18.3.1 The alarm example

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

AlarmSystem: obj
   class Stakeholder(id: var String): Observer
      ObservedSubject::< ShoppingMall.Alarm
      notify::<
          theSubject.whatWentWrong
          ...
      setUp:
         ShoppingMall.fireAlarm.subscribe(this(StakeHolder))
   FireStation: obj Stakeholder("FireStation")
   CenterOwner: obj Stakeholder("CenterOwner")
   ShoppingMall: obj
      class Alarm(id: var String): Subject
         whatWentWrong::
            " - smoke detected\n".print
         alert:
            notifyObservers
      fireAlarm: obj Alarm("fireAlarm")
      aSensor: obj Sensor
         ...; fireAlarm.alert; ...
      ShopA: obj StakeHolder("ShopA")
      ShopB: obj StakeHolder("ShopB")
      ShopC: obj StakeHolder("ShopC")
      setUp:
         ShopA.setUp
         ShopB.setUp
         ShopC.setUp
   FireStation.setUp
   CenterOwner.setUp
   ShoppingMall.setUp

• The AlarmSystem has a class StakeHolder that is a subclass of Observer.
  • It has an id identifying the Observer.
  • It makes further binding of notify, which invokes theSubject.whatWentWrong. The dots ('...') indicates that some further actions may be needed. We discuss that at the end of this section.
  • 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 has a method setUp, which invokes ShoppingMall.fireAlarm.subscribe(this(StakeHolder)) and thereby makes this(StakeHolder) subscribe to events from fireAlarm.
• The AlarmSystem has Observer objects FireStation, and CenterOwner, which are declared as instances of class StakeHolder.
• The AlarmSystem has a singular object ShoppingMall.
  • It has a local class Alarm, which 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.
  • It has an object fireAlarm that is declared as an instance of class Alarm.
  • It has a local object aSensor, which is an instance of class Sensor (not shown).
  • It has local objects ShopA, ShopB, and ShopC representing three shops in the mall.
  • It has method setUp, which invokes setUp for the three Shop objects and thereby makes them subscribe to events from the fireAlarm.
• Finally, the AlarmSystem invokes setUp for the FireStation, CenterOwner and ShoppingMall.

In the example we have shown just one instance of a a Sensor object – in practice there may be many sensors in a shopping mall. We have not described how a Sensor object is activated to invoke fireAlaram.alert – this depends on the actual sensor hardware.

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.

The ShoppingMall is an example of composition where fireAlarm, aSensor, ShopA, ShopB and ShopC are components being parts of the ShoppingMall.

Stakeholder objects subscribe to events from the fireAlarm. This includes the fireStation, the centerOwner, and the shop stake holders:

Fire sensors located at places in the shopping center are represented by objects of class Sensor. When a sensor detects a fire, the corresponding Sensor object invoke fireAlarm.alert, which then invokes notifyObservers, which in turn invoke notify on all Stakeholder objects. This is illustrated by the following OSD, however, just showing one Sensor object.

The notify invocations 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 FireStation will then observe all the fireAlarms whereas the shops and owner of a given shopping center only observe the alarms in that center.

Connections to the physical shopping center

As mentioned in the start of this section, a notification of an observer may involve more action than described in the further binding of notify in class StakeHolder, since the different stakeholders may need to be informed in different ways.

For the FireStation, an alarm may be activated in the living room of the fire fighters, the address of the ShoppingMall must be communicated to the GPS in the fire engines, etc. For the ShopOwner, an sms may have to sent to the shop owner. For the shops, the owner and personel of a shop must be informed, etc.

As an example we may add an inner to invoke in StakeHolder and introduce a further binding of invoke in CenterOwner:

AlarmSystem: obj
   class Stakeholder(id: var String): Observer
      ObservedSubject::< Alarm 
      notify::<
          theSubject.whatWentWrong
          inner(notify)
      -"-
   -"-
   CenterOwner: obj Stakeholder("CenterOwner")
      notify::<
          -- send sms to center owner

We have just inserted a comment about sending an sms to the owner of the center. In order to do that, we need an object that is connected to a real mobile phone device.

Remember that the objects in the example are representations of the real physical phenomena and their connection to these depends on the details of how the AlarmSystem is connected to the physical shopping mall.