Trouble calling async function with #escaping handler - swift

Question

I'm currently working with Firebase and Firestore and I'm working with their distributed counter system and have created a function with a #escaping handler that should return a value I can assign and then display. here's the code:
func getCount(ref: DocumentReference, handler: #escaping(_ querySnapshot: QuerySnapshot,_ err: Error) -> Void) {
ref.collection("shards").getDocuments() { (querySnapshot, err) in
var totalCount = 0
if err != nil {
print("error getting total count")
} else {
for document in querySnapshot!.documents {
let count = document.data()["count"] as! Int
totalCount += count
}
}
}
}
I'm having trouble calling the function and there isn't too much information that gives me a solid grasp about this. Can someone point me in the right direction?
I call it here:
getCount(ref: Featuredlikes {
ref.collection("shards").getDocuments() { (querySnapshot, err) in
var totalCount = 0
if err != nil {
print("error getting total count")
} else {
for document in querySnapshot!.documents {
let count = document.data()["count"] as! Int
totalCount += count
}
}
}
}
But evidently it isn't right I'm getting since I'm getting unrelated errors in the swiftui part of my code.

Answer 1

Basically your code cannot work because it's impossible to return (strictly spoken the closure does not return anything) a non-optional snapshot and non-optional error simultaneously. Declare both parameters as optional. By the way an underscore character and parameter label in the closure declaration is Swift 2 legacy code.
You have to call handler either with nil snapshot and the error instance or vice versa.
func getCount(ref: DocumentReference, handler: #escaping(QuerySnapshot?, Error?) -> Void) {
ref.collection("shards").getDocuments() { (querySnapshot, err) in
if let error = err {
handler(nil, error)
} else {
handler(querySnapshot, nil)
}
}
}
Or with the modern Result API
func getCount(ref: DocumentReference, handler: #escaping(Result<QuerySnapshot, Error>) -> Void) {
ref.collection("shards").getDocuments() { (querySnapshot, err) in
if let error = err {
handler(.failure(error))
} else {
handler(.success(querySnapshot))
}
}
}