CSS API to support custom UI Controls

Introduction

CSS is an integral part of JavaFX but heretofore has been implemented exclusively in private API within, primarily, the com.sun.javafx.css package. Adding a styleable property or pseudo-class state in JavaFX is fairly straight-forward and exposing certain elements of the internal API will facilitate the development of custom UI controls in the open source environment. This document outlines this public API.

Use Case

Say a contributor wished to develop a Watermark control which could overlay a region with copyright information. The Watermark API has a DoubleProperty that controls then angle at which the copyright text is displayed. The contributor wishes to make this copyrightAngle styleable through CSS. The contributor also wants to be able to modify some visual aspect of the Watermark once the copyright has been viewed; perhaps making the Watermark more transparent. The contributor wants to use a 'viewed' pseudo-class state to achieve this.

Making copyrightAngle styleable

First, the copyrightAngle is made styleable by having it extend one of the StyleableProperty classes; in this case, StyleableDoubleProperty. The code shown here follows the typical pattern. Notice that the only difference between a DoubleProperty and the StyleableDoubleProperty is the addition of the getCssMetaData method which links the copyrightAngle property to its corresponding CssMetaData instance.

private DoubleProperty copyrightAngle = new StyleableDoubleProperty(45d) {

                 /** Link this property with its CssMetaData */
                @Override
                public CssMetaData getCssMetaData() {
                    return COPYRIGHT_ANGLE;
                }

                @Override
                public Object getBean() {
                    return Node.this;
                }

                @Override
                public String getName() {
                    return "copyrightAngle";
                }
            };

    public final void setCopyrightAngle(double value) {
        copyrightAngle.set(value);
    }
    public final double getCopyrightAngle() {
        return copyrightAngle.get();
    }

    public final DoubleProperty copyrightAngleProperty() {
        return copyrightAngle;
    }

The CssMetaData instance provides information about the CSS style and some methods that allow CSS to set the property's value. The convention is to instantiate the CssMetaData as a singleton (note that this example code is not thread safe, but is sufficient). Here, the CSS property name is "-my-copyright-angle" and the initial value of 45 degrees with the default value of the copyrightAngle property.

private static final CssMetaData<Watermark,Number> COPYRIGHT_ANGLE = new CssMetaData("-my-copyright-angle", 45d) {

        public abstract boolean isSettable(Watermark node) {
            return copyrightAngle == null || copyrightAngle.isBound() == false;
        }

        public abstract WritableValue<Number> getWritableValue(Watermark node) {
            return copyrightAngleProperty();
        }
    };

In order for the CSS engine to know about the styleable properties of Watermark, the methods getClassCssMetaData() and getCssMetaData() need to be implemented. The first is a static method that returns the CssMetaData of the Watermark class and of its super class; in Watermark's case, the super class is Control. The second method returns the same value but is implemented as an instance method so that it is not necessary to use reflection to call getClassCssMetaData(). These methods are called quite frequently; for efficiency, the List<CssMetaData> is only created once. The following code shows a typical implementation.

private static final List<CssMetaData> CSS_META_DATA;
    static {
             final List<CssMetaData> metaData = new ArrayList<CssMetaData>(Control.getClassCssMetaData());
             Collections.addAll(metaData,
                 COPYRIGHT_ANGLE
             );
             CSS_META_DATA = Collections.unmodifiableList(metaData);

    }

    public static List<CssMetaData> getClassCssMetaData() {
        return CSS_META_DATA;
    }

    @Override public List<CssMetaData> getCssMetaData() {
        return getClassCssMetaData();
     }

At this point, the copyrightAngle can be styled through CSS. For example, to display the copyright from lower left to upper right:

