Shows how to apply the CSS to Java™ objects.

The CSS selector mechanism was designed to match elements in HTML or XML documents. It can also be used to match a hierarchy of Java™ objects accessible from a model interface. In this context, the CSS level 2 recommendation is transposed for the Java language and used to set bean properties according to the Java object hierarchy and state.

In applying CSS to Java objects, the term model objects is used as the equivalent of the term elements in the W3C recommendation.

The CSS declarations for each model object are sorted and used according to the application that controls the CSS engine. The declarations represent property settings on a target object. The target object concerned depends on the way the CSS engine is used.

Rogue Wave® JViews Diagrammer uses CSS declarations to create and customize one graphic object for each object in the data model, and to create and customize renderers according to user settings.

The CSS engine has different responsibilities at load time and at run time:

Usually the left side of a declaration represents a bean property of the graphic object. The right side is a literal and, if it needs type conversion, the Java method s etAsText is invoked on the Property Editor associated with the bean property.

The input data model represents the seed of the “CSS for Java™” engine. It provides three important kinds of information to the CSS engine, required to resolve the selectors:

The target object is the graphic object associated with the model object. The declarations change property values of the graphic object that corresponds to the matching model object, thereby customizing the graphic appearance given by the rendering.

In Rogue Wave® JViews Diagrammer the target object is an IlvGraphic instance (see Graphic objects inThe Essential JViews Framework,for information about IlvGraphic objects) or a composite graphic.

Object types and attribute matching

The following code example shows a rule that matches the object of class (type) test_Vehicle, with the attribute model equal to sport, and sets the property icon of the graphic object associated with this object (defined elsewhere) to sport-car.gif.

Attribute matching can be used to add dynamic behavior: a PropertyChange event occurring on the model can activate the CSS engine to set new property values on the graphic objects.

The following code example shows a rule that changes the color of any object of CSS class computer whenever the model attribute state is set to down.

For more information on attribute matching see also Attribute matching.

Object identifiers and CSS classes

The Java model provides a getID method which represents the ID of a model object. This ID can be checked against the # selector of a rule.

If there is a single CSS class in the selector, it is resolved with the getTag method in the model.

Additional CSS classes can be set for an object in a property called CSSclass. The engine automatically merges the result of getTag with the value of the CSSclass property.

CSS classes are not necessarily related to data model semantics; they are devices to add to the pattern-matching capabilities in the style sheet. An object belongs to only one class (its type) but can belong to several (or no) CSS classes. A check on a CSS class is for its presence or absence. Therefore a CSS class can be seen as an attribute without a value.

By default, an XML element name is defined as a CSS class. If, for example, a simple XML file contains the element names root and leaf, then the following code example shows how to change the color of leaf nodes to an RGB color specification.

The RGB color specification shown for the foreground (border) color is magenta.

Class name

The class property is a reserved keyword indicating the class name of the generated graphic object. Obviously the class declaration is applied only when there is a creation request. If the model state is changed, the graphic objects are customized by applying only new declarations coming from new matching rules of the style sheet. The class declaration is then simply ignored.

To change the class, you must remove the model object and add it again.

The right side of a class declaration may be:

Pseudo-classes and pseudo-elements

Pseudo-classes are the minimal building blocks of a selector which match model objects according to an external context. The syntax is like a CSS class but with a colon instead of a dot. For example, node:selected matches a node only if the node is selected. The user agent can resolve this pseudo-class at run time according to the state of each node.

A pseudo-class has the same specificity as a CSS class.

Pseudo-elements are metaclasses, like pseudo-classes, but match document structure instead of the user agent state.

Model indirection

The right side of a declaration resolves to a literal that is determined at run time by a Property Editor. However, if the literal is prefixed by @, the remainder of the string is interpreted as a model attribute name. The declaration takes the value from the model object, as shown in the following code example.

The label property will be set to the value of the attribute called caption in the model. If the specified attribute does not exist for the object, it is searched for recursively in the model ascendancy. The title property will be set to the literal CSS rocks.

Such indirection is also used in the opposite direction, that is, to retrieve the name of the model attribute that controls a graphic property. This allows user interactions to modify the data model correctly. Two special names, @_ID and @_TAG, represent values returned by the model method calls getID and getTag getTag respectively.

In a palette symbol CSS, the indirection cannot directly access the properties of the SDM object that is rendered by the palette symbol. In this case, the indirection can only access the symbol parameters. If you want a property of a graphic element of the symbol to be mapped to a property of the corresponding SDM object, you have to define a parameter and use it as an intermediate property. The ID of this parameter can then be accessed in the symbol CSS as an object property. In the outer CSS, you can use this parameter ID as a bean property.

Resolving URLs

Sometimes declaration values are URLs relative to the style sheet location. A special construct, standard in CSS level2, allows you to create a URL from the base URL of the current style sheet. For example:

This declaration extends the path of the current style sheet URL with images/icon.gif. This construct is very useful for creating a style sheet with images located relative to it, because the URL remains valid even if the style sheet is cascaded or imported elsewhere.

