iOS -Using Pull-To-Refresh How to know when new data is added to Firebase Node based on User Location - ios

Question

My app lets users sell things like sneakers etc. The sneakers that appears in the user's feed is based on the sellers who has posted items that are nearby to the user. I use GeoFire to get the seller's location and everything works fine. When the user uses pullToRefresh if there isn't any new data/sneakers that have been added nearby then there is no need to refresh the list.
The place where I am stumped is when the user pullsToRefresh, how do I determine that new items have been added by either a completely new seller who is nearby or the the same seller's who have added additional pairs of sneakers?
For eg. userA lives in zip code 10463 and there are 2 seller's within a 20 mi radius. Any sneakers that those seller's have for sale will appear in the user's feed. But a 3rd seller can come along and post a pair of sneakers or any of the first 2 seller's can add an additional pair. If the user pullsToRefesh then those items will appear but if nothing is added then pullToRefresh shouldn't do anything.
I don't want to unnecessarily rerun firebase code if I don't have to. The only way to do that would be to first check the postsRef to check to see if any new sneakers were added by the 2 sellers or a completely new seller who is also nearby.
code:
let refreshControl: UIRefreshControl = {
let refreshControl = UIRefreshControl()
refreshControl.addTarget(self, action: #selector(pullToRefresh), for: .valueChanged)
return refreshControl
}()
#objc func pullToRefresh() {
// if there aren't any new nearby sellers or current sellers with new items then the two lines below shouldn't run
arrOfPosts.removeAll() // this is the array that has the collectionView's data. It gets populated in thirdFetchPosts()
firstGetSellersInRadius(miles: 20.0)
}
override func viewDidLoad() {
super.viewDidLoad()
firstGetSellersInRadius(miles: 20.0) // the user can change the mileage but it's hardcoded for this example
}
// 1. I get the user's location and then get all the nearby sellers
func firstGetSellersInRadius(miles: Double) {
// user's location
guard let currentLocation = locationManager.location else { return }
let lat = currentLocation.coordinate.latitude
let lon = currentLocation.coordinate.longitude
let location = CLLocation(latitude: lat, longitude: lon)
let radiusInMeters = (miles * 2) * 1609.344 // 1 mile == 1609.344 meters
let region = MKCoordinateRegion(center: location.coordinate, latitudinalMeters: radiusInMeters, longitudinalMeters: radiusInMeters)
let geoFireRef = Database.database().reference().child("geoFire")
regionQuery = geoFireRef?.query(with: region)
queryHandle = regionQuery?.observe(.keyEntered, with: { (key: String!, location: CLLocation!) in
let geoModel = GeoModel()
geoModel.userId = key
geoModel.location = location
self.arrOfNearbySellers.append(geoModel)
})
regionQuery?.observeReady({
self.secondLoopNearbySellersAndGetTheirAddress(self.arrOfNearbySellers)
})
}
// 2. I have to grab the seller's username and profilePic before I show their posts because they're shown along with the post
func secondLoopNearbySellersAndGetTheirAddress(_ geoModels: [GeoModel]) {
let dispatchGroup = DispatchGroup()
for geoModel in geoModels {
dispatchGroup.enter()
if let userId = geoModel.userId {
let uidRef = Database.database().reference().child("users")?.child(userId)
uidRef.observeSingleEvent(of: .value, with: { [weak self](snapshot) in
guard let dict = snapshot.value as? [String: Any] else { dispatchGroup.leave(); return }
let profilePicUrl = dict["profilePicUrl"] as? String
let username = dict["username"] as? String
let userModel = UserModel()
userModel.profilePicUrl = profilePicUrl
userModel.username = username
self?.arrOfSellers.append(userModel)
dispatchGroup.leave()
})
}
}
dispatchGroup.notify(queue: .global(qos: .background)) { [weak self] in
self?.thirdFetchPosts(self!.arrOfSellers)
}
}
// 3. now that I have their address I fetch their posts
func thirdFetchPosts(_ userModels: [UserModel]) {
let dispatchGroup = DispatchGroup()
var postCount = 0
var loopCount = 0
for userModel in userModels {
dispatchGroup.enter()
if let userId = userModel.userId {
let postsRef = Database.database().reference().child("posts")?.child(userId)
postsRef?.observe( .value, with: { [weak self](snapshot) in
postCount = Int(snapshot.childrenCount)
guard let dictionaries = snapshot.value as? [String: Any] else { dispatchGroup.leave(); return }
dictionaries.forEach({ [weak self] (key, value) in
print(key, value)
loopCount += 1
guard let dict = value as? [String: Any] else { return }
let postModel = PostModel(userModel: userModel, dict: dict)
self?.arrOfPosts.append(postModel)
if postCount == loopCount {
dispatchGroup.leave()
postCount = 0
loopCount = 0
}
})
})
}
}
dispatchGroup.notify(queue: .global(qos: .background)) { [weak self] in
self?.fourthRemoveQueryObserverReloadCollectionView()
}
}
// 4. now that I have all the posts inside the arrOfPosts I can show them in the collectionView
func foutrhRemoveQueryObserverReloadCollectionView() {
DispatchQueue.main.async { [weak self] in
self?.arrOfPosts.sort { $0.postDate ?? 0 > $1.postDate ?? 0}
self?.refreshControl.endRefreshing()
if let queryHandle = self?.queryHandle {
self.regionQuery?.removeObserver(withFirebaseHandle: queryHandle)
}
self?.collectionView.reloadData()
self?.arrOfNearbySellers.removeAll()
self?.arrOfSellers.removeAll()
}
}