Event Listeners

Abteilung für Telekooperation
Übung Softwareentwicklung 2
für Wirtschaftsinformatik
Event Listeners
Dr. Wieland Schwinger, MSc.
Abteilung für Telekooperation
Don't call us!
we call you!
-- the Hollywood principle
Abteilung für Telekooperation
Handling Events
• Every time the user types a character or pushes a mouse
button, an event occurs
• Objects can be notified by the occurrence of the event.
• For this the object has to implement the event listener
interface.
• Some examples of swing component generated events:
–
–
–
–
–
–
–
ActionListener,
WindowListener,
MouseListener,
MouseMotionListener,
ComponentListener,
FocusListener,
ListSelectionListener, …
Abteilung für Telekooperation
Java Event Mechanism
• Event Source:
– holds list of Event Listerner interested in the Event Source's
signaling of the occurance of an Event and fires an Event on it's demand
– to subscribe an Event Listeners
• addXXXEventListener (XXXEventListener)
– to descubsribe an Event Listeners
• removeXXXEventListener (XXXEventListener)
– to signal the occurence of an event to the subscribed Event Listeners
• fireXXXEvent()
• Event Object:
– the signalled event is an object of type EventObject
• class XXXEvent extends java.util.EventObject { ... }
• Event Listener:
– the Event Listeners are informed about an event's occrence by calling the
dedicated method and passing the event object
• interface XXXEventListener extends java.util.EventListener {
XXXEventOccurred (XXXEvent e) {
// react on the event } }
Abteilung für Telekooperation
Beispiel: Interfaces für Ereignisbehandlung
• Implementieren Sie ein geeignetes Java-Ereignisszenario, das auf Eingaben
von der Konsole mit einem Ereignis reagiert.
Hinweise:
Es sollen die folgenden Teile des Ereignismechanismus implementiert werden:
– InputEvent (Ereignisobjekt mit Daten)
– InputEventListener (Definition einer Schnittstelle für Ereignisempfänger)
– InputTrigger (Ereignisquelle mit Registrierungsmöglichkeit für Empfänger
und Mechanismus zum Auslösen des Ereignisses)
Die Ereignisquelle (InputTrigger) liest die Eingabe in folgender run()-Methode:
public void run () {
try {
do {
int in = System.in.read();
fireInputEvent(this, (byte) in);
} while (in != (int) 'q');
} catch (Exception e) { /* handle exception here */ }
}
Abteilung für Telekooperation
Beispiel: Interfaces für Ereignisbehandlung
Datei InputEventListener.java
Datei InputEvent.java
import java.util.*;
public interface InputEventListener extends EventListener {
public void inputReceived (InputEvent e);
4
}
import java.util.*;
public class InputEvent extends EventObject {
1
public char data;
InputEvent (Object sender, char data) {
2
super(sender);
this.data = data;
}
}
Datei MyHandler.java
public class MyHandler extends Thread
implements InputEventListener {
1
MyHandler(InputTrigger
t) {
t.addInputEventListener(this);
}
2
public void inputReceived (InputEvent e) {
// here I do what ever I want
// if a new input has been received, e.g.
System.out.println("new input: " + e.data);
}
}
3
Abteilung für Telekooperation
Beispiel: Interfaces für Ereignisbehandlung
Datei InputTrigger.java
3
import java.util.*;
public class InputTrigger implements Runnable {
protected Vector listeners = new Vector();
public void addInputEventListener (InputEventListener l) {
listeners.addElement(l);
}
public void removeInputEventListener(InputEventListener l) {
listeners.removeElement(l);
}
private void fireInputEvent (Object sender, byte data) {
System.out.println("InputEvent triggered");
Iteration iter = listeners.iterator();
while (iter.hasNext()) {
InputEvent e = new InputEvent(sender, data);
((InputEventListener) iter.next()).inputReceived(e);
}
}
4
public void run () {
try {
char in;
do {
in = (char) System.in.read();
fireInputEvent(this, in);
} while (in != (char) 'q');
} catch (Exception e) { /* some error handling */ }
}
public static void main (String[] args) {
InputTrigger t = new InputTrigger();
t.run();
MyHandler theListener = new MyHandler(t);
theListener.start();
}
}
2
1
Abteilung für Telekooperation
How to implement an Event Handler
• To realize e.g. an ActionListerner:
– Declaration of the event handler class
public class MyClass implements ActionListener { ...
– Register an instance of the event handler class as a listener upon one
(more) event source (components, e.g. a Button)
Button signallingComponent = new Button("Click Me");
signallingComponent.addActionListener(instanceOfMyClass);
– Implementing methods of the listener interface, e.g.:
public void actionPerformed(ActionEvent e) {
... // code that reacts to the action
}
• The signaling of the event (by generating the event object and
invoking the actionPerformed method) is realized in the AWT/Swing
package
Abteilung für Telekooperation
Event Object
• An event is represented by an instance of
EventObject.
• EventObject is the superclass for all AWT and
Swing events.
• The method getSource() returns the object
that fired the event.
Abteilung für Telekooperation
Mouse Listener Example
/* Example 1: implementing the MouseListener Interface */
public class MyClass implements MouseListener {
signaler of the event, e.g. component like a button
...
someObject.addMouseListener(this);
pass object as listener to
...
subscribe to the signaler
public void mousePressed(MouseEvent e) { ... }
public void mouseReleased(MouseEvent e) { ... }
public void mouseEntered(MouseEvent e) { ... }
public void mouseExited(MouseEvent e) { ... }
public void mouseClicked(MouseEvent e) { ...}
}
Abteilung für Telekooperation
Mouse Listener Example
/* Example 2: extending the MouseAdapter class */
public class MyClass extends MouseAdapter {
class of AWT package
...
implementing the
someObject.addMouseListener(this);
EventListerner interface
...
public void mouseClicked(MouseEvent e) {
...//Event handler implementation goes here...
…e.getX()
…e.getY())
}
}
Abteilung für Telekooperation
Mouse Listener Example
/* Example 3: using an inner class */
public class MyClass extends Applet {
...
someObject.addMouseListener(new MyAdapter());
...
realized as inner class
class MyAdapter extends MouseAdapter {
public void mouseClicked(MouseEvent e) {
...//Event handler implementation goes here...
}
}
}
Abteilung für Telekooperation
Events Handling and Threads
• Event-handling code executes in a single thread,
the event-dispatching thread.
• "Painting code" also executes in the eventdispatching thread.
• Consequently, program's GUI frozen while
executing event handler
– for long-running event handler additional thread recommended
Abteilung für Telekooperation
Further Readings
• Java.sun.com tutorial
• Important Classes and APIs:
–
–
–
–
–
EventObject
Mouse Event
Mouse Listener API
Mouse-Motion Event
Mouse-Motion Listener API