1. JFC/Swing lectures O BSERVER PATTERN – general form Idea: decouple event from event handling Concrete Observable Abstract Observable Concrete Observer 0..* Abstract Observable attach(Observer) detach(Observer) notifyObservers() Abstract Observable Abstract Observer update()

2. JFC/Swing lectures JFC use of O BSERVER  The observer pattern is used for event notification.  Observables (classes like JButton) generate events.  An observable can have many observers.  The pattern is applied several times in design of class library.  Pattern is tailored to application: method names are suitable for each application.

3. JFC/Swing lectures O BSERVER PATTERN - applied In JFC/Swing, Observer is realized for different event types, with methods named accordingly. Abstract Observable 0..* JButton addActionListener(ActionListener) removeActionListener(ActionListener) fireActionPerformed(ActionEvent) Abstract Observable ActionListener actionPerformed(ActionEvent ) attach  addActionListener detach  removeActionListener notifyObservers  fireActionPerformed update  actionPerformed

4. JFC/Swing lectures Multiple Observers We can add more than one observer to our button. All observers are notified when a button event occurs. Let us add an observer that will also change the color of the label. button.addActionListener(new ActionListener() { public void actionPerformed(ActionEvent e) { label.setForeground(Color.RED); } });