HOWTO: Fire 'Events' from a Java Applet on a Web Page

ID: Q178994

The information in this article applies to:

SUMMARY

In Internet Explorer 3.0x, there is no way for a Java Applet to "fire" an event that can be handled through scripting on a Web page. This article shows how to use combination of Java class methods and scripting methods to simulate events.

NOTE: In Internet Explorer 4.0, events can be fired from a Java applet and caught in the Web pages script. See the Microsoft SDK for Java 2.0x or later documentation for more information.


MORE INFORMATION

In order to simulate an event from a Java applet, you will need to create a set of methods in your Java Applet class and a polling function on the Web page. The polling function will check the applet to see if the event has occurred and act on that event if it has occurred.

For example, if you wanted to fire an event when the users clicks on the applet, you would code the class as follows: 


import java.awt.*;
import java.applet.*;
public class MyApplet extends Applet
{
   // fire is the private member that determines if the event fired
   private boolean fire = false;

   public MyApplet()
   {
      // Set the background to black so it can be seen
      setBackground(Color.black);
   }

   public boolean handleEvent(Event e)
   {
      if(e.id == e.MOUSE_DOWN)
      {
         // When the user clicks, set fire to True.
         fire = true;
      }
      return false;
   }

   // Scripting will call this method to see if the event occurred.
   public boolean clicked()
   {
      // If fire is True, then 'reset' it to False and return True
      if(fire)
      {
         fire = false;
         return true;
      }
      // Otherwise, return False
      return false;
   }
}
Then, you need to create HTML: 

<HTML>
<HEAD>
<SCRIPT LANGUAGE="VBScript">
<!--
Sub window_onLoad()
   ' Set the initial timeout for polling
   window.setTimeout "checkClicked", 1000
end sub

' This is the method that the timeout calls
Sub checkClicked()
   if document.app.clicked then
      ' The applets clicked method returned True
      msgbox "You clicked my applet!"
      ' Reset the timeout
      window.setTimeout "checkClicked", 1000
   else
      ' The applets clicked method returned False
      ' Reset the timeout
      window.setTimeout "checkClicked", 1000
   end if
end sub
-->
</SCRIPT>

<TITLE>Document Title</TITLE>
</HEAD>

<BODY>

<APPLET CODE=MyApplet.class ID=app WIDTH=320 HEIGHT=200>
</APPLET>

</BODY>
</HTML>
When the Web page is loaded, the window_onLoad function executea. It sets a "timeout" to execute the checkClicked VBScript function. The checkClicked function "polls" the Applet by calling the clicked method on the Applet. If the clicked method returns True, then the checkClicked displays a message box and resets the timeout. If the clicked method returned False, then the checkClicked function just resets the timeout.

Both VBScript and JScript can access public members and methods of the Java Applet due to a feature of the Microsoft virtual machine called AutoIDispatch. If you needed to simulate "Event Parameters," such as coordinates of the mouse click, you could add additional public methods on the Applet that returned these values.


REFERENCES

For more information about the Microsoft virtual machine's AutoIDispatch feature, please see the following article in the Microsoft Knowledge Base:

Q172202 INFO: Implementing Java Automation Objects Using AutoIDispatch
For the latest Knowledge Base articles and other support information on Visual J++ and the SDK for Java, see the following pages on the Microsoft Technical Support site:
http://support.microsoft.com/support/visualj/

http://support.microsoft.com/support/java/

Additional query words: fire events applet script 


Keywords          : kbIE301 kbJScript100 kbJScript200 kbVBScript200 kbIE302 JMisc 
Version           : WINDOWS:1.0,2.0,3.01,3.02
Platform          : WINDOWS 
Issue type        : kbhowto

Last Reviewed: July 26, 1999