March Ron McFadyen1 Observer P Also known as Publish-Subscribe Applied in order to implement the Model-View Separation principle (see pages 471+) model and view are separated non-GUI objects are not directly coupled to GUI components domain objects are not directly coupled to window objects same as Model-View-Controller (MVC) principle that came from Smalltalk

March Ron McFadyen2 Observer Problem: There are many objects (subscribers) needing to know of the state changes, or events, of another object (publisher), and we want to keep the coupling low. Solution: Define a subscriber or listener interface that is implemented by the subscribers. Situations: Text example: A user interface object, a window, needs to be informed when a domain object changes In some distributed meta-data environments, replicas need to be notified when the source changes Alarm systems need notification of alarms being triggered...

March Ron McFadyen3 Figure the display must reflect the correct total There is a requirement for a window to be updated whenever the total value of the sale changes

March Ron McFadyen4 Observer Text example. There is a requirement for a window to be updated whenever the total of the sale changes A subscriber interface, PropertyListener, is defined. SaleFrame1 is defined to inherit the PropertyListener interface. This will allow SaleFrame1 to be alerted of changes in the value of the sale total A reference to the Sale is passed to SaleFrame1 when SaleFrame1 is initialized. This allows SaleFrame1 to subscribe to the Sale instance The Sale only knows of objects that subscribe to it; it does not know what class of object they are - so, coupling is kept low.

March Ron McFadyen5 Figure The Observer Pattern in a DCD

March Ron McFadyen6 Figure a window subscribing When a SaleFrame1 (the subscriber) is initialized, it subscribes to the Sale (the publisher)

March Ron McFadyen7 Figure a sale publishing a change A Sale receives a message changing its state. The Sale invokes its method, publishPropertyEvent, which will in turn notify any subscribers of the change Note the activations for the sale

March Ron McFadyen8 Figure a window receiving notification The window receives notification of the state change and modifies its display appropriately Notice that this is a continuation from the previous sequence diagram

March Ron McFadyen9 Observer Pattern Example From: Designed Patterns Explained by Shalloway & Trott; Addison-Wesley; P Observers: objects that want to be notified of a certain event. It must have an update method whereby it is notified of an event. Subject: the object that triggers the event. It must implement: attach (observer) - add an observer to its list of observers detach (observer) - remove an observer from … notify () - goes through its list of observers calling each observer’s update method various methods to allow an observer to get additional information The Observer Pattern defines a one to many dependency between objects so that when one object changes state, all its dependents are notified automatically

March Ron McFadyen10 Observer Pattern Example Whenever a new customer is entered send a welcome letter verify the customer’s address with the post office send letter with coupons to new customers within 20 miles of the company’s “brick and mortar” stores. Four classes from the problem domain: Customer, WelcomeLetter, AddrVerification, BrickAndMortar Applying the Observer pattern results in Customer being the Subject; the others will be Observers Altogether: 6 classes

March Ron McFadyen11 Class Diagram Customer +attach() +detach() +notify() +getState() +setState() Observer +update() WelcomeLetter AddrVerification BrickAndMortar Subject +attach() +detach +notify()

March Ron McFadyen12 Observer Pattern Example: a special situation for courses is: when enrollment reaches the capacity and no more seats are available, the course is full. (for simplicity we ignore sections) Suppose when a course is full: The instructor is informed. The Registration Office is informed

March Ron McFadyen13 Example: Suppose our class model is: DepartmentRegOffice Course * Faculty 0,1* 0..*

March Ron McFadyen14 Example: Filling out some methods Department RegOffice Course * 0..* update() Attach() Detach() Notify() checkFull() myObs 0,1 Instructor Faculty and RegOffice will be observers, and must implement the update method Course will be the subject, and will implement attach, detach, and notify methods. Course keeps track of its observers. 0..* Faculty update()

March Ron McFadyen15 Course > Observer * Recall from Larman that the subject is loosely couple to a set of observers; Course will have a set of references to observers. FacultyRegOffice Faculty and RegOffice must implement the Observer interface

March Ron McFadyen16 Example: interactions i:Facultyc:Course When the Instructor is assigned to a course, the Instructor must register his/her interest with the course attach(i) assign(c) myObs:Object The course adds the Instructor to its list add(i)

March Ron McFadyen17 Example: interactions c:CoursemyObs:Object The Registration Office registers its interest in the course add(r) r:RegOffice attach(r) newCourse(c) The course adds the Registration Office to its list

March Ron McFadyen18 Example: interactions – updates sent to two observers i:Facultyc:Courser:RegOffice update() [full] notify() checkFull() The course realizes it is full, and notifies its two observers The two objects will do whatever is appropriate for them when they are notified via the update message

March Ron McFadyen19 Example: interactions – similar to previous slide, but in more general terms c:CoursemyObs:Object * : update() [full] notify() checkFull() The course realizes it is full, and notifies its observers In general, the course sends the update message to each object that is registered for the event

March Ron McFadyen20 Observer Pattern Summary/Review Responsibility for monitoring an event is with the object that creates the event (the Subject) Subject knows its observers (but not the class they belong to) Don’t need to modify the Subject if the observers change More detail needed if the subscribers need to get specific information after being notified

March Ron McFadyen21 Objects that participate in the observer pattern must either be publishers (subject) or subscribers (observers) publishers publish events of interest to others notify objects, who previously subscribed, when events occur subscribers subscribe to information about these events these objects must be allowed to unsubscribe

March Ron McFadyen22 Publishers must have Attach method (Larman text: addPropertyListener) Detach method Notify method (Larman text: publishPropertyEvent) Subscribers must have Update method (Larman text: onPropertyEvent)