subscribe method
Implementation
Future subscribe(String topic, Map<String, dynamic> options,
IMqttMessageListener listener) async {
// Check for open connection
checkOpen();
// Subscribe to topic
await Future(() {
var res = connection_!.subscribe(
topic,
options['qos']!,
);
if (res == null) throw Exception('Subscribe error');
});
// Determine if messages shall be filtered (topic without wildcarts)
var filter = !topic.contains('*');
// Add the subscription
var subscription = MqttSubscription(
topic: topic, options: options, filter: filter, listener: listener);
subscriptions_.add(subscription);
}