blueflood-core/src/main/java/com/rackspacecloud/blueflood/eventemitter/Emitter.java
public Emitter once(final String event, final Listener<T> fn) {
        Listener on = new Listener<T>() {
            @Override
            public void call(T... args) {
                Emitter.this.off(event, this);
                fn.call(args);
            }
        };

        this.onceCallbacks.put(fn, on);
        this.on(event, on);
        return this;
    }
public Emitter once(final String event, final Listener<T> fn) {
        Listener on = new Listener<T>() {
            @Override
            public void call(T... args) {
                Emitter.this.off(event, this);
                fn.call(args);
            }
        };

        this.onceCallbacks.put(fn, on);
        this.on(event, on);
        return this;
    }
Adds a one time listener for the event.

@param event an event name.
@param fn
@return a reference to this object.