.watermark { -my-copyright-angle: -45; }

Making 'viewed' a pseudo-class

The 'viewed' pseudo-class will be implemented as a BooleanProperty. Although BooleanProperty is most commonly used for pseudo-class state, any property type can be a pseudo-class, even a Styleable*Property. When the 'viewed' property changes value, the code needs to notify CSS that the state has changed. The place to do this is in the invalidated() method of the property which calls the pseudoClassStateChanged method, passing it then PseudoClass.State of the pseudo-class that changed state. Again, this code follows the typical pattern of implementing a property. Note that a "simple" property could be used here, but the invalidated method would still need to be overridden. Since the anonymous class will be created anyway, the few bytes needed for the bean and name will be saved by using BooleanPropertyBase instead of SimpleBooleanProperty.

public final void setViewed(boolean value) {
        viewed.set(value);
    }

    public final boolean isViewed() {
        return viewed.get();
    }

    public final BooleanProperty viewedProperty() {
        return viewed;
    }

    private static final PseudoClass.State VIEWED_PSEUDO_CLASS = PseudoClass.getState("viewed");

    private BooleanProperty viewed = new BooleanPropertyBase(false) {

                @Override
                protected void invalidated() {
                    pseudoClassStateChanged(VIEWED_PSEUDO_CLASS);
                }

                @Override
                public Object getBean() {
                    return Node.this;
                }

                @Override
                public String getName() {
                    return "viewed";
                }
            };

CSS calls getPseudoClassStates() which returns the pseudo-class state of the node. The node's pseudo-class state should include the state from the node class and all its super classes. Therefore, the first thing getPseudoClassStates() should do is call super.getPseudoClassStates().

public PseudoClass.States getPseudoClassStates() {
        PseudoClassStates pseudoClassStates = super.getPseudoClassStates();
        if (isViewed()) pseudoClassStates = pseudoClassStates.addState(VIEWED_PSEUDO_CLASS);
        return pseudoClassStates;
    }

With this framework in place, the following style can be used to make the watermark more transparent if it has been viewed.

.watermark:viewed { -fx-opacity: 30%; }

Design Goals

The primary goal is to create an API that allows CSS styling to be applied to a JavaFX property and to support the use of pseudo-class state in the open-software environment.
The API should support the following javafx.beans.property types: BooleanProperty, FloatProperty, DoubleProperty, IntegerProperty, LongProperty, and StringProperty. Support for ObjectProperty is desired but would, possibly, require additional hooks into the parser or public API to convert a parsed value to the parameterized type which may not be feasible at this time. However, the ability to style an ObjectProperty with a parameterized type such as Insets or Paint is essential. Likewise, ObjectProperty<MyEnumType> presents challenges, particularly in parsing and converting, that make support for this type of property impracticable at this time.
A secondary goal is to make the API such that an IDEs whould be able to automatically generate much of the necessary code. It is not a goal, however, to create the IDE boilerplate.
While it is possible to have classes other than instances of Node be styleable through CSS, this support requires an additional interface which exists in private implementation. Including this interface in the public API is not a goal.
The current implementation of CSS in JavaFX is only a small portion of the W3C standards and is only partially compliant with those standards. It is beyond the scope of this API implementation to rectify differences between the W3C standards and the current JavaFX implementation.

Architecture

There are two main pieces to the styleable property architecture. First is a CssMetaData whose value can be represented syntactically in a .css file. A CssMetaData encapsulates the CSS property name, the type into which the string value is converted, and the default value of the property. Second is the JavaFX property to which the parsed CssMetaData value applies. Any JavaFX property that supports this styling is a StyleableProperty. A StyleableProperty also incorporates additional logic to ensure that values set by the user through calls to set methods are not overridden by styles in a user agent stylesheet.

There is a one-to-one correspondence between a CssMetaData and a StyleableProperty. A CssMetaData is scoped to a class whereas a StyleableProperty is an attribute of a class instance. Typically, a node will assume the CssMetaData of its ancestors. During CSS processing, the CSS engine iterates over a List<CssMetaData> (particular to the node) and looks up the parsed value of each CssMetaData in turn. If the CssMetaData has a parsed value, the parsed value is converted to the type of the StyleableProperty and the StyleableProperty is set.

