Waiting for an initial value

_README.md

In Firebase you can listen for a single value event, with once()/addListenerForSingleValueEvent()/observeSingleEventOfType(). This adds a temporary listener that will receive a single value event and then unregister it.

But if no value currently exists at the location, this will fire with null. This is valid behavior. But what if you want to wait for the first value at the location?

The files below show how to do that for the three main languages: Java, JavaScript and Swift.

WaitForInitialValue.java

public class Main {
    static ValueEventListener mListener;

    public static void main(String[] args) throws Exception {
        Firebase ref = new Firebase("https://<your-app>.firebaseio.com/");
        mListener = ref.addValueEventListener(new ValueEventListener() {
            @Override
            public void onDataChange(DataSnapshot snapshot) {
                if (snapshot.exists()) {
                    System.out.println("The value is now "+snapshot.getValue());
                    ref.removeEventListener(mListener);
                }
            }

            @Override
            public void onCancelled(FirebaseError firebaseError) {

            }
        });

WaitForInitialValue.js

var ref = new Firebase('https://<your-app>.firebaseio.com');
var listener = ref.on('value', function(snapshot) { 
    if (snapshot.exists()) {
        console.log('Value is now '+snapshot.val());
        ref.off('value', listener);
    }
});

WaitForInitialValue.swift

let ref = Firebase(url: "https://<your-app>.firebaseio.com/")
var handle: FirebaseHandle!
handle = ref.observeEventType(.Value, withBlock: { snapshot in
    if snapshot.exists() {
        print("The value is now \(snapshot.value)")
        ref.removeObserverWithHandle(handle)
    }
})