Now you'll be adding support for bound properties to NervousText04.
An object that supports bound properties can notify other objects when
certain property values change. Each time a bound property changes,
it notifies registered listener objects by calling specified property
change handler methods defined for the listener. These methods are
called with the new and old values of the property as well as name of
the property which has changed.
Step 1. Add import Statement to Support Property Change Events
To enable a bean to generate or receive property change events,
you must add the following import statement to the top of the file.
import java.beans.*;
The package and import statements at the top of the file should
be as follows:
package sun.beanbox.beans;
import java.awt.event.*;
import java.awt.Graphics;
import java.awt.Font;
import java.awt.*;
import java.io.ObjectOutputStream;
import java.io.ObjectInputStream;
import java.io.IOException;
import java.beans.*;
Step 2. Add Code to Track and Notify Interested Property Change Listeners
Add the following code to maintain a list of listeners potentially
wanting to be notified of changes for the text property.
public void addPropertyChangeListener(
PropertyChangeListener l) {
support.addPropertyChangeListener(l);
}
public void removePropertyChangeListener(
PropertyChangeListener l) {
support.removePropertyChangeListener(l);
}
private PropertyChangeSupport support =
new PropertyChangeSupport(this);
The last statement in this listing declares an instance variable,
support
, for NervousText04 objects. This variable
holds a collection of listener objects that will be notified about
property changes to the text property. The two supporting methods,
addPropertyChangeListener
and
removePropertyChangeListener
provide a public interface
allowing interested listeners to register themselves with
NervousText04 objects.
Step 3. Add Code to Fire text
Property Change Events
Now modify the text property setter method to fire a property
change event whenever it is called. To do this, you need to save
copies of both the old and new values of the property being changed.
To save the old property, Add the following line to the end of the
setText
method in NervousText04
.
String oldstring = s;
Recall that s is the value of the text property accessed by the
getText
and setText
getter and setter
methods for the text property.
The instance variable s
is declared at the top of the class
public class NervousText04 extends Panel
implements Runnable, MouseListener {
...
String s = null;
...
Now that you have both old a new values saved in local variables Add
the following line to the end of the setText
method as
well.
support.firePropertyChange(
"text", oldstring, newstring);
The full method now looks like this:
public void setText(String newstring){
String oldstring = s;
s=new String(newstring);
separated= new char[s.length()];
s.getChars(0,s.length(),separated,0);
support.firePropertyChange(
"text", oldstring, newstring);
}
Step 4. Define Event Handler Methods for Property Change Events
Now define a handler method that can potentially be called by a
listener object to report that a text property change has occurred.
public void reportChange(PropertyChangeEvent evt) {
System.err.println(
"ENTER---> NervousText04 reportChange");
String oldValue = evt.getPropertyName() +
" := " + evt.getOldValue();
String newValue = evt.getPropertyName() +
" := " + evt.getNewValue();
System.err.println("New value: " + oldValue);
System.err.println("Old value: " + newValue);
System.err.println(
"EXIT----> NervousText04 reportChange");
}
Add the above code for reportChange
at the end of the
class definition for NervousText04 immediately after the declaration
for the support variable you added earlier:
private PropertyChangeSupport support =
new PropertyChangeSupport(this);
Now NervousText04 is both a source for property change events and
a handler for property change events. We haven't made it a listener.
That's because the BeanBox will automatically generate a listener
class as an adapter. An adapter is created when you connect any
object that generates a property change to a NervousText04 object as a
target when building a BeanBox application.
The reportChange
method does not actually cause a
change to take place in the target object. To do that, define an
additional method called makeChange
as follows:
public void makeChange(PropertyChangeEvent evt) {
System.err.println(
"ENTER---> NervousText04 makeChange");
String oldValue = (String)evt.getOldValue();
// (String) cast required since
String newValue = (String)evt.getNewValue();
// evt.getOldValue() returns an Object
setText("*" + newValue + "*");
System.err.println("New value: " + oldValue);
System.err.println("Old value: " + newValue);
System.err.println(
"EXIT----> NervousText04 makeChange");
}
Add the method definition for makeChange
immediately after the code for reportChange
. The
actual change in the target NervousText object occurs when the setter
method, setText
is called in the makeChange
event handler method..
Step 4. Build the JAR and Install in BeanBox
Compile NervousText04.java:
javac -d . NervousText04.java
Create the mainfest:
echo "Manifest-Version: 1.0" > manifest.tmp
echo "" >>
manifest.tmp
echo "Name: sun/beanbox/beans/NervousText04.class" >>
manifest.tmp
echo "Java-Bean: True" >>
manifest.tmp
The resulting temporary manifest file should look
like this:
Manifest-Version: 1.0
Name: sun/beanbox/beans/NervousText04.class
Java-Bean: True
Create the JAR file:
jar cfm NervousText04.jar
manifest.tmp sun/beanbox/beans/NervousText04.class
Remove the temporary manifest and install the JAR in the
BeanBox jars directory (substituting your BDK installation
directory for BDK_HOME:
rm manifest.tmp
cp -p NervousText04.jar BDK_HOME/beans/jars
Step 5. Test NervousText04 in BeanBox
NO: Use the exiting app, and modify it.
Start up the BeanBox. Add one OurButton, and two NervousText04 beans. You're
going to bind changes in the first NervousText04 bean to the second. The changes
to the first NervousText04 will be effected by pressing the OurButton. Change
the label for OurButton to "change direction." As before, you're going to
make an action event in OurButton cause the text to be reversed in the
first NervousText04 bean.
------------------------------------------------
diffs
/matisse/usr/home/sparky_gvoss/work97/06/test/
beans-example-code/NervousText/1.1/dodiffs.out
/matisse/usr/home/sparky_gvoss/work97/06/test/
beans-example-code/NervousText/1.1/dodiffs.out
-------------------------------------------------
Program Source Code
The makefile
for this lesson automates source code
compilation, JAR file construction, and copying of JAR files to the
appropriate BeanBox directory. You'll have to edit several of the
variables in the makefile to indicate the location of your JDK 1.1
and BDK installation directories.
You may want to look at the final source file for NervousText Bean, Version 04 to
verify the changes you have made to the original NervousText.java
file.