blueflood-core/src/main/java/com/rackspacecloud/blueflood/eventemitter/Emitter.java
public Emitter on(String event, Listener fn) {
        ConcurrentLinkedQueue<Listener> callbacks = this.callbacks.get(event);
        if (callbacks == null) {
            callbacks = new ConcurrentLinkedQueue<Listener>();
            ConcurrentLinkedQueue<Listener> _callbacks = this.callbacks.putIfAbsent(event, callbacks);
            if (_callbacks != null) {
                callbacks = _callbacks;
            }
        }
        callbacks.add(fn);
        return this;
    }
public Emitter on(String event, Listener fn) {
        ConcurrentLinkedQueue<Listener> callbacks = this.callbacks.get(event);
        if (callbacks == null) {
            callbacks = new ConcurrentLinkedQueue<Listener>();
            ConcurrentLinkedQueue<Listener> _callbacks = this.callbacks.putIfAbsent(event, callbacks);
            if (_callbacks != null) {
                callbacks = _callbacks;
            }
        }
        callbacks.add(fn);
        return this;
    }
Listens on the event.
@param event event name.
@param fn
@return a reference to this object.