I think everybody's talking about messages, but you can choose to encode messages as direct method-calls versus indirect data-objects, e.g.:

watchers[0].customerNameChanged(this, old_name, new_name);

emit_for_whomever(new CustomerNameChanged(this,old_name, new_name));

The second version actually has more classes being defined and objects being created. However, it comes with quite a few benefits in terms of documentation, duplication, persistence, deferring, batching, etc.