unSubscribe method
Implementation
Future<void> unSubscribe(String topic) async {
_subList.remove(topic);
_dataArriveCallBack.remove(topic);
_finishedSubCache.remove(topic);
var msg = MqttMessageUnSubscribe.withTopic([topic]);
msg.withMessageID(MessageIdentifierDispenser().getNextMessageIdentifier());
_send(msg);
return;
}