Making a property styleable, then, consist of:

defining the javafx.beans.property as a StyleableProperty
creating a corresponding CssMetaData
ensuring the CssMetaData is returned in the List<CssMetaData>

Pseudo-class state support consists of notifying CSS of a pseudo-class state change via the invalidated() method of a property, and implementation of a method that returns the pseudo-class state of the node.

API

StyleableProperty

StyleableProperty is an interface that is implemented by various classes that extend from javafx.beans.property properties; for example,

public class StyleableBooleanProperty extends BooleanPropertyBase implements StyleableProperty<Boolean> { ... }

.

StyleableProperty.java

interface StyleableProperty<T> {

    /**
     * This method is called from CSS code to set the value of the property.;
     */
    void applyStyle(Origin origin, T value);

    /**
     * Tells the origin of the value of the property. This is needed to
     * determine whether or not CSS can override the value.
     */
    Origin getOrigin();

    /**
     * Reflect back the CssMetaData that corresponds to this;
     * <code>javafx.beans.property.StyleableProperty</code>
     */
    CssMetaData getCssMetaData();

}

The getCssMetaData method is useful for getting from a StyleableProperty to the corresponding CssMetaData, which is useful for tooling and unit testing. For example, one can get the CssMetaData of the fillProperty of a Rectangle:

Rectangle rect = new Rectangle(50,50);
    CssMetaData fillCssMetaData = ((StyleableProperty)rect.fillProperty()).getCssMetaData();
    System.out.println("Use " + fillCssMetaData.getProperty() + " to style Rectangle fill");

There is an implementing classes for each of the javafx.beans.property.*PropertyBase types and for each of the javafx.beans.property.Simple*Property types. The developer should choose the simple variety unless there is some method that needs to be overridden (but even then, it is a matter of choice).

public class StyleableBooleanProperty extends BooleanPropertyBase implements StyleableProperty<Boolean>
public class StyleableFloatProperty extends FloatPropertyBase implements StyleableProperty<Float>
public class StyleableDoubleProperty extends DoublePropertyBase implements StyleableProperty<Double>
public class StyleableIntegerProperty extends IntegerPropertyBase implements StyleableProperty<Integer>
public class StyleableLongProperty extends LongPropertyBase implements StyleableProperty<Long>
public class StyleableStringProperty extends StringPropertyBase implements StyleableProperty<String>
public class StyleableObjectProperty<T> extends ObjectPropertyBase<T> implements StyleableProperty<T>

Each of the classes have a constructor taking a CssMetaData arg and another taking a CssMetaData arg and an initial value. For example, StyleableBooleanProperty has the following constructors:

/**
  * The constructor of the {@code StyleableBooleanProperty}.
  */
  public StyleableBooleanProperty(CssMetaData CssMetaData) {
    super();
    this.CssMetaData = CssMetaData;
  }

  /**
  * The constructor of the {@code StyleableBooleanProperty}.
  *
  * @param CssMetaData
  *      the {@code CssMetaData} that corresponds to this {@code StyleableProperty}
  * @param initialValue
  *      the initial value of the wrapped {@code Object}
  */
  public StyleableBooleanProperty(CssMetaData CssMetaData, boolean initialValue) {
    super(initialValue);
    this.CssMetaData = CssMetaData;
  }

public class SimpleStyleableBooleanProperty extends SimpleBooleanProperty implements StyleableProperty<Boolean>
public class SimpleStyleableFloatProperty extends SimpleFloatProperty implements StyleableProperty<Float>
public class SimpleStyleableDoubleProperty extends SimpleDoubleProperty implements StyleableProperty<Double>
public class SimpleStyleableIntegerProperty extends SimpleIntegerProperty implements StyleableProperty<Integer>
public class SimpleStyleableLongProperty extends SimpleLongProperty implements StyleableProperty<Long>
public class SimpleStyleableStringProperty extends SimpleStringProperty implements StyleableProperty<String>
public class SimpleStyleableObjectProperty<T> extends SimpleObjectProperty<T> implements StyleableProperty<T>

Each of these simple classes have a constructor taking an Object which is the property bean and a String which is the property name in combination with a CssMetaData arg and another taking a CssMetaData arg and an initial value. For example, SimpleStyleableBooleanProperty has the following constructors:

