups[0].notify(queue: DispatchQueue.main) {
dispatch_groups[1].notify(queue: DispatchQueue.main) {
completion(events)
}
}
}