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Event Handling and Listeners in SWING

Event Handling and Listeners in SWING. The practice of event handling. Some swing component events and listeners. Implementing listeners. To write an Action Listener, follow the steps given below:

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Event Handling and Listeners in SWING

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  1. Event Handling and Listeners in SWING The practice of event handling

  2. Some swing component events and listeners

  3. Implementing listeners • To write an Action Listener, follow the steps given below: • Declare an event handler class and specify that the class either implements an ActionListener interface or extends a class that implements an ActionListener interface. • For example: public class MyClass implements ActionListener { • Register an instance of the event handler class as a listener on one or more components. • For example: someComponent.addActionListener(instanceOfMyClass); • Include code that implements the methods in listener interface. • For example: public void actionPerformed(ActionEvent e) { ...//code that reacts to the action... } • Three key bits of code • 1) add interface • 2) register • 3) handle • Components can have multiple listeners • A simple JButton ActionListener…

  4. 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

  5. Implementing listeners (2) import java.awt.*; import java.awt.event.*; public class AL extends Frame implements WindowListener,ActionListener { TextField text = new TextField(20); Button b; private int numClicks = 0; public static void main(String[] args) { AL myWindow = new AL("My first window"); myWindow.setSize(350,100); myWindow.setVisible(true); } public AL(String title) { super(title); setLayout(new FlowLayout()); addWindowListener(this); b = new Button("Click me"); add(b); add(text); b.addActionListener(this); } public void actionPerformed(ActionEvent e) { numClicks++; text.setText("Button Clicked " + numClicks + " times"); } public void windowClosing(WindowEvent e) { dispose(); System.exit(0); } public void windowOpened(WindowEvent e) {} public void windowActivated(WindowEvent e) {} public void windowIconified(WindowEvent e) {} public void windowDeiconified(WindowEvent e) {} public void windowDeactivated(WindowEvent e) {} public void windowClosed(WindowEvent e) {} Set up things: public class AL extends Frame implements windowListener,ActionListener { TextField text = new TextField(20); Button b; private int numClicks = 0;

  6. Implementing listeners (2) We would like to handle the button-click event, so we add an action listener to the button b as below: b = new Button("Click me"); b.addActionListener(this); import java.awt.*; import java.awt.event.*; public class AL extends Frame implements WindowListener,ActionListener { TextField text = new TextField(20); Button b; private int numClicks = 0; public static void main(String[] args) { AL myWindow = new AL("My first window"); myWindow.setSize(350,100); myWindow.setVisible(true); } public AL(String title) { super(title); setLayout(new FlowLayout()); addWindowListener(this); b = new Button("Click me"); add(b); add(text); b.addActionListener(this); } public void actionPerformed(ActionEvent e) { numClicks++; text.setText("Button Clicked " + numClicks + " times"); } public void windowClosing(WindowEvent e) { dispose(); System.exit(0); } public void windowOpened(WindowEvent e) {} public void windowActivated(WindowEvent e) {} public void windowIconified(WindowEvent e) {} public void windowDeiconified(WindowEvent e) {} public void windowDeactivated(WindowEvent e) {} public void windowClosed(WindowEvent e) {}

  7. Implementing listeners (2) public void actionPerformed(ActionEvent e) { numClicks++; text.setText("Button Clicked " + numClicks + " times"); Now, when the user clicks the Button b, the button fires an action event which invokes the action listener's actionPerformed method. import java.awt.*; import java.awt.event.*; public class AL extends Frame implements WindowListener,ActionListener { TextField text = new TextField(20); Button b; private int numClicks = 0; public static void main(String[] args) { AL myWindow = new AL("My first window"); myWindow.setSize(350,100); myWindow.setVisible(true); } public AL(String title) { super(title); setLayout(new FlowLayout()); addWindowListener(this); b = new Button("Click me"); add(b); add(text); b.addActionListener(this); } public void actionPerformed(ActionEvent e) { numClicks++; text.setText("Button Clicked " + numClicks + " times"); } public void windowClosing(WindowEvent e) { dispose(); System.exit(0); } public void windowOpened(WindowEvent e) {} public void windowActivated(WindowEvent e) {} public void windowIconified(WindowEvent e) {} public void windowDeiconified(WindowEvent e) {} public void windowDeactivated(WindowEvent e) {} public void windowClosed(WindowEvent e) {}

  8. Types of event listeners • Global component listeners • may be used for any Swing components • Types of global listeners • 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)

  9. 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

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

  11. 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)

  12. Low-level and semantic events • 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

  13. Low-level and semantic events • 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.

  14. Adapters for event handling • 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….

  15. 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... } }

  16. Inner classes for event handling • 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

  17. 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 } }); ... }

  18. Threads and Swing • 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.

  19. Threads and Swing • 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 }

  20. Threads and Swing • 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);

  21. 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); }

  22. Summary • 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

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

  24. 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

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