Firebase-for循环Swift内部嵌套的observeSingleEvent方法

Question

I have been trying to fetch data from Firebase using Realtime database. I want to check the contacts in iPhone and then if any contact number matches with that of any number in "numbers" table in db, then I have to get the user_key from it and then using that key, I have to obtain the corresponding details from users table.

for number in numbers {
    Database.database().reference().child("numbers/\(number)").observeSingleEvent(of: .value, with: { (snapshot) in
        if snapshot.exists() {
            let userKey = snapshot.value as! String

            // We found this user, no determine there name, (TODO has_image?)
            Database.database().reference().child("users/\(userKey)/public/name").observeSingleEvent(of: .value, with: { (namesnapshot) in
                if namesnapshot.exists() {
                    let name = namesnapshot.value as! String
                    print("FOUND \(name)")
                    complete(.success((userID: userKey, name: name)))
                }
            })

        } else {
            if numbers.index(of: number) == numbers.count - 1 {  // Last Number checked and not found yet, so fail
                complete(.failure(UserApiError.UserNotFound))
            }
        }
    })
}

numbers is the array of contact numbers for a particular contact. For a contact having single number, this works fine. But for contacts having multiple numbers, the

Database.database().reference().child("users/\(userKey)/public/name").observeSingleEvent(of: .value, with: { (namesnapshot) in

will call after some time due to which the next index in the for loop gets called. So even if I have the data in first number in a contact, it will return failure because the next number will be iterated before the success of the observeSingleEvent.

I have been sitting for hours now, no ideas left with me. Please help!

Answer 1

I think a better approach is:
1 - Get all numbers from DB.
2 - Get all contact numbers that exists on DB.
3 - Finally, get the name of that contacts.(Exactly the way you are doing).
OBS: To do that you must change your DB. Your numbers must be saved as key-value pair. For exemple "555-0000" : true.

Database.database().reference().child("numbers").observeSingleEvent(of: .value, with: { (snapshot) in
        guard let numbersFromDB = snapshot.value as? [String: Any] else{
            print("Fail get numbers from db")
        }
        let numbersMatchedOnDB = numbersFromDB.keys.filter{ numbers.contains($0) }//get numbers from contact that exist on DB.
        if numbersMatchedOnDB.isEmpty{
            complete(.failure(UserApiError.UserNotFound))
        }
        else{
            //For each contact number that exist on DB. it gets its name.
            numbersMatchedOnDB.forEach{ numberMatchedOnDB in
                Database.database().reference().child("numbers/\(numberMatchedOnDB)").observeSingleEvent(of: .value, with: { (snapshot) in
                        if snapshot.exists() {
                            let userKey = snapshot.value as! String
                            // .... nothing changed here ....
                        }
                    })
            }
        }
    })