Software Design Lecture : 40

Observer Design Pattern OR Publish and Subscribe

Motivation for Observer Design Pattern When we partition a system into a collection of cooperation classes, it is desired that consistent state between participating objects is to be maintained. This should be not achieved via tight coupling as against our basis design principle because for obvious reason this will reduce reusability

One Data multiple Representations

Myth about the Diagram Bar Graph and Pie Chart don’t know about each other so that any one can be reused independent of each other, but the interesting thing is that it seems that they know each other. How??? When the data in the spreadsheet is changed it is reflected in pie chart and bar graph also immediately

Myth about the diagram This behavior implies that they are dependent on data of the spreadsheet and when ever there is a change in spreadsheet pie chart and bar graph is notified to update the change. There seems to be no reason to believe that the number of objects representing the data is to be limited may be i- e may be line graph is to be used in future to represent data

Observations There exists a consistent communication model between a set of dependent objects and an object that they are dependent on. This allows the dependent objects to have their state synchronized with the object that they are dependent on.

Observers and Subjects (Observable) The set of dependent objects are referred to as Observers i-e Graphs in our example The object on which Observer dependent is referred to as the subject. i-e Spreadsheet in our example

Newspaper Subscription Example

Case for Observer Design Pattern Observer pattern suggests a publisher-subscriber model leading to a clear boundary between the set of Observer objects and the Subject object. A typical observer is an object with interest or dependency in the state of the subject.

Case for Observer Design Pattern The subject cannot maintain a static list of such observers as the list of observers for a given subject could change dynamically. Hence any object with interest in the state of the subject needs to explicitly register itself as an observer with the subject Whenever the subject undergoes a change in its state, it notifies all of its registered observers. Upon receiving notification from the subject, each of the observers queries the subject to synchronize its state with that of the subject’s.

One – to Many Relationship In other words, the scenario contains a one-to-many relationship between a subject and the set of its observers. Each of the observer objects has to register itself with the subject to get notified when there is a change in the subject’s state

Observer Pattern Defined “This pattern defines a one-to-many relationship between objects so that when there is change in the state of the one object it should be notified and automatically updated to all of it’s dependent”

Communication Mechanism The subject should provide an interface for registering and unregistering for change notifications. In the pull model — The subject should provide an interface that enables observers to query the subject for the required state information to update their state. In the push model — The subject should send the state information that the observers may be interested in.

Class Diagram

Sequence Diagram

Consequences Support for event broadcasting Minimal coupling between the Subject and the Observer . Reuse obervers without using subject and vice verca

Consequences Liabilities: Possible cascading of notifications Observers are not necessarily aware of each other and must be careful about triggering updates

Observer Pattern in Java Java provides the Observable/Observer classes as built-in support for the Observer pattern. The java.util.Observable class is the base Subject class. Any class that wants to be observed extends this class. Provides methods to add/delete observers Provides methods to notify all observers A subclass only needs to ensure that its observers are notified in the appropriately Uses a Vector for storing the observer references

Observer Interface The java.util.Observer interface is the Observer interface. It must be implemented by any observer class.

Sample Code

public class ConcreteSubject extends Observable { private String name; // To be observed private float price; public ConcreteSubject(String name, float price) { this.name = name; this.price = price; System.out.println("ConcreteSubject created: " + name + " at “ + price); }

public String getName() {return name; } public float getPrice() {return price;} public void setName(String name) { this.name = name; setChanged(); // Methods Implemented in Java notifyObservers(name); public void setPrice(float price) { this.price = price; setChanged(); notifyObservers(new Float(price)); }}

// An observer of name changes // An observer of name changes. public class NameObserver implements Observer { private String name; public NameObserver() { name = null; System.out.println("NameObserver created: Name is " + name);} public void update(Observable obj, Object arg) { if (arg instanceof String) { name = (String)arg; System.out.println("NameObserver: Name changed to " + name); } else { System.out.println("NameObserver: Some other change to subject!");}}}

// An observer of price changes // An observer of price changes. public class PriceObserver implements Observer { private float price; public PriceObserver() { price = 0; System.out.println("PriceObserver created: Price is " + price); } public void update(Observable obj, Object arg) { if (arg instanceof Float) { price = ((Float)arg).floatValue(); System.out.println("PriceObserver: Price changed to " + price); } else { System.out.println(”PriceObserver: Some other change to subject!"); }

public class TestObservers { public static void main(String args[]) { // Create the Subject and Observers. ConcreteSubject s = new ConcreteSubject("Corn Pops", 1.29f); NameObserver nameObs = new NameObserver(); PriceObserver priceObs = new PriceObserver(); // Add those Observers! s.addObserver(nameObs); s.addObserver(priceObs); // Make changes to the Subject. s.setName(“Corn Flakes"); s.setPrice(4.57f); s.setPrice(9.22f); s.setName("Sugar Crispies");}}

Output of the Program ConcreteSubject created: Corn Pops at 1.29 NameObserver created: Name is null PriceObserver created: Price is 0.0 PriceObserver: Some other change to subject NameObserver: Name changed to Corn Flakes PriceObserver: Price changed to 4.58 NameObserver: Some other change to subject! PriceObserver: Price changed to 9.22 NameObserver: Name changed to Sugar Crispies