Bound Properties

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Trail: JavaBeans(TM)
Lesson: Properties

Bound Properties

Here is a list of background reading to prepare you for learning bound properties:

Chapter 7 of the JavaBeans API Specification
PropertyChangeListener interface
PropertyChangeSupport class
PropertyChangeEvent class

Sometimes when a Bean property changes, another object may want to be notified of the change, and react to the change. Whenever a bound property changes, notification of the change is sent to interested listeners.
A Bean containing a bound property must maintain a list of property change listeners, and alert those listeners when the bound property changes. The convenience class PropertyChangeSupport implements methods that add and remove PropertyChangeListener objects from a list, and fires PropertyChangeEvent objects at those listeners when the bound property changes. Your Beans can inherit from this class, or use it as an inner class.
An object that wants to listen for property changes must be able to add and remove itself from the listener list on the Bean containing the bound property, and respond to the event notification method that signals a property change. By implementing the PropertyChangeListener interface the listener can be added to the list maintained by the bound property Bean, and because it implements the PropertyChangeListener.propertyChange method, the listener can respond to property change notifications.
The PropertyChangeEvent class encapsulates property change information, and is sent from the property change event source to each object in the property change listener list via the propertyChange method.
The following sections provide the details of implementing bound properties.

Implementing Bound Property Support Within a Bean

To implement a bound property, take the following steps:
Import the java.beans package. This gives you access to the PropertyChangeSupport class.
Instantiate a PropertyChangeSupport object:
private PropertyChangeSupport changes =
            new PropertyChangeSupport(this);
This object maintains the property change listener list and fires property change events. You can also make your class a PropertyChangeSupport subclass.
Implement methods to maintain the property change listener list. Since PropertyChangeSupport implements these methods, you merely wrap calls to the property-change support object's methods:
public void addPropertyChangeListener(
                PropertyChangeListener l)
{
    changes.addPropertyChangeListener(l);
}
public void removePropertyChangeListener(
                PropertyChangeListener l)
{
    changes.removePropertyChangeListener(l);
}
Modify a property's setter method to fire a property change event when the property is changed. OurButton's setLabel method looks like this:
public void setLabel(String newLabel) {
    String oldLabel = label;
    label = newLabel;
    sizeToFit();
    changes.firePropertyChange("label", oldLabel, newLabel);
}
Note that setLabel stores the old label value, because both the old and new labels must be passed to firePropertyChange.
public void firePropertyChange(String propertyName,
                               Object oldValue, Object newValue)
The firePropertyChange method bundles its parameters into a PropertyChangeEvent object, and calls propertyChange(PropertyChangeEvent pce) on each registered listener. The old and new values are treated as Object values. If your property values are primitive types such as int, you must use the object wrapper version such as java.lang.Integer. Also, property change events are fired after the property has changed.
When the BeanBox (or Beans-aware builder tool) recognizes the design patterns for bound properties within your Bean, you will see a propertyChange interface item when you select the Edit|Events menu.
Now that you have given your Bean the ability to broadcast events when a bound property has changed, the next step is to create a listener.

Implementing Bound Property Listeners

To listen for property change events, your listener Bean must implement the PropertyChangeListener interface. This interface contains one method:
public abstract void propertyChange(PropertyChangeEvent evt)
This is the notification method that the source Bean calls on all property change listeners in its property change listener list.
So to make your class able to listen and respond to property change events, you must:
Implement the PropertyChangeListener interface.
public class MyClass
       implements java.beans.PropertyChangeListener,
		  java.io.Serializable
Implement the propertyChange method in the listener. This method needs to contain the code that handles what you need to do when the listener receives property change event. Very often, for example, this is a call to a setter method in the listener class: a property change in the source Bean propagates a change to a property in a listener Bean.
To register interest in receiving notification about a Bean property change, the listener Bean calls the listener registration method on the source Bean. For example:
button.addPropertyChangeListener(aButtonListener);
Or, you can use an adapter class to catch the property change event, and subsequently call the correct method within the listener object. Here is an example taken from comments in the beans/demo/sunw/demo/misc/ChangeReporter.java file.
OurButton button = new OurButton();
...
PropertyChangeAdapter adapter = new PropertyChangeAdapter();
...
button.addPropertyChangeListener(adapter);
...
class PropertyChangeAdapter implements PropertyChangeListener
{
    public void propertyChange(PropertyChangeEvent e)
    {
        reporter.reportChange(e);
    }
}

Bound Properties in the BeanBox

The BeanBox handles bound property events as it handles all events: by using an event hookup adapter. Event hookup adapters classes are generated by builder tools when you connect an event source Bean to an event listener Bean. These objects interpose between event sources and event listeners to provide control and filtering over event delivery. Since an event listener can register with multiple listeners that fire the same event type, event hookup adapters can be used to intercept an event from a particular event source, and forward it to the correct event listener. This saves the event listener from implementing code that would examine each event to determine if it is from the correct source. See section 6.7 of the JavaBeans API Specification for a complete discussion of event hookup adapters.
The OurButton and ChangeReporter Beans can be used to illustrate this technique. To see how this works, take the following steps:

Drop OurButton and ChangeReporter instances on the BeanBox.
Select the OurButton instance and choose the Edit|Events|propertyChange|propertyChange menu item.
Connect the rubber band line to the ChangeReporter instance. The EventTargetDialog will be displayed.
Choose reportChange from the EventTargetDialog. The event hookup adapter source will be generated and compiled
Select OurButton and change some of it's properties. You will see change reports in ChangeReporter.

Behind the scenes the BeanBox generated the event hookup adapter. This adapter implements the PropertyChangeListener interface, and also generates a propertyChange method implementation that calls the ChangeReporter.reportChange method. Here's the generated adapter source code:
// Automatically generated event hookup file.

package tmp.sunw.beanbox;
import sunw.demo.misc.ChangeReporter;
import java.beans.PropertyChangeListener;
import java.beans.PropertyChangeEvent;

public class ___Hookup_14636f1560 implements
            java.beans.PropertyChangeListener, java.io.Serializable {

    public void setTarget(sunw.demo.misc.ChangeReporter t) {
        target = t;
    }

    public void propertyChange(java.beans.PropertyChangeEvent arg0) {
        target.reportChange(arg0);
    }

    private sunw.demo.misc.ChangeReporter target;
}
The ChangeReporter Bean need not implement the PropertyChangeListener interface; instead, the BeanBox generated adapter class implements PropertyChangeListener, and the adapter's propertyChange method calls the appropriate method in the target object (ChangeReporter).
The BeanBox puts the event adapter classes in the beans/beanbox/tmp/sunw/beanbox directory. When an adapter class is generated, you can view the adapter source in that directory.

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