Bean Customization

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Trail: JavaBeans(TM)

Lesson: Bean Customization

To prepare yourself for learning about property editors and customizers, read the following documentation:

PropertyEditor interface
PropertyEditorSupport class
PropertyEditorManager class
Customizer interface
BeanInfo interface

A Bean's appearance and behavior can be customized at design time within Beans-compliant builder tools. Typically there are two ways to customize a Bean:

By using a property editor. Each Bean property has its own property editor. A builder tool usually displays a Bean's property editors in a property sheet. A property editor is associated with, and edits a particular property type.
By using customizers. Customizers give you complete GUI control over Bean customization. Customizers are used where property editors are not practical or applicable. Unlike a property editor, which is associated with a property, a customizer is associated with a Bean.

Property Editors

A property editor is a tool for customizing a particular property type. Property editors are displayed in, or activated from property sheets. A property sheet will determine a property's type, search for a relevant property editor, and display the property's current value in a relevant way.

Property editors must implement the PropertyEditor interface. PropertyEditor provides methods that specify how a property should be displayed in a property sheet.

Here is the BeanBox's Properties sheet containing OurButton properties:

You begin the process of editing these properties by clicking on the property entry in the sheet.

The label and fontSize properties are displayed in an editable text box. Changes can be made in place.
The largeFont and debug properties are selection boxes with discrete choices.
Clicking on the foreground, background, and font entries brings up separate panels.

How each of these is displayed depends on which PropertyEditor methods you implement to return non-null (or equivalent) values.

For example, the int property editor implements the setAsText method. This indicates to the property sheet that the property can be displayed as a String, hence an editable text box will be used.

The Color and Font property editors use a separate panel, and merely use the property sheet to display the current property value. The editor is displayed by clicking on that value. To display the current property value "sample" within the property sheet you need to override isPaintable to return true, and override paintValue to paint the current property value in a rectangle in the property sheet. Here's how ColorEditor implements paintValue:

public void paintValue(java.awt.Graphics gfx, java.awt.Rectangle box) {
    Color oldColor = gfx.getColor();
    gfx.setColor(Color.black);
    gfx.drawRect(box.x, box.y, box.width-3, box.height-3);
    gfx.setColor(color);
    gfx.fillRect(box.x+1, box.y+1, box.width-4, box.height-4);
    gfx.setColor(oldColor);
}

To support the custom property editor, you need to override two more methods: Override supportsCustomEditor to return true, and override getCustomEditor to return a custom editor instance. ColorEditor.getCustomEditor returns this.

Additionally, the PropertyEditorSupport class maintains a PropertyChangeListener list, and fires property change event notifications to those listeners when a bound property is changed.

How Property Editors are Associated with Properties

Property editors are discovered and associated with a given property by

Explicit association via a BeanInfo object. The Molecule demo Bean uses this technique. Within the MoleculeBeanInfo class, the Molecule Bean's property editor is set with the following line of code:
```
pd.setPropertyEditorClass(MoleculeNameEditor.class);
```
Explicit registration via java.Beans.PropertyEditorManager.registerEditor. This method takes a pair of arguments: The class type, and the editor to be associated with that type.
Name search. If a class has no explicitly associated property editor, then the PropertyEditorManager searchs for that class's property editor by:
- Appending "Editor" to the fully qualified class name. For example, for the java.beans.ComplexNumber class, the property editor manager would search for the java.beans.ComplexNumberEditor class.
- Appending "Editor" to the class name and searching a class search path. The default class path for the BeanBox is sun.beans.editors.

The BDK Property Editors

The BDK provides property editors for the primitive data types like int, boolean, and float, and Color and Font class types. The source code for these property editors is in beans/apis/sun/beans/editors. These sources make a good starting point for writing your own property editors. Some things to note about the BDK property editors:

All the "number" properties are represented as String objects. The IntEditor overrides PropertyEditorSupport.setAsText.
The boolean property editor is a menu of discrete choices that overrides the PropertyEditorSupport.getTags method to return a String[] containing "True" and "False":
```
public String[] getTags() {
    String result[] = { "True", "False" };
    return result;
}
```
The Color and Font property editors implement custom property editors. Because these objects require a more sophisticated interface to be easily edited a separate component pops up to do the property editing. Overriding supportsCustomEditor to return true signals the property sheet that this property's editor is a custom component. The isPaintable and paintValue methods are also overridden to provide color and font painting in the editors property sheet sample areas.

Note that if no property editor is found for a property, the BeanBox will not display that property in the Properties sheet.

Customizers

When you use a Bean Customizer, you get complete control over how to configure or edit a Bean. A Customizer is like an application that specifically targets a Bean's customization. Sometimes properties are insufficient for representing a Bean's configurable attributes. Customizers are used where sophisticated instructions would be needed to change a Bean, and where property editors are too primitive to achieve Bean customization.

All customizers must:

Extend java.awt.Component or one of its subclasses.
Implement the java.beans.Customizer interface This means implementing methods to register PropertyChangeListener objects, and firing property change events at those listeners when a change to the target Bean has occurred.
Implement a default constructor.
Associate the customizer with its target class via BeanInfo.getBeanDescriptor.

If a Bean that has an associated Customizer is dropped into the BeanBox, you will notice a "Customize..." item on the Edit menu.

BDK Customizers

The OurButtonCustomizer serves as an example that demonstrates the mechanics of building a customizer. OurButtonCustomizer:

Extends java.awt.Panel (a Component subclass).
Implements the Customizer interface, and uses a PropertyChangeSupport object to manage PropertyChangeListener registration and notification. See the bound property section for a PropertyChangeSupport description.

Implements a default constructor:

public OurButtonCustomizer() {
    setLayout(null);
}

Is associated with its target class, ExplicitButton, by the following ExplicitButtonBeanInfo code:

public BeanDescriptor getBeanDescriptor() {
    return new BeanDescriptor(beanClass, customizerClass);
}
...
private final static Class customizerClass =
                            OurButtonCustomizer.class;

The BridgeTester and JDBC Select demo Beans also have customizers.

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