Manipulating data after retrieved it from Firebase - ios

Question

Here's my code:
//Get data from Firebase
func getData(withBlock completion:#escaping() ->Void){
let ref = Database.database().reference().child("hobbies")
let query = ref.queryOrdered(byChild: "cost").queryEqual(toValue: "low")
query.observe(.childAdded, with: {(snapshot) in
self.user_choice_Cost.append((snapshot.childSnapshot(forPath: "hobbyName").value as? String)!)
completion()
//print(self.user_choice_Cost)
})
{ (error) in
print(error)
}
//Manipulating data
getData{
let set2:Set<String> = ["a"]
let set1:Set<String> = Set(self.user_choice_Cost)
print(set1.union(set2))}
This works correctly! But is there any way I can get the user_choice_Cost with all value(["a","b"]) instead of one by one(["a"],["a","b")] and manipulate user_choice_Cost array without putting it inside inside getData{}. Because if I put that outside it will return only "a"

Answer 1

When you observe .childAdded, your completion handler gets called for each individual child that matches your query. If you want to get called only once for all matching children, you should observe .value:
query.observe(.value, with: {(snapshot) in
Since your completion handler gets called only once, the snapshot in this case contains all matching nodes. So you need to loop over snapshot.children:
query.observe(.value, with: {(snapshot) in
for child in snapshot.children.allObjects as! [DataSnapshot] {
let value: String = (child.childSnapshot(forPath: "hobbyName").value as? String)!;
user_choice_Cost.append(value)
}
print("value \(user_choice_Cost)")
})
With this code you'll only see one logging output, with all matching hobby names.