Observer design pattern


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Observer design pattern

  1. 1. Research week #2
  2. 2.  Introduction Intent Class diagram Sequence diagram Benefit Implementations Issues Observer pattern in java
  3. 3.  Observer design pattern is behavioral pattern Used to assure consistency between objects. Separation of the objects that is dependent on each other. It used to make relation between objects at run time not compile time. Used in MVC(mainly Model & GUI part)
  4. 4.  Define a one-to-many dependency between objects so that when one object changes state, all its dependents are notified and updated automatically. Object that changes called “Subject”. Object that receives updates called “Object”.
  5. 5.  Sequence diagram observer Subject Register/att ach Change state trigger notify()/upda notification te getstate()
  6. 6.  Minimal coupling between the Subject and the Observer: Can reuse subjects without reusing their observers and vice versa. Observers can be added without modifying the subject. All subject knows is its list of observers so make decoupling. Subject does not need to know the concrete class of an observer.
  7. 7.  How does the subject keep track of its observers? Array, linked list, Vector What if an observer wants to observe more than one subject? subject tell the observer who it is via the update interface. Who triggers the update? The subject whenever its state changes. Can an observer also be a subject? Yes! Because class can implement more than one interface.
  8. 8.  How does Subject send the changed data to Object? Two ways: 1-Pull model: Observer invoke method requesting data SubjectName.getdata(); 2-Push model: Subject passes data to observer as argument at update () Object[i].update(SubjectName,data);
  9. 9.  We could implement the Observer pattern from scratch in Java. But 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 Uses a Vector for storing the observer references
  10. 10.  The java.util.Observer interface is the Observer interface. It must be implemented by any observer class. Java.util.Observable class public synchronized void addObserver(Observer o) public synchronized void deleteObserver(Observer o) protected synchronized void setChanged() public void notifyObservers() public void notifyObservers(Object arg)
  11. 11.  Java.util.Observer interface • public abstract void update(Observable o, Object arg) Let’s see sample code:
  12. 12. import java.util.Observable;import java.util.Observer;public class ConcreteSubject extends Observable { private String name; private float price; public ConcreteSubject(String name, float price) { = name; this.price = price; System.out.println("ConcreteSubject created: " + name + " at " + price); } public String getName() {return name;} public float getPrice() {return price;} public void setPrice(float price) { this.price = price; setChanged(); notifyObservers(new Float(price)); }}
  13. 13. import java .util.Observer;import java .util.Observable;public class ConcreteObserver implements Observer { private float price; public void NameObserver() { price =0; System.out.println("price observer is created is"+price);} public void update(Observable obj, Object a) { price = ((Float)a).floatValue(); System.out.println("PriceObserver: Price changed to " + price); }}
  14. 14. public class TestObserver { /** * @param args */ public static void main(String[] args) { ConcreteSubject s = new ConcreteSubject("GUI team",1.29f); ConcreteObserver o = new ConcreteObserver(); s.addObserver(o); s.setPrice(4.56f); s.setPrice(2.3f); }}
  15. 15.  Program outputConcreteSubject created: GUI team at 1.29PriceObserver: Price changed to 4.56PriceObserver: Price changed to 2.3
  16. 16.  /patterns/observer.html ml us/library/ee817669.aspx s/Observer.pdf
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