15.6 Defining a Simple Property Editor

A bean can also provide auxiliary PropertyEditor classes for use by a beanbox tool. PropertyEditor is a flexible interface that allows a bean to tell a beanbox how to display and edit the values of certain types of properties.

A beanbox tool always provides simple property editors for common property types, such as strings, numbers, fonts, and colors. If your bean has a property of a nonstandard type, however, you should register a property editor for that type. The easiest way to "register" a property editor is through a simple naming convention. If your type is defined by the class X, the editor for it should be defined in the class X Editor. Alternatively, you can register a property editor by calling the PropertyEditorManager.registerEditor( ) method, probably from the constructor of your BeanInfo class. If you call this method from the bean itself, the bean then depends on the property editor class, so the editor has to be bundled with the bean in applications, which is not desirable. Another way to register a property editor is by using a PropertyDescriptor object in a BeanInfo class to specify the PropertyEditor for a specific property. The YesNoPanelBeanInfo class does this for the messageText property, for example.

The PropertyEditor interface can seem confusing at first. Its methods allow you to define three techniques for displaying the value of a property and two techniques for allowing the user to edit the value of a property. The value of a property can be displayed:

As a string

If you define the getAsText( ) method, a beanbox can convert a property to a string and display that string to the user.

As an enumerated value

If a property can take only on values from a fixed set of values, you can define the getTags( ) method to allow a beanbox to display a drop-down menu of allowed values for the property.

In a graphical display

If you define paintValue( ), a beanbox can ask the property editor to display the value using some natural graphical format, such as a color swatch for colors. You also need to define isPaintable( ) to specify that a graphical format is supported.

The two editing techniques are:

String editing

If you define the setAsText( ) method, a beanbox knows it can simply have the user type a value into a text field and pass that value to setAsText( ). If your property editor defines getTags( ), it should also define setAsText( ), so that a beanbox can set the property value using the individual tag values.

Custom editing

If your property editor defines getCustomEditor( ), a beanbox can call it to obtain some kind of GUI component that can be displayed in a dialog box and serve as a custom editor for the property. You also need to define supportsCustomEditor( ) to specify that custom editing is supported.

The setValue( ) method of a PropertyEditor is called to specify the current value of the property. It is this value that should be converted to a string or graphical representation by getAsText( ) or paintValue( ).

A property editor must maintain a list of event listeners that are interested in changes to the value of the property. The addPropertyChangeListener( ) and removePropertyChangeListener( ) methods are standard event-listener registration and removal methods. When a property editor changes the value of a property, either through setAsText( ) or through a custom editor, it must send a PropertyChangeEvent to all registered listeners.

PropertyEditor defines the getJavaInitializationString( ) for use by beanbox tools that generate Java code. This method should return a fragment of Java code that can initialize a variable to the current property value.

Finally, a class that implements the PropertyEditor interface must have a no-argument constructor, so it can be dynamically loaded and instantiated by a beanbox.

Most property editors can be much simpler than this detailed description suggests. In many cases, you can subclass PropertyEditorSupport instead of implementing the PropertyEditor interface directly. This useful class provides no-op implementations of most PropertyEditor methods. It also implements the methods for adding and removing event listeners.

A property that has an enumerated value requires a simple property editor. The alignment property of the YesNoPanel bean is an example of this common type of property. The property has only the three legal values defined by the Alignment class. The AlignmentEditor class shown in Example 15-7 is a property editor that tells a beanbox how to display and edit the value of this property. Because AlignmentEditor follows a JavaBeans naming convention, a beanbox automatically uses it for any property of type Alignment.

Example 15-7. AlignmentEditor.java

package je3.beans;
import java.beans.*;
import java.awt.*;

/**
 * This PropertyEditor defines the enumerated values of the alignment property
 * so that a bean box or IDE can present those values to the user for selection
 **/
public class AlignmentEditor extends PropertyEditorSupport {
    /** Return the list of value names for the enumerated type. */
    public String[  ] getTags( ) {
        return new String[  ] { "left", "center", "right" };
    }
    
    /** Convert each of those value names into the actual value. */
    public void setAsText(String s) {
        if (s.equals("left")) setValue(Alignment.LEFT);
        else if (s.equals("center")) setValue(Alignment.CENTER);
        else if (s.equals("right")) setValue(Alignment.RIGHT);
        else throw new IllegalArgumentException(s);
    }
    
    /** This is an important method for code generation. */
    public String getJavaInitializationString( ) {
        Object o = getValue( );
        if (o == Alignment.LEFT)
            return "je3.beans.Alignment.LEFT";
        if (o == Alignment.CENTER)
            return "je3.beans.Alignment.CENTER";
        if (o == Alignment.RIGHT)
            return "je3.beans.Alignment.RIGHT";
        return null;
    }
}