Quartz::Observer
The Observer module is intended to be included in a class as a mixin.
It provides a protocol so that objects can register to Observable objects
and receives their updates, along with additional optional information.
Observers must define an #update method.
Instance methods
update(observable : Observable, info)
This method is called whenever the observed object (observable) is
changed. A dictionary representing additional information, info,
may be available, or nil otherwise.