Querying GeoHashes in Firestore returns nothing - swift

Question

The code that retrieves all locations from Firestore within a 50km location of a given point is given on the Firebase website. Here it is:
// Find cities within 50km of London
let center = CLLocationCoordinate2D(latitude: 51.5074, longitude: 0.1278)
let radiusInM: Double = 50 * 1000
// Each item in 'bounds' represents a startAt/endAt pair. We have to issue
// a separate query for each pair. There can be up to 9 pairs of bounds
// depending on overlap, but in most cases there are 4.
let queryBounds = GFUtils.queryBounds(forLocation: center,
withRadius: radiusInM)
let queries = queryBounds.map { bound -> Query in
return db.collection("cities")
.order(by: "geohash")
.start(at: [bound.startValue])
.end(at: [bound.endValue])
}
var matchingDocs = [QueryDocumentSnapshot]()
// Collect all the query results together into a single list
func getDocumentsCompletion(snapshot: QuerySnapshot?, error: Error?) -> () {
guard let documents = snapshot?.documents else {
print("Unable to fetch snapshot data. \(String(describing: error))")
return
}
for document in documents {
let lat = document.data()["lat"] as? Double ?? 0
let lng = document.data()["lng"] as? Double ?? 0
let coordinates = CLLocation(latitude: lat, longitude: lng)
let centerPoint = CLLocation(latitude: center.latitude, longitude: center.longitude)
// We have to filter out a few false positives due to GeoHash accuracy, but
// most will match
let distance = GFUtils.distance(from: centerPoint, to: coordinates)
if distance <= radiusInM {
matchingDocs.append(document)
}
}
}
// After all callbacks have executed, matchingDocs contains the result. Note that this
// sample does not demonstrate how to wait on all callbacks to complete.
for query in queries {
query.getDocuments(completion: getDocumentsCompletion)
}
The issue that I am having is that matchingDocs (the empty array that the locations from the database are supposed to append to) returns empty every time.
I have double checked that the center and example locations in my database are within 50km of each other. The code is able to retrieve the four documents in my database and I know this because if I put a print statement in the last for loop, I get something printed 4 times.
I need an explanation on exactly what the getDocumentsCompletion function does because I don't understand fully what it does, or how the call works. There are no arguments passed in where the function is called.
I have also added print statements within the getDocumentsCompletion function, but nothing ever gets printed out, so I believe that my issue lies there. I would like an explanation on what exactly is going on there so I can better address the issue.