Specify event types to listen for. Note, this is not a filter out but rather a "filter in," that is, when a filter is applied only the filtered values are received. Multiple filters on a socket connection are allowed.
Source
Future<Reply> filter(String eventHeader, String valueToFilter,
{int timeoutSeconds: 10}) =>
_subscribeAndSendCommand('filter $eventHeader $valueToFilter',
new Duration(seconds: timeoutSeconds));