add method
Add or update a StreamSubscription to the list with a specified key. Will unsubscribe a Subscription when updating a existing key.
For example:
subxMap.add('key', observable.listen(...));
Implementation
SubxMap add(Object key, StreamSubscription<dynamic> value) {
final oldSubscription = _subscriptionMap[key];
if (oldSubscription != null && oldSubscription != value) {
oldSubscription.cancel();
}
_subscriptionMap[key] = value;
return this;
}