/**
  * The constructor of the {@code SimpleStyleableBooleanProperty}.
  */
  public SimpleStyleableBooleanProperty(Object bean, String name, CssMetaData CssMetaData) {
    super(bean, name);
    this.CssMetaData = CssMetaData;
  }

  /**
  * The constructor of the {@code SimpleStyleableBooleanProperty}.
  *
  * @param CssMetaData
  *      the {@code CssMetaData} that corresponds to this {@code StyleableProperty}
  * @param initialValue
  *      the initial value of the wrapped {@code Object}
  */
  public SimpleStyleableBooleanProperty(Object bean, String name, CssMetaData CssMetaData, boolean initialValue) {
    super(bean, name, initialValue);
    this.CssMetaData = CssMetaData;
  }

/**
  * The constructor of the {@code SimpleStyleableBooleanProperty}.
  */
  public SimpleStyleableBooleanProperty(CssMetaData CssMetaData) {
    super();
    this.CssMetaData = CssMetaData;
  }

  /**
  * The constructor of the {@code SimpleStyleableBooleanProperty}.
  *
  * @param CssMetaData
  *      the {@code CssMetaData} that corresponds to this {@code StyleableProperty}
  * @param initialValue
  *      the initial value of the wrapped {@code Object}
  */
  public SimpleStyleableBooleanProperty(CssMetaData CssMetaData, boolean initialValue) {
    super(initialValue);
    this.CssMetaData = CssMetaData;
  }

Issues:
In the future, StyleableProperty may need to incorporate support for attribute selectors and animations.

CssMetaData

CssMetaData encapsulates the data needed to lookup a value and apply that value to a StyleableProperty. This includes the CSS property name, the default property value, and a link back to the corresponding StyleableProperty. The class is abstract and it is necessary to implement two methods which are invoked from the CSS engine:

/**
  * Check to see if the corresponding property on the given node is
  * settable. This method is called before any styles are looked up for the
  * given property. It is abstract so that the code can check if the property
  * is settable without expanding the property. Generally, the property is
  * settable if it is not null or is not bound.
  *
  * @param node The node on which the property value is being set
  * @return true if the property can be set.
  */
  public abstract boolean isSettable(N node);
  /**
  * Return the corresponding <code>javafx.beans.value.WriteableValue</code> for
  * the given Node. Note that calling this method will cause the property
  * to be expanded.
  * @param node
  * @return
  */
  public abstract WritableValue<V> getWritableValue(N node);

The StyleableProperty is required to be a WritableValue. WritableValue is the interface which has the setValue method. If it isn't possible to setValue on the property, then it cannot be styled from CSS. The implementation is fairly consistent throughout the existing code, typically:

public abstract boolean isSettable(Watermark node) {
    return corner == null || corner.isBound() == false;
  }
  public abstract WritableValue<String> getWritableValue(Watermark node) {
    return cornerProperty();
  }

Note that isSettable should be implemented in a way that does not cause the expansion of the property. The isSettable check is performed before the CSS value is looked up. If the property is not settable, then no further CSS processing is done for that property (on any given pulse). The getWritableValue method is invoked only if there is a CSS value to apply. Thus, if the property is not settable or there is no CSS value, the property is not expanded.

The CSS engine does not call setValue directly on the WritableValue. Rather, the code calls a set method on the CssMetaData which, in turn, calls applyStyle on the StyleableProperty. This level of indirection allows a CssMetaData to intercept the value before the calculated style value is applied (in other words, before setValue is called on the corresponding property). This is used primarily for Number based properties where the parameterized type is Number but the actual type might be Integer and so the value's intValue() method needs to be invoked.

