循环添加后，Swift数组为空

Question

我正在将Firebase中的用户信息附加到数组中，但是循环后所有打印中的array（usersArray）似乎都是空的。数组在函数外部声明。有什么想法吗？谢谢。

    func fetchAllUsers(completion: @escaping (_ message: String) -> Void){

    //User or advertiser?
    let uid = Auth.auth().currentUser?.uid

    Database.database().reference(withPath: "Advertiser").child(uid!).observeSingleEvent(of: .value, with: { (snapshot) in

        if snapshot.exists(){

            myAdvertiserVar.advertiser = true

            // Fetch all users
            self.ref?.child("Users").observe(DataEventType.childAdded, with: { (snapshot) in


                if let dictionary = snapshot.value as? [String: AnyObject] {
                    let user = User()
                    user.nameLabel = dictionary["Username"] as? String
                    user.occupationLocation = dictionary["Occupation"] as? String
                    user.ageLabel = dictionary["Age"] as? String
                    user.infoText = dictionary["Bio"] as? String
                    user.emailText = dictionary["email"] as? String

                    let email = user.emailText
                    let ref = Database.database().reference().child("Users")
                    ref.queryOrdered(byChild: "email").queryEqual(toValue: email).observeSingleEvent(of: .childAdded, with: { (snapshot) in
                        user.toId = snapshot.key

                    })

                    self.usersArray.append(user)

                }
            })
            dump(self.usersArray)   //This print is empty
            completion("FetchAllUsers")

        }
}