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Event driven programmingEvent driven programming
Listeners, sources, and events
GUI programmingGUI programming
z Asynchronous programing / event-based programming
z Events
z Must respond to events in arbitrary order
z Simplistic approach: make a class to extend a window object
Wright State University, College of Engineering Dr. T. Doom, Computer Science & Engineering
CS 241 Computer Programming II
z Simplistic approach: make a class to extend a window object
- Manage layout
- Create menus
- Application areas
- Manage user interaction
What is the role of an event?What is the role of an event?
import javax.swing.*; … public static void main (String[] args) { JFrame frame = new JFrame(); frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE); JPanel contentPane = (JPanel) frame.getContentPane();
JButton button = new JButton("Click me"); contentPane.add(button);
Wright State University, College of Engineering Dr. T. Doom, Computer Science & Engineering
CS 241 Computer Programming II
z What happens when we click the button?
z We need two things:
- A way to know when something happens (an event)
- A method to be called to perform the appropriate task in response
z We’re interested in the user-takes-action-on-a-button event!
frame.setSize(50,50); frame.setVisible(true); } // end method main
Events have sources and handlers/listenersEvents have sources and handlers/listeners
private int numClicks = 0; … public void processButtonClick() { numClicks++;
button.setText("I’ve been clicked " + numClicks + " times. ");
} // end method processButtonClick
Wright State University, College of Engineering Dr. T. Doom, Computer Science & Engineering
CS 241 Computer Programming II
z Design/implement the “what to do” code as appropriate
z Then set up a relationship between the code and the event-generating
object
Hey, I care about
what happens to you!
I’m listening
The user clicked me!
Here are the details on the event!
The listener interfaceThe listener interface
z An event source (like a Swing GUI button) creates an event object when
the user does something that matters (like click the button)
z Every event type has a matching listener interface
- If your code implements a listener interface and registers as a listener
- then you’ve set up a contract that allows communication from the
event source to your code.
R ll i f 100% b l i
Wright State University, College of Engineering Dr. T. Doom, Computer Science & Engineering
CS 241 Computer Programming II
z Recall: interfaces are 100% pure abstract classes, you must write
implementations for the methods declared in the interface
ActionListener actionPerformed (e : ActionEvent )
ItemListener itemStateChanged (e : ItemEvent )
KeyListener keyPressed (e : KeyEvent) keyReleased (e : KeyEvent) keyTyped (e : KeyEvent)
listenerList
Register: Add me to your List of listeners!
OK. I’ll call you when I have an event!
I’ll use the method name in the contract!
addActionListener(this)
actionPerformed(e)
An eventAn event--handling listener object!handling listener object!
import javax.swing.; import java.awt.event.;
public class ButtonHandler implements ActionListener { private JButton button; private int numClicks = 0; public void buildGUI () { JFrame frame = new JFrame (); frame.setDefaultCloseOperation (JFrame.EXIT_ON_CLOSE); JPanel contentPane = (JPanel) frame.getContentPane ();
Wright State University, College of Engineering Dr. T. Doom, Computer Science & Engineering
CS 241 Computer Programming II
( ) g (); button = new JButton ("Click me"); contentPane.add (button); button.addActionListener (this); // register as listener frame.setSize (300,300); frame.setVisible (true); } // end method GUI
public void actionPerformed (ActionEvent event) { numClicks++; button.setText ("I’ve been clicked " + numClicks + " times. "); } // end method actionPerformed } // end class ButtonHandler
Nested classesNested classes
z Most OO-languages allow you to nest object code structures
- In Java, this is often referred to as an inner class
z Inner classes have access to all members (even private members) of the
outer class
class Outer { private int x = 0; public void go() {
z Using inner classes can greatly
simplify registering multiple
Wright State University, College of Engineering Dr. T. Doom, Computer Science & Engineering
CS 241 Computer Programming II
p g Inner inner = new Inner(); inner.y = 3; inner.go(); } // end method go
class Inner { private int y = 0; public void go() { x = y; } // end method go } // end nested class Inner } // end class Outer
p y g g p
listeners.
