Observer Pattern. (Java listeners!)

Transcription

1 Observer Pattern (Java listeners!)

2 Variable Behavior? S pose we have a SETI program searching for ETs. Originally, would notify US president when discovered life. New requirements: must also notify news agencies (to help prevent world-wide panic). SETISearch +notifypresident() Original SETISearch +notifypresident() Press +notifypress() Possible revision? What s the problem? SETISearch +notifypresident() +notifypress() Possible revision? What s the problem?

3 Wrap It? How would you do this? Possibly the Adapter pattern. adds functionality to existing class Now s pose new requirement: must send response message to ET. And later another requirement Adapters on adapters on adapters possible but awkward. And what if new behavior requires whole new class (not just a method)? SETISearch +notifypres() SETISearch2 +notifypress() SETISearch3 +sendresponse() And what if later we decide not to notify the press?

4 Another Possibility What is varying? The behavior that needs to happen when find ET. behavior = methods. Encapsulate varying methods! We ve seen an example of this e.g., Strategy Create separate class. All new behaviors inherit from this class.

5 Encapsulate Behavior Observer pattern. Make abstract class (Observer) that gets notified by SETISearch when finds ET. Contains abstract update() method. Observer performs the necessary task(s). Concrete implementations of update() do the work. Put notify() method in SETISearch class (Subject). notify() calls the update() method of the observer. Subject has to know that the Observer exists. Otherwise doesn t know who to notify. Put attach(observer o) method in Subject.

6 SETI Example SETISearch +attach(:observer) +notify() Observer +update() attach() keeps a list of all Observers that are passed to it. Can have multiple Observers. notify() calls the update() method of each Observer. The update() method contains the appropriate code. NotifyPresident +update() SendResponse +update() Modular: can add new observers without changing any code!

7 Observer and Refactoring Observer is a great pattern for design stage. Observer is less good for refactoring. Why? Major changes to structure of methods. Irritates QA because have to retest original class. Along these lines: Adapter pattern might be useful if concrete Observers (like NotifyPresident) already exist. But probably won t bother. Just rewrite class since already making other big changes.

8 Common Pattern, So Java Makes Easy interface is like an abstract class. Have an interface so that it doesn t interfere with possibility of NotifyPress needing to inherit from another class. Allows for multiple inheritance. public class SETISearch extends Observable { Now SETISearch contains the } Observable functions. public class Observable //not abstract in Java { //attach public void addobserver(observer o) { //pre-written by SUN } } //notify public void notifyobservers() { //pre-written by SUN } public interface Observer { } public void update(observable o) ; //notify public class NotifyPress implements Observer { } public void update(observable o) { } //press code gets passed the Observable object in case needs any state info from SETISearch. implements is used with interfaces. So NotifyPress could still extend another class.

9 Same as Java Listeners! This is the Java Event model. The Java ActionListener, etc. use different method names but it is the same concept. addactionlistener() for attach(). actionperformed() for update(). For example, look at the JButton. Has addactionlistener(). Then you create a ButtonListener implements ActionListener which implements the actionperformed() method. You feed your ButtonListener to the addactionlistener().

10 What Sets This Pattern Apart? Can add/remove new behaviors without changing code in the future. Different from Strategy! Strategy decides which variable behavior (method) is appropriate. Often (not always) Strategy assumes that each method does the same thing (just in a different way). heap sort bubble sort Observer does a list of (variable) actions/behaviors. Doesn t decide which is appropriate. Just does them all. Often, Observer behaviors can be radically different. send thank you pay online bill launch ICBM

11 Flexibility More flexible than adapter. Do not always have to attach the Observer! So Observer s functionality can be added/removed in each different instantiation. Adapter would require major changes to eliminate functionality (after added).

12 Another Way to Think About Observer When find ET, triggers events. NotifyPresident, NotifyPress, etc. Can t always anticipate what will need to know about the event. Will we later add a requirement to notify riot police as well? Need a technique that can handle anything that needs to know about the event. Create Observer class. Another example. GUI code. President presses nuke em button. Can you anticipate all the necessary actions when this button is pressed? What if add later requirements to ensure certain follow-up actions (e.g., send Red Cross).

13 Details Purpose: Encapsulate variability in methods, especially when they need to act after the object changes state. e.g., after the nuke em button is pressed, it changes the state variable called redalert. e.g., after the cancel button is pressed, it changes the state variable called redalert. Consequences: increased cohesion (relegate tasks to their own classes). annoying method requirements. Solution: see UML, next slide.

14 Observer UML also called Subject Observable +attach(:observer) +notify() ConcreteObservable Observer +update() ConcreteObserver +update() This is like the NotifyPresident class. This is like the SETISearch class.

15 Observer Code Example Give me an idea for a Subject class. Gimme anything! (But no inheritance.) e.g., Car Give it an instance variable with a setter (e.g., speed). Give it constructor. Now make it extend Observable. Imagine a class that might want to observe the Observable. e.g. Cop Make it implement the Observer interface. Write an update method. (e.g., println( You re busted! );) Add this Observer to Observable by attach(new Cop()) Call attach() from Car s constructor, or from elsewhere. Have the setter call the notify method to indicate a state change. public void setspeed(int speed) { this.speed = speed; if(speed > 55) notify(); //calls the Cop note notify() was pre-written } //by SUN and is part of the Observable class.

16 OK, Your Turn Two Teams Create an example of the Observer pattern. Present on board.