March 2003 91.3913 Ron McFadyen 1

Observer P. 368+

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

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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

...

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Figure 23.20 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

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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.

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Figure 23.21 The Observer Pattern in a DCD

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Figure 23.22 a window subscribing

When a SaleFrame1 (the subscriber) is initialized, it subscribes to the Sale (the publisher)

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Figure 23.23 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

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Figure 23.24 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

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Observer Pattern Example

From: Designed Patterns Explained by Shalloway & Trott; Addison-Wesley; P 265-277

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

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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

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Class Diagram

Customer

+attach()+detach()+notify()+getState()+setState()

Observer+update()

WelcomeLetter

AddrVerification

BrickAndMortar

Subject

+attach()+detach+notify()

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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

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Example: Suppose our class model is:

Department RegOffice

Course*

Faculty0,1 *

0..*

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Example: Filling out some methods

Department

RegOffice

Course*

0..*update()

Attach()

Detach()

Notify()

checkFull()

myObs

0,1Instructor

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()

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Course

<<interface>>Observer

*

Recall from Larman that the subject is loosely couple to a set of observers; Course will have a set of references to observers.

Faculty RegOffice

Faculty and RegOffice must implement the Observer interface

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Example: interactions

i:Faculty c: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)

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Example: interactions

c:Course myObs: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

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Example: interactions – updates sent to two observers

i:Faculty c:Courser:RegOffice

update()

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

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Example: interactions – similar to previous slide, but in more general terms

c:Course myObs: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

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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

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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

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Publishers must have

•Attach method (Larman text: addPropertyListener)

•Detach method

•Notify method (Larman text: publishPropertyEvent)

Subscribers must have

•Update method (Larman text: onPropertyEvent)