z Each button can have:
z its own (inner) class that
z it registers as an action listener
z handles only that button’s
function
z With full access to outer class
members
Handling multiple event sourcesHandling multiple event sources
using inner classesusing inner classes
button = new JButton ("Click me"); contentPane.add (BorderLayout.CENTER,button); button.addActionListener ( new ClickListener() );
JButton exitButton = new JButton ("Exit"); contentPane.add (BorderLayout.SOUTH,exitButton); exitButton.addActionListener ( new ExitListener() ); … class ClickListener implements ActionListener {
Wright State University, College of Engineering Dr. T. Doom, Computer Science & Engineering
CS 241 Computer Programming II
p { public void actionPerformed (ActionEvent event) { numClicks++; button .setText ("I’ve been clicked " + numClicks + " times. "); } // end method actionPerformed } // end inne class ClickListener
class ExitListener implements ActionListener { public void actionPerformed (ActionEvent event) { System.exit (0); } // end method actionPerformed } // end inner class ClickListener …
ExamplesExamples
z Add “Speed up” and “Slow down” buttons to our simple ball animation
z Use the keyboard to move in a general frame w/o buttons
z SWING Layout
Wright State University, College of Engineering Dr. T. Doom, Computer Science & Engineering
CS 241 Computer Programming II
z SWING Layout
- Add a button to each region
- Next multiple buttons in one panel in central region
- Etc
- JLabel
- JTextfield
- JCheckbox
- JList
Example: Simple WindowsExample: Simple Windows--style GUIstyle GUI
Swing buttons
Swing Menus
Responding to button click events
Creating a simple WindowsCreating a simple Windows--Style GUIStyle GUI
We will create a simple GUI application that
changes the color in the central panel on
demand.
We need to know:
Wright State University, College of Engineering Dr. T. Doom, Computer Science & Engineering
CS 241 Computer Programming II
- How to create a menu
- How to respond to events
- button clicks and menu selections
- How to output text to a text field
This is our goal!
Example Goal
Wright State University, College of Engineering Dr. T. Doom, Computer Science & Engineering
CS 241 Computer Programming II
Using JLabel and JTextField
import javax.swing.*;
JTextField colorName;
JPanel colorNamePanel;
public ColorWindow() {
colorNamePanel = new JPanel();
Jlabel colorLabel = new Jlabel("Current
color:“));
Wright State University, College of Engineering Dr. T. Doom, Computer Science & Engineering
CS 241 Computer Programming II
color: ));
colorNamePanel.add(colorLabel);
colorName = new JTextField(5);//5char wide
colorName.setEditable(false); // read only
colorName.setText("Red");
colorNamePanel.add(colorName);
// add to Layout
add(colorNamePanel,BorderLayout.NORTH);
Layout complete
No functionality
Inner class implements ActionListener
z Recall: A listener can be implemented using an
inner class, implementing the ActionListener
interface
- An inner class is encapsulated within a
larger class
- An inner class may only be used within the
larger class
Public Class
ColorWindow
Class Listener
(implements
Wright State University, College of Engineering Dr. T. Doom, Computer Science & Engineering
CS 241 Computer Programming II
larger class
- There can be several inner classes inside a
outer class
ActionListener
interface)
Implementing the Listener
class Listener implements ActionListener {
public void actionPerformed(ActionEvent e)
// find source of event and respond accordingly
if( (e.getSource() == redButton) ||
(e.getSource() == redMenuItem) ) {
colorPanel.setBackground(Color.RED);
colorName.setText("Red");
Wright State University, College of Engineering Dr. T. Doom, Computer Science & Engineering
CS 241 Computer Programming II
// repeat for blue and green colors
All listeners must implement the actionPerformed(ActionEvent e) method
defined in the ActionListener interface.
This would work… can we do better?
Register the listener
z The listener must be
registered with any buttons
and menu items that use the
listener.
z If desired, you can
implement different
listeners with different GUI
import javax.swing.*;
// repeat for each color:
// register the menu item
redMenuItem.addActionListener(new
Listener());
// i h b
Wright State University, College of Engineering Dr. T. Doom, Computer Science & Engineering
CS 241 Computer Programming II
listeners with different GUI
components.
z When registering an
ActionListener, create a
new instance of the
Listener class.
and menu item
// register the button
redButton.addActionListener(new
Listener());
An elegant way to implement listeners is to
extend the JObjects and implement the interface
in the extended objects!
Example 2: a KeyboardExample 2: a Keyboard--based “game”based “game”
z The basic form of a
games is pretty
standard
- Make a JFrame
- Make a JPanel
z move()
import java.awt.; import javax.swing.; public class GameFrame { public GameFrame() { play(); } // end constructor private void pause(int millis) { try { Thread.sleep(millis); } catch (Exception e) {
Wright State University, College of Engineering Dr. T. Doom, Computer Science & Engineering
CS 241 Computer Programming II
z paintComponent()
loop
z force move
z check game logic
z force repaint
z pause/repeat
public class Main { public static void main(String[] args) { GameFrame game = new GameFrame(); } // end method main } // end class Main
} ( p ) { e.printStackTrace(); } } // method pause … // needs play() } // end class GameFrame
Main loop skeletonMain loop skeleton
private void play() {
JFrame frame = new JFrame(”CS241 Game");
frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
GamePanel panel = new GamePanel(40,4);
frame.getContentPane().add(BorderLayout.CENTER,panel);
frame.addKeyListener(panel);
frame.setSize(1024,600);
frame setVisible(true);
Wright State University, College of Engineering Dr. T. Doom, Computer Science & Engineering
CS 241 Computer Programming II
frame.setVisible(true);
while (true) {
pause(5);
panel.move();
panel.repaint();
} // end method play
