FO Tree
Layout Managers
Layout Process
Handling Attributes
Area Tree

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FO Tree

The FO Tree is a representation of the XSL:FO document. This represents the Objectify step from the spec. The Refinement step is part of reading and using the properties which may happen immediately or during the layout process.

classes are in*, for elements in the flow they are in* and some others are in*.

The base class for all objects in the tree is FONode. The base class for all FO Objects is FObj.

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There is a class for each element in the FO set. An object is created for each element in the FO Tree. This object holds the properties for the FO Object.

When the object is created it is setup. It is given its element name, the FOUserAgent - for resolving properties etc. - the logger and the attributes. The methods handleAttributes() and setuserAgent(), common to FONode, are used in this process. The object will then be given any text data or child elements. Then the end() method is called. The end method is used by a number of elements to indicate that it can do certain processing since all the children have been added.

Some validity checking is done during these steps. The user can be warned of the error and processing can continue if possible.

The FO Tree is simply a heirarchy of java objects that the layout or structure process only in the flow elements.


The XML attributes on each element are passed to the object. The objects that represent FO objects then convert the attributes into properties.

Since properties can be inherited the PropertyList class handles resolving properties for a particular element. All properties are specified in an XML file. Classes are created automatically during the build process.

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In some cases the element may be moved to have a different parent, for example markers, or the inheritance could be different, for example initial property set.

Foreign XML

The base class for foreign XML is XMLObj. This class handles creating a DOM Element and the setting of attributes. It also can create a DOM Document if it is a top level element, class XMLElement. This class must be extended for the namespace of the XML elements. For unknown namespaces the class is UnknowXMLObj.

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If some special processing is needed then the top level element can extend the XMLObj. For example the SVGElement makes the special DOM required for batik and gets the size of the svg.

Foreign XML will usually be in an fo:instream-foreign-object, the XML will be passed to the render as a DOM where the render will be able to handle it. Other XML from an unknwon namespace will be ignored.

By using element mappings it is possible to read other XML and either

  • set information on the area tree
  • create pseudo FO Objects that create areas in the area tree
  • create FO Objects

Unknown Elements

If an element is in a known namespace but the element is unknown then an Unknown object is created. This is mainly to provide information to the user. This could happen if the fo document contains an element from a different version or the element is misspelt.

Page Masters

The first elements in a document are the elements for the page master setup. This is usually only a small number and will be used throughout the document to create new pages. These elements are kept as a factory to create the page and appropriate regions whenever a new page is requested by the layout. The objects in the FO Tree that represent these elements are themselves the factory. The root element keeps these objects as a factory for the page sequences.


The elements that are in the flow of the document are a set of elements that is needed for the layout process. Each element is important in the creation of areas.

Other Elements

The remaining FO Objects are things like page-sequence, title and color-profile. These are handled by their parent element; i.e. the root looks after the declarations and the declarations maintains a list of colour profiles. The page-sequences are direct descendents of root.

Associated Tasks
  • Create diagrams
  • Setup all properties and elements for XSL:FO
  • Setup user agent for property resolution
  • Verify all XML is handled appropriately

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