function fnOnce() {
			this.unsubscribe(sChannelId, sEventId, fnOnce, undefined); // 'this' is always the control, due to the context 'undefined' in the attach call below
			fnFunction.apply(oListener || this, arguments);
		}
Attaches an event handler, called one time only, to the event with the given identifier on the given event channel.

When the event occurs, the handler function is called and the handler registration is automatically removed afterwards.

@param {string}
           [sChannelId] The channel of the event to subscribe to. If not given, the default channel is used.
                        The channel <code>"sap.ui"</code> is reserved by the UI5 framework. An application might listen to
                        events on this channel but is not allowed to publish its own events there.
@param {string}
           sEventId The identifier of the event to listen for
@param {function(string, string, Object)}
           fnFunction The handler function to call when the event occurs. This function will be called in the context of the
                      <code>oListener</code> instance (if present) or on the event bus instance. The channel is provided as first argument of the handler, and
                      the event identifier is provided as the second argument. The parameter map carried by the event is provided as the third argument (if present).
                      Handlers must not change the content of this map.
@param {object}
           [oListener] The object that wants to be notified when the event occurs (<code>this</code> context within the
                       handler function). If it is not specified, the handler function is called in the context of the event bus.
@since 1.32.0
@return {this} Returns <code>this</code> to allow method chaining
@public