CityStaffordshireU 2008-09 CE00371-1

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Event

in the TwoRedRoseApplet, whenever you scroll or resize your display screen, all RedRoses have blue background
- what is going on? something is happening
- doing multiple task at the same time
- Thread : "A thread is a thread of execution in a program"
- applets and GUI event handling inherently use multiple threads
- we will adopt the first rule of using threads: "avoid them when you can"
- learn more when you study topics like concurrent programming and animation

Events occur when users interact with your program

HTML element
<INPUT TYPE="button" VALUE="press me">

Occurence of Events must be communicated among a set of processes and handled
the <INPUT> element and the JavaScript function UpdateCells()

<SCRIPT LANGUAGE="JavaScript">
<!--
function UpdateCells() {
  top.location = "http://www.cityu.edu.hk/";
}
// -->
</SCRIPT>
<INPUT TYPE="button" 
	VALUE="press me to goto City U" 
		OnClick="UpdateCells()">

CityButton Applet

//filename: CityButton.java
import java.awt.*;
import java.applet.*;
import java.awt.event.*;
import java.net.*;
public class CityButton extends Applet implements ActionListener
{	
  public Button bCityU;
  public void init( )
  {
    bCityU = new Button("CityU");
    add(bCityU);
    bCityU.addActionListener(this);
  }

  public void actionPerformed(ActionEvent e)
  {
   try {
    URL url = new URL("http://www.cityu.edu.hk");
    if (e.getActionCommand().equals("CityU"))
	getAppletContext().showDocument(url);
	 } catch(MalformedURLException mue) {}
  }
}

In JAVA, events are handled using the delegation event model e.g. A button when pressed generated an ActionEvent object. ActionListener is an interface that contains a single method actionPerformed(ActionEvent e) which is automatically invoked when a button is pressed.
Delegation-based Model
- Any component can be notified of the event. All it has to do is implement the appropriate interface and be registered as an event listener on the appropriate event source.
- Listeners "listen" for an event. When an event occurs, the listener identifies the target of the event, then delegates the processing of the event to the proper event handler. Each object in a program upon which events can be generated "register" themselves with one or more listeners.
Different types of listeners listen for different types of events
ActionListener
clicks a Button, or presses Return while typing in a text field
For example:
- Time3 Tester applet
- Rational Tester applet
WindowListener
closes a windows Frame
MouseListener
presses a mouse button while the cursor is over a component
ItemListener
selects List or Choice items

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