{"id":7963,"date":"2024-10-03T12:27:33","date_gmt":"2024-10-03T10:27:33","guid":{"rendered":"https:\/\/oopm.org\/?page_id=7963"},"modified":"2024-12-30T10:19:31","modified_gmt":"2024-12-30T09:19:31","slug":"17-3-2-representing-the-observer-pattern-as-aspects","status":"publish","type":"page","link":"https:\/\/oopm.org\/?page_id=7963","title":{"rendered":"18.3.2 Representing the observer pattern as aspects"},"content":{"rendered":"
\"image_pdf\"<\/a>\"image_print\"<\/a><\/div>\n

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<\/code> is a subclass of Subject<\/code> and that FireDepartment<\/code>, etc. are subclasses of Observer<\/code>.<\/p>\n\n\n\n

In this section, we will show how the observer pattern may be represented as aspects. For this purpose, we will use the bank example. 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<\/em> 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.<\/p>\n\n\n\n

Account<\/code> objects have a subject aspect represented by the intrinsic object asSubject<\/code>, and Customer<\/code> objects and the alarm<\/code> object have an observer aspect represented by the intrinsic object asObserver<\/code>:<\/p>\n\n\n\n

BankSysEx: obj<\/strong>\n   class<\/strong> Account(owner: ref<\/strong> Customer):   \n      -\"-\n      asSubject: obj<\/strong> Subject\n\n   class<\/strong> Customer(id: var<\/strong> Integer):\n      -\"-\n      asObserver: obj<\/strong> Observer\n\n   alarm: obj<\/strong>\n      asObserver: obj<\/strong> Observer\n   ... <\/code><\/pre>\n\n\n\n
The subject and observer aspects are now represented as properties of intrinsic objects of Account<\/code> objects, of Customer<\/code> objects, and of the alarm<\/code> object. This is illustrated in the following figure, in the first round with the types of the intrinsic objects being Subject<\/code> and Observer<\/code>:<\/p>\n\n\n\n
\"\"<\/figure>\n\n\n\n
As for the Firealarm<\/code> example, we also have to extend ObservedSubject<\/code> to include information about the accounts. The type of asSubject<\/code> object cannot just be Subject<\/code>, as it has to report events that has to do with Account<\/code>-objects. Since we have two types of observers, Customer<\/code> objects and the alarm<\/code>, we introduce a class AccountSubject<\/code> being a subclass of Subject<\/code>, and we use the class as the type of asSubject<\/code>:<\/p>\n\n\n\n
class<\/strong> AccountSubject(whichAccount: ref<\/strong> Account): Subject\n   issueWithTransactions:\n      \"There is an issue with the account of: \".print\n      whichAccount.owner.print\n   inner(AccountSubject)\n\nclass<\/strong> Account(owner: ref<\/strong> Customer):\n   -\"-\n   asSubject: obj<\/strong> AccountSubject(this(Account))<\/code><\/pre>\n\n\n\n
In a similar way, the asObserver<\/code> objects of Customer<\/code> and alarm<\/code> are defined as singular objects derived from class Observer<\/code>:<\/p>\n\n\n\n
class<\/strong> Customer(name: var<\/strong> String):\n   asObserver: obj<\/strong> Observer\n      class<\/strong> ObservedSubject:: AccountSubject  \n      notify::\n         theSubject.issueWithTransactions   \n...\nalarm: obj<\/strong>\n   asObserver: obj<\/strong> Observer\n      class<\/strong> ObservedSubject:: AccountSubject \n      notify::\n         theSubject.issueWithTransactions<\/code><\/pre>\n\n\n\n
\"\"<\/figure>\n\n\n\n
Next we show how Customer<\/code> objects and alarm<\/code> may subscribe to events in Account<\/code> objects. This is done in a method newAccount<\/code> in BankSysEx<\/code>:<\/p>\n\n\n\n
newAccount(owner: ref<\/strong> Customer):\n   acc: ref<\/strong> Account\n   acc := Account(owner)\n   theAccountsFile.insert(acc)\n   acc.asSubject.subscribe(owner.asObserver)\n   acc.asSubject.subscribe(alarm)<\/code><\/pre>\n\n\n\n
The method newAccount<\/code> creates a new Account<\/code> for the Customer<\/code> referred to by owner<\/code>.<\/p>\n\n\n\n