I’d like to second this response, but for the record the answer to your question is the runEvent() method. Functions in handlers are generally events and are intended to be invoked by the framework. If you simply have a chunk of re-usable utility code you wish to make available to multiple handlers, then put it in your model and inject it into all the handlers that need it. If it is something extremely simple such as a formatting function or something, you could also use the UDF library.
~Brad