使用iOS中的Firebase数据库检索数据

Question

In my app I have a simple user base that looks like this:

What I'm trying to do is to simply fetch this list once, to check wether a username is valid when a new user signs up with a new username.
The thing is that the only ways I found to retrieve data utilize some sort of observer methods, which are not good for me.

The logic I'm trying to achieve (with the retrieving method that doesn't work) :

// When user tries to sign up with a new username

let username = nicknameField.text?.stringByTrimmingCharactersInSet(NSCharacterSet.whitespaceAndNewlineCharacterSet())    

self.usersRef.observeEventType(.Value) { (snapshot: FIRDataSnapshot) in
    let dict = snapshot.value as! NSDictionary
    for val in dict.allValues {
        if username == val as! String {
            // Present alert
            return
        }
    }
}   

self.usersRef.child(username).setValue(username) { (error, dbRef) in
    if error == nil {
        // Continue
    }
}

How can I simply just fetch the list of users once? Thanks in advance!

Answer 1

I had to change the observeEventType method to observeSignleEventOfType .
I have also updated my code to make it work (regardless):

self.usersRef.observeSingleEventOfType(.Value) { (snapshot: FIRDataSnapshot) in
let dict = snapshot.value as! NSDictionary
for val in dict.allValues {
    if username == val as! String {
        // Present alert
        return
    }
    else {
        self.usersRef.child(username).setValue(username) { (error, dbRef) in
        if error == nil {
            // Continue
        }
    }
}