The parameterization of CssMetaData is that it requires a Node since Node is the visible element of the scene graph. The V parameter is the type of the property's value, for example Boolean.

public abstract class CssMetaData<N extends Node, V> {

  /**
  * Check to see if the corresponding property on the given node is
  * settable. This method is called before any styles are looked up for the
  * given property. It is abstract so that the code can check if the property
  * is settable without expanding the property. Generally, the property is
  * settable if it is not null or is not bound.
  *
  * @param node The node on which the property value is being set
  * @return true if the property can be set.
  */
  public abstract boolean isSettable(N node);

  /**
  * Return the corresponding <code>javafx.beans.value.WriteableValue</code> for
  * the given Node. Note that calling this method will cause the property
  * to be expanded.
  * @param node
  * @return
  */
  public abstract WritableValue<V> getWritableValue(N node);

  /**
  * Set the value of the corresponding property on the given Node.
  * @param node The node on which the property value is being set
  * @param value The value to which the property is set
  */
  public void set(N node, V value, Origin origin) {
    // details omitted, but this method ends up calling applyStyle in the StyleableProperty interface.
  }

  private final String property;
  /**
  * @return the CSS property name
  */
  public final String getProperty() {
    return property;
  }

  private final V initialValue;
  /**
  * The initial value of a CssMetaData corresponds to the default
  * value of the WritableValue in code.
  * For example, the default value of Shape.fill is Color.BLACK and the
  * initialValue of Shape.CssMetaData.FILL is also Color.BLACK.
  * <p>
  * There may be exceptions to this, however. The initialValue may depend
  * on the state of the Node. A ScrollBar has a default orientation of
  * horizontal. If the ScrollBar is vertical, however, this method should
  * return Orientation.VERTICAL. Otherwise, a vertical ScrollBar would be
  * incorrectly set to a horizontal ScrollBar when the initial value is
  * applied.
  * @return The initial value of the property, possibly null
  */
  public V getInitialValue(N node) {
    return initialValue;
  }

  private final List<CssMetaData> subProperties;
  /**
  * The sub-properties refers to the constituent properties of this property,
  * if any. For example, "-fx-font-weight" is sub-property of "-fx-font".
  */
  public final List<CssMetaData> getSubProperties() {
    return subProperties;
  }

  private final boolean inherits;
  /**
  * If true, the value of this property is the same as
  * the parent's computed value of this property.
  * @default false
  * @see <a href="http://www.w3.org/TR/css3-cascade/#inheritance">CSS Inheritance</a>
  */
  public final boolean isInherits() {
    return inherits;
  }

  /**
  * Construct a CssMetaData with the given parameters and no sub-properties.
  * @param property the CSS property
  * @param initalValue the default value of the corresponding property which may be null
  * @param inherits true if this property uses CSS inheritance
  * @param subProperties the sub-properties of this property. For example,
  * the -fx-font property has the sub-properties -fx-font-family,
  * -fx-font-size, -fx-font-weight, and -fx-font-style.
  */
  protected CssMetaData(
      final String property,
      final V initialValue,
      boolean inherits,
      final List<CssMetaData> subProperties) {

    this.property = property;
    this.initialValue = initialValue;
    this.inherits = inherits;
    this.subProperties = subProperties != null ? Collections.unmodifiableList(subProperties) : null;

    if (this.property == null)
      throw new IllegalArgumentException("property cannot be null");
  }

  /**
  * Construct a CssMetaData with the given parameters and no sub-properties.
  * @param property the CSS property
  * @param initalValue the default value of the corresponding property which may be null
  * @param inherits true if this property uses CSS inheritance
  */
  protected CssMetaData(
      final String property,
      final V initialValue,
      boolean inherits) {
    this(property, initialValue, inherits, null);
  }

  /**
  * Construct a CssMetaData with the given parameters, inherit set to
  * false and no sub-properties.
  * @param property the CSS property
  * @param initalValue the default value of the corresponding property which may be null
  */
  protected CssMetaData(
      final String property,
      final V initialValue) {
    this(property, initialValue, false, null);
  }

  /**
  * Construct a CssMetaData with the given parameters, initialValue is
  * null, inherit is set to false, and no sub-properties.
  * @param property the CSS property
  */
  protected CssMetaData(
      final String property) {
    this(property, false, null);
  }
}

