on method
Subscribe a function to a topic
in the event system. If the callback
is not already subscribed, adds it to the subscription list. Returns true
if the function was added to the list, false if it was already subscribed.
Implementation
MPS on(String topic, Function? callback) {
if (!_cache.containsKey(topic)) {
_cache[topic] = [];
}
if (!_cache[topic]!.contains(callback)) {
_cache[topic]!.add(callback);
}
return this;
}