18.3.1 The alarm example

In order to simplify this first example, we do not represent the subject and observer aspects as aspects in the form of objects as in the previous sections. We give an example of this later in this section.

The fire alarm is represented by an object of class Alarm, a subclass of Subject. The fire department, center owner, and shops are represented by instances of class Stakeholder which in turn is a subclass of Observer.

   class Alarm(id: var String): Subject 
      whatWentWrong:
         " - smoke detected\n".print
      alert:
         notifyObservers      
   class Stakeholder(id: var String): Observer
      ObservedSubject::< Alarm 
      notify::
          theSubject.whatWentWrong	
      fireAlarm.subscribe(this(StakeHolder))
   fireAlarm: obj Alarm("fireAlarm")
   fireDepartment: obj Stakeholder("FireDepartment")
   centerOwner: obj Stakeholder("CenterOwner")
   shopA: obj StakeHolder("ShopA")
   shopB: obj StakeHolder("ShopB")
   shopC: obj StakeHolder("ShopC")

Class Alarm has an id identifying the alarm.
It has a method whatWentWrong that may supply information to an Observer when notified.
It has a method alert that is invoked by a fire censor in case a fire is detected.
Class StakeHolder is a subclass of Observer.
It has an id identifying the Observer.
It makes further binding of notify, which invokes theSubject.whatWentWrong.
It makes a further binding of class ObservedSubject to Alarm.
Note that theSubject (the parameter of notify) is of type ObservedSubject, and since it is bound to Alarm, the method whatWentWrong may be invoked.
It invokes fireAlarm.subscribe(this(StakeHolder)) and thereby subscribe to events from fireAlarm.
The object fireAlarm is declared as an instance of class Alarm.
The Observer objects FireDepartment, etc. are declared as instances of class Stakeholder.

The next diagram illustrates the situation where the subject fireAlarm has references to all the Observers, and the argument, theSubject of invocations of notify on an Observer refers to the fireAlarm object.

When the Stakeholder objects are generated, they all subscribe to events from the fireAlarm. At some point in time an external sensor may detect a possible fire and invoke fireAlarm.alert, which then invokes notifyObservers, which in turn invoke notify on all Stakeholder objects. This is illustrated by the following OSD.

The invocations of notify all invoke whatWentWong on the FireAlarm, but this is not shown in the above diagram.

In the above example, the FireAlarm is the only subject being observed. It is of course possible to add more FireAlarm objects at different places in the shopping center or at other locations. The FireDepartment will then observe all the FireAlarms whereas the shops and owner of a given shopping center only observe the alarm in that center.