Issues:
One major aspect of the private interface has been omitted from the public interface and that is the notion of a Conveter. A Converter takes a value from the parser (a ParsedValue) and converts it to the corresponding java type. So, for example, a ColorConverter would convert a ParsedValue representing a Color into a Color. If possible, the parser performs the conversion. But this is not always possible such as when converting an em size to an absolute pixel value. The Converter was omitted because including it in the API would result in dragging much more of the private implementation than is desired. ParsedValue, for instance, is (in my opinion) not suitable for public API in its current state because it is tightly coupled with the parser and the Converters. I'm not sure, however, that this public API can exist without a Converter member, but I believe I can infer the Converter from the WritableValue. Alternatively, some type indicator could be used or a set of CSS*Property classes (e.g., CSSBooleanProperty) could be created.

Origin

/**
* Enumeration of the possible source or origin of a stylesheet and styles.
*/
public enum Origin {
  /** The stylesheet is a user-agent stylesheet */
  USER_AGENT,
  /** The value of a property was set by the user through a call to a set method */
  USER,
  /** The stylesheet is an external file */
  AUTHOR,
  /** The style is from the Node via setStyle */
  INLINE
}

Additional Node API.

List<CssMetaData> getCssMetaData()
static List<CssMetaData> getClassCssMetaData()

These methods return a List of CssMetaData supported by the node. The list should include not only the properties of this node, but the properties of the node's super-class(es) as well. This method is called frequently and, by convention, it returns a static list. Since this static list may be used by other node classes, the convention is to provide a public static method that returns the static list: public static List<CssMetaData> getClassCssMetaData(). The getCssMetaData() method is used by the CSS engine to avoid reflection. A typical implementation would be:

private static class StyleableProperties {

  private static final CssMetaData<Watermark, String> CORNER = new CssMetaData<Watermark, String>("-my-corner", "lower-right") {
    public abstract boolean isSettable(Watermark node) {
      return corner == null || corner.isBound() == false;
    }
    public abstract WritableValue<String> getWritableValue(Watermark node) {
      return cornerProperty();
    }
  }
  private static final List<CssMetaData> META_DATA;
  static {
    final List<CssMetaData> data = new ArrayList<CssMetaData>();
    Collections.addAll(data,
      Control.getClassCssMetaData(),
      CORNER
    );
    META_DATA = Collections.unmodifiableList(data);
  }
}

public static List<CssMetaData> getClassCssMetaData() {
  return StyleableProperties.META_DATA;
}

public static List<CssMetaData> getCssMetaDataMetaData() {
  return getClassCssMetaData();
}

Pseudo-class

The pattern for handling pseudo-class state is to override the invalidated method of the property in order to invoke pseudoClassStateChanged which sets a flag indicating that the node needs updating by CSS. During a pulse, pseudoClassStateChanged may be called many times, but only if there is a style with the given pseudo-class will the update flag be set. Once the update flag is set, subsequent calls to pseudoClassStateChanged are effectively no-ops.

When CSS processes the update, it gets the node's current pseudo-class state by calling getPseudoClassStates(). Each node that has pseudo-class state overrides getPseudoClassStates().

PseudoClass

PseudoClass contains two inner classes, State and States TBD: bad naming?. PseudoClass.State is an immutable object that represents one pseudo-class state. PseudoClass.States is a mutable object that is returned from getPseudoClassStates() and represents the summation of individual PseudoClass.State values that are in effect.

A pseudo-class state is unique according to the string value of the pseudo-class that might appear in a CSS file. In other words, there is only one PseudoClassState for "hover", even if the "hover" pseudo-class is found in more than one class. Care must be exercised when choosing names for pseudo-classes so they do not conflict with other pseudo-classes.

