Recitation 11 November 11, 2011

Today’s Goals:  Automatic refresh  Learn and apply the Observer pattern

Fixing the Refresh Problem  Tell Object Editor when changes have occurred  Observer pattern: “Observable” Object Obervers / Listeners Events / Notifications

Analogy: AlertCarolina  You register / listen for alerts  When the state changes, you receive a notification indicating:  State has changed  Information relevant to the change

Two Steps in Observer Pattern  1) Observers register with observable objects to listen for events (one time only)  2) When observable object undergoes an appropriate change, it sends an informative notification to all registered observers

Step 1: Registration (Properties)  Observable class must implement: public void addPropertyChangeListener( PropertyChangeListener listener)  This method adds the provided listener to a collection of listeners (as long as the collection doesn’t already contain this listener – avoid double notifications)  If ObjectEditor.edit() is called on a class that implements this method, ObjectEditor will automatically register itself for notifications from this class

Step 2: Notification (Properties)  When the observed object changes state, it calls propertyChange( PropertyChangeEvent event) on each of its observers  Construct a PropertyChangeEvent with:  this (i.e. a pointer to the observed object)  The name of the changed property as a String  “width”, “x”, “knight”, etc.  The old value of the property  The new value of the property

Step 2 Example: setWidth(int newWidth) int oldWidth = width; width = newWidth; PropertyChangeEvent event = new PropertyChangeEvent(this, “width”, oldWidth, newWidth); /* call propertyChange(event) on all observers */

Collection Observers  Very similar to property observers  Must implement public void addVectorListener(VectorListener listener);  When collection changes, call on all registered listeners: updateVector(VectorChangeEvent event)

Vector Change Event package util.models; public class VectorChangeEvent { // constants to be used for event type public static final int AddComponentEvent = 1, DeleteComponentEvent = 2, ChangeComponentEvent = 3, InsertComponentEvent = 4, CompletedComponentsEvent = 5, ClearEvent = 6, UndefEvent = 1000; // constructor, oldObject can be null when no value is replaced public VectorChangeEvent(Object theSource, int type, int posn, Object oldObject, Object newObject, int newSize) { … } … }

Vector Change Event  To create a VectorChangeEvent within an Observer, pass the following into the VectorChangeEvent constructor (in order):  this  The int corresponding to the change that occurred, e.g. VectorChangeEvent.AddComponentEvent  The index of the affected element  The Object that was previously at that location (null for push())  The Object that is now at that location (null for pop())  The new size of the collection

Recitation Specification  Download Recitation11.zip from the Recitations page  Implement ObservableAlignedShapeStack:  Create an ArrayList instance variable that stores all objects listening to / observing this stack (VectorChangeListener objects)  public void addVectorListener(VectorListener listener) /* adds the provided listener to the list of listeners, but only if the listener is not already in the list */  Hint: Use the add() and contains() methods from ArrayList  Create: void notifyAllListeners(VectorChangeEvent event)  For each listener in the ArrayList, call propertyChange on that listener with the provided event  Override push() and pop() so that they notify all listeners of the change made to the collection (but only IF a change was made!)  If done correctly, you won’t need to manually refresh