UID – Event Handling and Listeners

UID – Event Handling and Listeners Boriana Koleva (bnk@cs.nott.ac.uk)

Event handling and listeners • What is an event? • user actions and context • event sources and listeners • Why should my programs be event-driven? • User interaction with the GUI

Some typical component events and listeners

Implementing listeners (1) • Three key bits of code • 1) add interface • 2) register • 3) handle • Components can have multiple listeners • A simple JButton ActionListener…

Implementing listeners (2) public class myClass … implements ActionListener { … // where setting up occurs (e.g. constructor) JButton button = new JButton(“I am a button”); button.addActionListener(this); … public void actionPerformed(ActionEvent e) { … // respond to event } // end response method } // end class

Types of event listeners (1) • Global component listeners • may be used for any Swing components • Types • ComponentListener (changes in size, position, visibility) • FocusListener (whether ability for keyboard input) • KeyListener (key press events, only with focus) • MouseListener (clicks and movement into/out of component area) • MouseMotionListener (changes in position over component)

Types of event listeners (2) • Component-specific listeners • relevant to specific components’ actions • Types • ActionListener • CaretListener • ChangeListener • DocumentListener • ItemListener • ListSelectionListener • WindowListener • etc. • See: http://java.sun.com/docs/books/tutorial/uiswing/events/eventsandcomponents.html

Working with event listeners • Getting event information • Low-level events • Semantic events • Adapters for event handling • Inner classes for event handling

Getting event information • EventObject class - use sub classes of this to determine what’s happened. • Get the firing object with getSource(); • Actual event classes sometimes have specific types • e.g. the ComponentListener uses a sub-class of EventObject : ComponentEvent that has getComponent(); • Event classes may define methods that return more information • e.g. ActionEvent has a method for getting modifiers (Shift, Alt, Ctrl)

Low-level and semantic events (1) • Low-level events - window-system level • e.g. mouse, key, component, container, focus, window • trigger component-independent • Semantic events • everything else! – e.g. action, item, list selection • trigger can differ by component • e.g. button click and textfield ‘return’ action events

Low-level and semantic events (2) • Listen for semantic events whenever possible • Gives robust and portable code • eg Button - listen for action event rather than mouse event. Means that button responds to keyboard shortcuts. • Compound components • eg combo box - no real way of guaranteeing low level listeners on all look and feel specific components used to form the compound component.

Adapters for event handling (1) • Classes which implement listener interfaces must implement all listener methods • e.g. MouseListener has 5 methods: mouseClicked, mouseReleased, mousePressed, mouseEntered, mouseExited • This leads to cluttered code • Say you only want mouseClicked to do something then all others have to be implemented but empty • Alternative….

Adapters for event handling (2) • ... is to extend a MouseAdapter class • inherits empty definitions of all five mouseListener methods. Eg: public class MyClass extends MouseAdapter { ... someObject.addMouseListener(this); ... public void mouseClicked(MouseEvent e) { //Event handler implementation goes here... } }

Inner classes for event handling (1) • Don’t want to / cant inherit from an adapter class? • there’s no multiple inheritance in Java • eg can’t extend JPanel AND MouseAdapter • Solution: use an inner class • public class MyClass extends JPanel { … anObject.addMouseListener(new myAdapter()); … class myAdapter extends MouseAdapter { public void mouseClicked(MouseEvent e) { // blah } // end mouseClicked } // end inner class } // end MyClass

Inner classes for event handling (2) • Anonymous classes - • used to simplify code • good when only 1 instance will ever be needed • public class MyClass extends JPanel { ... someObject.addMouseListener(new MouseAdapter() { public void mouseClicked(MouseEvent e) { //Event handler implementation goes here } }); ... }

Threads and Swing (1) • Why use them? • Improved perceived performance • Can remove time consuming task from event thread to keep GUI responsive • Initialisation of program so GUI appears faster • Potential problems • Deadlock the application if access any realised swing components from non event threads.

Threads and Swing (2) • Remember the rule: • Once a Swing component has been realised, all code that might affect or depend on the state of that component should be executed in the event-dispatching thread. • If code does not need to be in event thread then: public void actionPerformed(ActionEvent e) { final SwingWorker worker = new SwingWorker() { public Object construct() { //---code that might take a while to execute is here... return someValue; } }; worker.start(); //required for SwingWorker 3 }

Threads and Swing (3) • invokeLater() • requests that event thread runs certain code • can be called from any thread • code goes in run method of Runable object • returns immediately without waiting for event thread to execute code. Runnable updateAComponent = new Runnable() { public void run() {component.doSomething(); } }; SwingUtilities.invokeLater(updateAComponent);

Threads and Swing (4) • invokeAndWait() • identical to invokeLater() except doesn’t return till event thread has finished executing the code. • Should use this if possible - less chance of deadlock. void showHelloThereDialog() throws Exception { Runnable showModalDialog = new Runnable() { public void run() { JOptionPane.showMessageDialog(myMainFrame, "Hello There"); } }; SwingUtilities.invokeAndWait(showModalDialog); }

Summary - but not the end... • Implementing event listeners • Types of event listeners • Handling event listeners • getting event information • low-level and semantic events • adapters • inner classes - named and anonymous • Threads

What Covered So Far? • What is Swing? • Containers • Frames • Dialogs • (applets) • Components • Loads to choose from • Layout Managers • ‘Educated Trial and Error’ • Events and User Interaction

A simple Swing program • Uses components in containers • Lays components out correctly • Listens for events • An example: • SwingExample.java (revisited)… • Code on Course Website…

A (Slightly) More Complex Swing program • Uses components in containers (again) • Lays components out correctly (again - but more complex) • Listens for events - Multiple listeners • Another example: • SwingExample2.java

UID – Event Handling and Listeners