subscribe method
Implementation
bool subscribe(String topic) {
if (_closed) throw Exception('Subscribe on closed Subscriber');
var ret = _subscriptions.putIfAbsent(topic, () => <Subscriber>{}).add(this);
_localSubs.add(topic);
if (_sticky.containsKey(topic)) {
var stickyVal = _sticky[topic];
if (stickyVal != null) {
_send(stickyVal);
}
}
return ret;
}