17.3.2 Representing the observer pattern as aspects

The example in the previous section is meant for illustrating the observer pattern, but from a modeling point-of-view, it does not make sense that fireAlarm is a subclass of Subject and that FireDepartment, etc. are subclasses of Observer.

In this section, we will show how the observer pattern may be represented as aspects. For this purpose, we will use the bank system. A scenario that calls for adding subject/observer aspects to the bank system is the following: Every day the bank checks if there has been some suspicious transactions on accounts, and in case, both a special alarm part of the bank and the actual customer are notified of the event. This is done by giving each account a subject aspect and both alarm and each customer an observer aspect.

Account objects have a subject aspect represented by the object asSubject, and Customer objects and the alarm object have an observer aspect represented by an object asObserver:

BankSysEx: obj
   class Account(owner: ref Customer):   
      -"-
      asSubject: obj Subject

   class Customer(id: var Integer):
      -"-
      asObserver: obj Observer

   alarm: obj
      asObserver: obj Observer
   ... 

The subject and observer aspects are now represented as properties of part-objects of Account objects, Customer objects, and the alarm object. This is illustrated in the following figure, in the first round with the types of the part objects being Subject and Observer:

Recall that an object defined by obj is generated and executed as part of the generation of the object in which the object is. So, each time an Account object is generated, the asSubject object is generated and executed, with the implication that the alarm and the owner are subscribed as observers of this account. Note that the parameters to the invocation of subscribe are not references to alarm or owner, but rather to their asObserver aspect objects.

As for the Firealarm example, we also have to extend ObservedSubject to include information about the accounts. The type of asSubject object cannot just be Subject, as it has to report events that has to do with Account-objects. Since we have two types of observers, Customer objects and the alarm, we introduce a class AccountSubject being a subclass of Subject, and we use the class as the type of asSubject:

class AccountSubject(whichAccount: ref Account): Subject
   issueWithTransactions:
      "There is an issue with the account of: ".print
      whichAccount.owner.print
   inner(AccountSubject)

class Account(owner: ref Customer):
   -"-
   asSubject: obj AccountSubject(this(Account))

In a similar way, the asObserver objects of Customer and alarm are defined as subclasses of Observer:

class Customer(name: var String):
   asObserver: obj Observer
      class ObservedSubject:: AccountSubject  
      notify::
         theSubject.issueWithTransactions   
...
alarm: obj
   asObserver: obj Observer
      class ObservedSubject:: AccountSubject 
      notify::
         theSubject.issueWithTransactions

Next we show how Customer objects and alarm may subscribe to events in Account objects. This is done in a method newAccount in BankSysEx:

newAccount(owner: ref Customer):
   acc: ref Account
   acc := Account(owner)
   theAccountsFile.insert(acc)
   acc.asSubject.subscribe(owner.asObserver)
   acc.asSubject.subscribe(alarm)

The method newAccount creates a new Account for the Customer referred to by owner.

• First a new Account object is generated and a reference to it is assigned to acc.
• Next this reference is inserted in the theAccountsFile, which is the set of accounts of BankSysEx.
• Then the owner aspect (owner.asObserver) of the new Account is subscribed to the subject aspect of acc (acc.asSubject).
• Finally the alarm subscribes to the subject aspect of acc.

Skal der være en figur her?

To sum up, the overall structure of BankSysEx is:

BankSysEx: obj
   class AccountSubject(whichAccount: ref Account): Subject
      -"-
   class Account(owner: ref Customer):   
      -"-   
      asSubject: obj AccountSubject

   class Customer(id: var Integer):
      -"-
      asObserver: obj Observer
         ObservedSubject:: AccountSubject
         -"-  
 
   alarm: obj
      asObserver: obj Observer
         ObservedSubject:: AccountSubject
         -"- 
   newAccount(owner: ref Customer):
      -"-
   accountsFile: obj OrderedList(Account)
   ...