: Class AWTEvent

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ibm.techexplorer.awt
Class AWTEvent

java.lang.Object
  |
  +--ibm.techexplorer.util.EventObject
        |
        +--ibm.techexplorer.awt.AWTEvent

Direct Known Subclasses:: FocusEvent, InputEvent

public class AWTEvent
extends EventObject

Description

Java applets and scripts can interact with techexplorer through events. Events are occurrences such as a user clicking the mouse or pressing a key. All techexplorer AWT events are rooted at the abstract base class ibm.techexplorer.awt.AWTEvent

The techexplorer AWT event hierarchy provides the organization for several types of events that techexplorer instances can trigger. techexplorer events follow the Java 1.1 event model used by the Java AWT and Java Beans. Although the techexplorer event model is similar to that of Java 1.1, it can still be used in conjunction with applets the make use of Java 1.0 events. In fact, for purposes of Java 1.0 compatibility, techexplorer events do not subclass from Java 1.1 event classes. As a consequence, the techexplorer Java library provides all of the required classes for application development. Note that although techexplorer AWT events are based on the Java 1.1 AWT event model, several methods and attributes found in the Java 1.1 AWT classes are not used. In this sense, the techexplorer AWT event model can be viewed as a subset of the Java 1.1 AWT model.

Subclasses of the AWTEvent class provide event specific information. For example the ibm.techexplorer.awt.event.InputEvent AWTEvent subclass adds information such as pressed modifier keys. The ibm.techexplorer.awt.event.MouseEvent and ibm.techexplorer.awt.event.KeyEvent subclasses of InputEvent contain information about the mouse and keyboard respectively.

Events are generated by event sources. In our context, techexplorer is an event source that can fire one or more different types of events. For each type of event that techexplorer fires, techexplorer maintains a list of event listeners. One or more listeners can register to be notified about techexplorer events. An event listener registers with techexplorer by using the registration methods provided in ibm.techexplorer.techexplorer

This notification model is similar to the Observer design pattern [Gamma E.,Helm R.,Johnson R.,Vlissides J. Design Patterns, Elements of Reusable Object-Oriented Software, Addison-Wesley Reading Massachusetts, 1994]. This pattern defines a one to many dependancy between objects so that when one object changes state, all of its dependents are notified and updated automatically.

Event handlers can be instances of any class that implement an event listener interface. Each interface specifies the methods that a listener of a particular event type must implement. For example, the ibm.techexplorer.awt.event.FocusListener contains two methods, namely focusGained and focusLost. When a techexplorer document receives the focus, the focusGained method will be called for each registered listener. Consequently, when a techexplorer document looses focus, the focusLost method will be invoked. All methods defined in a listener interface must be implemented, even if a particular Java applet only requires one method in the interface. For those events not required by a Java applet simply provide empty bodies for the reaming listener methods. To alleviate the resulting collection of empty method bodies, the techexplorer AWT provides adapter classes for each listener interface. The techexplorer adaptor classes provides empty implementations of all of the corresponding interface methods. To use an adapter, subclass the appropriate adaptor class and provide methods for the events your applet requires.

Current Restriction: Note that although the techexplorer AWT event model allows a Java applet to handle events, the generation of techexplorer events from Java is not permitted in this version.

Java applets can enable or disable the flow of events to a particular techexplorer instance. techexplorer maintains an event mask for keeping track of the enabled events. Java applets can use the ibm.techexplorer.techexplorer enableEvents() and disableEvents() methods to allow and disallow event flow to techexplorer respectively. Note that this only effects the event flow to a particular techexplorer instance and not the event flow to techexplorer listeners. This is often used by Java applets to disable default and user defined context menus, hypermedia links, and text selection to name a few. Most events are delivered first to the techexplorer instance and then to any registered listeners. The only exception is the ibm.techexplorer.plugin.InstanceEvent. Since this event notifies a registered listener of techexplorer life cycle changes, techexplorer responds to the event after the applet has been notified.

Example

Every Java applet that makes use of techexplorer events contain four similar pieces of code. For example, let's look at four code fragments from the TEMessageListener applet:

Statements for importing the appropriate techexplorer classes.

  import ibm.techexplorer.techexplorer;
  import ibm.techexplorer.awt.AWTEvent;
  import ibm.techexplorer.awt.event.MouseEvent;
  import ibm.techexplorer.awt.event.MouseListener;

Code declaring that the class implements a listener interface.

  public class TEMessageListener extends Applet implements MouseListener {

Code that registers an instance of the event handling class with techexplorer
```
  Plugin.addMouseListener( (MouseListener)this );
 
```

The implementation of the methods in the listener interface.

  public void mouseClicked( MouseEvent e ) { }
  public void mouseEntered( MouseEvent e ) { }
  public void mouseExited ( MouseEvent e ) { }
  public void mousePressed( MouseEvent e ) { }
  public void mouseRelease( MouseEvent e ) { }

Press here to try out the TEMessageListener applet. Click here for the whole program. Note that since some browsers do not support Java 1.1, the TEMessageListener applet is Java 1.0 compliant.

See Also:: FocusEvent, InputEvent, KeyEvent, MouseEvent

Field Summary
`static int`	`FOCUS_EVENT_MASK` The event mask for selecting focus events.
`static int`	`KEY_EVENT_MASK` The event mask for selecting key events.
`static int`	`MOUSE_EVENT_MASK` The event mask for selecting mouse events.
`static int`	`MOUSE_MOTION_EVENT_MASK` The event mask for selecting mouse motion events.

Fields inherited from class ibm.techexplorer.util.EventObject

documentName,  
id,  
listenerClass,  
objectName,  
source

Constructor Summary
`AWTEvent()`

Methods inherited from class ibm.techexplorer.util.EventObject

getDocumentName, 
getID, 
getListenerClass, 
getObjectName, 
getSource

Methods inherited from class java.lang.Object

clone, 
equals, 
finalize, 
getClass, 
hashCode, 
notify, 
notifyAll, 
toString, 
wait, 
wait, 
wait

Field Detail

FOCUS_EVENT_MASK

public static final int FOCUS_EVENT_MASK

The event mask for selecting focus events.

KEY_EVENT_MASK

public static final int KEY_EVENT_MASK

The event mask for selecting key events.

MOUSE_EVENT_MASK

public static final int MOUSE_EVENT_MASK

The event mask for selecting mouse events.

MOUSE_MOTION_EVENT_MASK

public static final int MOUSE_MOTION_EVENT_MASK

The event mask for selecting mouse motion events.

Constructor Detail

AWTEvent

public AWTEvent()