You are likely to want to specify a Java™ object as the value of a declaration. A simple convention allows you to recurse in the style sheet, that is, to define a new Java object which has the same style sheet but is unrelated to the current data model.

@# construct

Prefix the value with @# to create new beans when required as shown in the following code example.

The @# operator extends the current data model by adding a dummy model object as the child of the current object. The object ID of the dummy object is the remainder of the string, beyond the @# operator. The type of the dummy object is Subobject. The dummy object inherits CSS classes and attributes from its parent.

The CSS engine creates and customizes a new subobject according to the declarations it finds for the dummy object. This means, in particular, that the Java class of the subobject is determined by the value of the class property. The newly created subobject becomes the value of the @# expression. In the declarations for the subobject, attribute references through the @ operator refer to the attributes of the parent object.

Once the subobject is completed, the previous model is restored so that normal processing is resumed.

In code example Creating a bean in a declaration, a java.util.Date object is created, with the time property set to 23849291. This new object is assigned to the date property of the form object.

@= and @+ constructs

There are two refinements of the @#ID operator:

The need for these refinements arises from a performance issue. The @# operator creates a new object each time a declaration is resolved. Usually a declaration is applied whenever a property changes. Under certain circumstances, the creation of objects may lead to expensive processing, so Rogue Wave® JViews Diagrammer provides an optional mechanism to minimize the creation of objects during property changes.

@| construct

A CSS declaration value starting with @| is interpreted as an expression (see Expressions).

@ construct

A CSS declaration value that is exactly @ means cancel the property setting made in a previous rule. This construct is useful to prevent a property from being modified, especially when the default value is unknown. For example:

These two rules say that the width property value should be set to 23, unless the node has the CSS class fixed. Without the @ ability, the default value of width would have to be written down in the CSS.

The value in a CSS declaration is usually a literal. However, it is possible to write an expression in place of a literal.

If the value begins with @|, then the remainder of the value is processed as an expression.

The syntax of the expressions, after the @| prefix, is close to the Java syntax. The expression type can be arithmetic (type int, long, float, or double ), Boolean, or String. Examples:

You can use the following regular arithmetic operators, listed here in decreasing precedence:

An expression can refer to model attributes. The syntax is the usual one:

@|@speed/100+@drift -> 1/100 of the value of speed plus the value of drift. speed and drift are attributes of the current object.

'@|"name is: " + @name' -> "name is: Bob", if the value of the current object attribute name is Bob. Note the use of quotation marks to keep the space characters. You could use the backslash (\) character instead, directly preceding the space characters to retain them. The backslash works to quote any character that directly follows it. The use of the backslash character makes sure that the character thus quoted is not interpreted by the expression parser.

For example, when you edit a style sheet directly and you use double quotation marks for the entire declaration value specification, you must use escaped double quotation marks for inner strings of CSS expressions.

Alternatively, you can use quotation marks:

The standard functions abs(), acos(), asin(), atan(), ceil(), cos(), exp(), floor(), log(), pi, rint(), round(), sin(), sqrt(), and tan() are accepted, as in, for example:

The following functions are also available:

 

Java™ objects are not HTML documents. The CSS2 syntax remains, so that a CSS editor can still be used to create the style sheet. However, the differences lead to adaptations of the CSS mechanism so that its power can be fully exploited and to some specific behavior.

Cascading

Cascading is explicit: the API offers a means of cascading style sheets. However, the !important and inherit tags are not supported for the sake of simplicity.

Pseudo-classes and pseudo-elements

The pseudo-class construct is fully implemented and used to represent renderer-specific states or GUI items. The list of predefined pseudo-classes and where they are used is as follows:

You can add custom pseudo-classes with the method IlvSDMEngine:setPseudoClasses.

The CSS2 predefined pseudo-elements and pseudo-classes ( :link, :hover, and so forth) are not implemented because they have no meaning in Java.

Attribute matching

The attribute pattern in CSS2 makes the following checks for strings: presence [ att ], equality [ att= val], and inclusion [ att~= val]. The |= operator is disabled.

For Java objects, there are the following numeric comparators >, >=, <>, <=, <, with the usual semantics.

There are also equal and not-equal comparators which make the distinction between string comparison and numerical comparison:

Syntax enhancement

CSS for Java requires the use of quotation marks when a token contains special characters, such as dot (.), colon (:), at sign (@), number sign (also known as hash sign, #), space (  ), and so on.

Quotes can be used almost everywhere, in particular to delimit a declaration value, an element type, or a CSS class with reserved characters.

The closing “;” is optional.

Null value

Sometimes it makes sense to specify a null value in a declaration. By convention, null is a zero-length string '' or "". For example:

When a null class name is specified, no object is created at all, and no error is reported as it would be for a malformed class name.

The notation '' is also used to denote a null array for properties expecting an array of values.

Empty string

The null syntax does not allow you to specify an empty string in the style sheet. Instead, you can create an empty string, as shown in the following code example.

Better still, you can use the sharing mechanism to avoid the creation of several strings. The @= construct will create the empty string the first time only and will then reuse the same instance for all other occurrences of @#emptyString, see Sharing an empty string.