Typical usage is:

private static final PseudoClassState MAGIC_PSEUDO_CLASS_STATE = PseudoClassState.getState("xyzzy");

    @Override public PseudoClass.States getPseudoClassStates() {
          PseudoClass.States states = super.getPseudoClassStates();
          if (isMagic()) states.addState(MAGIC_PSEUDO_CLASS_STATE);
          return states;
     }

The API is outlined below.

package javafx.scene;

public final class PseudoClass {

    /**
     * There is only one PseudoClass.State instance for a given pseudoClass.
     * @return The PseudoClass.State for the given pseudoClass. Will not return null.
     */
    public static PseudoClass.State getState(String pseudoClass) {
        // Use pseudoClass as key to find State in a Map<String,State>
        // If not found, create it and add to the map.
        // return the PseudoClass.State
    }

    /**
     * @return A new PseudoClass.States instance
     */
    public static PseudoClass.States createStatesInstance() {
    }

    public final class State {

        /** Cannot create an instance of State except through PseudoClass static methods */
        private State() {
        }

       @Override public String toString() {
            // return the String that was used to create this PseudoClassState pseudo-class, e.g. "hover"
        }

    }

    public final class States {

        /**
         * Add the state to the current set of states. The result of the operation will be the union of
         * the existing set of states and the added state.
         * @param state The state to add
         */
        public void addState(PseudoClass.State state) {
        }

        /**
         * Remove the state to the current states. The result of the operation will be the relative complement of
         * the removed state in the existing set of states. This implies that removing a state that is not in the
         * existing set of states had no effect.
         * @param state The state to remove
         */
        public void removeState(PseudoClass.State state) {
        }

        /**
         * Reset the set of states to the empty set.
         */
        public void clear() {
        }

        /** @return The list of PseudoClass.State that are represented by this States object */
        public List<PseudoClass.State> getStates() {
        }

        /** Cannot create an instance of States except through PseudoClass static methods */
        private States() {
        }

    }

}

pseudo-class Node API

/**
     * Used to notify that a pseudo-class has changed. Typically, this method is called from the invalidated
     * method of a property that is used as a pseudo-class.
     * <code>
     * private static final PseudoClass.State MY_PSEUDO_CLASS_STATE = PseudoClass.getState("my-state");
     *
     * BooleanProperty myPseudoClassState = new BooleanPropertyBase(false) {
     *
     *           @Override public void invalidated() {
     *                pseudoClassStateChanged(MY_PSEUDO_CLASS_STATE);
     *           }
     *
     *           @Override public Object getBean() {
     *               return MyControl.this;
     *           }
     *
     *           @Override public String getName() {
     *               return "myPseudoClassState";
     *           }
     *       };
     * <code>
     * @param pseudoClassState The PseudoClass.State that has changed.
     */
    protected void pseudoClassStateChanged(PseudoClass.State pseudoClassState) {
       // if there are styles that use the given pseudoClassState, mark this node as needing css update
    }

    /**
     * The pseudo-class state of a Node is the state pertaining to this class and all of its super classes. This method is
     * implemented by getting the pseudo-class state of the super class and then or'ing in the pseudo-class state of this class.
     * <code>
     * private static final PseudoClass.State MY_PSEUDO_CLASS_STATE = PseudoClass.getState("my-state");
     *
     * @Override public PseudoClass.States getPseudoClassStates() {
     *      PseudoClass.States states = super.getPseudoClassStates();
     *      if (isMyPseudoClassState()) { states.addState(MY_PSEUDO_CLASS_STATE); }
     *      return state;
     * }
     * <code>
     * @return PseudoClass.States representing the pseudo-class state of this node and the state of its super classes.
     *
     */
    public PseudoClass.States getPseudoClassStates() {
        PseudoClass.States states = PseudoClass.createStatesInstance(); ??[TBD - better to have a instance member and just clear it?]??
        if(isHover()) states.addState(HOVER_PSEUDOCLASS_STATE);
        if(isPressed()) states.addState(PRESSED_PSEUDOCLASS_STATE);
        if(isDisabled()) states.addState(DISABLED_PSEUDOCLASS_STATE);
        if(isFocused()) states.addState(FOCUSED_PSEUDOCLASS_STATE);
        if(impl_isShowMnemonics()) states.addState(SHOW_MNEMONICS_PSEUDOCLASS_STATE);
        return states;
    }