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<!-- by Arved Sandstrom -->
<s1 title="FOP Mechanics">
<s2 title="Introduction">
<p>
The overall process is controlled by <em>org.apache.fop.apps.Driver</em>. In
this class, a typical sequence is:</p>
<p><code>Driver driver = new Driver();</code></p>
<p><code>driver.setRenderer("org.apache.fop.render.pdf.PDFRenderer", version);</code></p>
<p><code>driver.addElementMapping("org.apache.fop.fo.StandardElementMapping");</code></p>
<p><code>driver.addElementMapping("org.apache.fop.svg.SVGElementMapping");</code></p>
<p><code>driver.addPropertyList("org.apache.fop.fo.StandardPropertyListMapping");</code></p>
<p><code>driver.addPropertyList("org.apache.fop.svg.SVGPropertyListMapping");</code></p>
<p><code>driver.setWriter(new PrintWriter(new FileWriter(args[1])));</code></p>
<p></p>
<p><code>driver.buildFOTree(parser, fileInputSource(args[0]));</code></p>
<p></p>
<p><code>driver.format();</code></p>
<p></p>
<p><code>driver.render();</code></p>
</s2>
<s2 title="Formatting Object Tree">
<p>The class <em>org.apache.fop.fo.FOTreeBuilder</em> is responsible for actually
constructing the FO tree. The key SAX events used are </p>
<p><code>startElement()</code>,</p>
<p><code>endElement()</code> and <code>characters()</code>.</p>
<p>All formatting objects derive from abstract class
<em>org.apache.fop.fo.FONode</em>. The other FO classes inherit from
<em>FONode</em> as follows:</p>
<p><code>            FONode</code></p>
<p><code>               |</code></p>
<p><code>     __________|________</code></p>
<p><code>    |                   |</code></p>
<p><code>   FObj               FOText</code></p>
<p><code>    |</code></p>
<p><code>    |___________________</code></p>
<p><code>    |                   |</code></p>
<p><code>  FObjMixed      SequenceSpecifier
</code></p>
<p>FO's extending FObj:</p>
<p>Package org.apache.fop.fo.pagination:</p>
<p><code> LayoutMasterSet</code></p>
<p><code> PageSequence</code></p>
<p><code> RegionAfter</code></p>
<p><code> RegionBefore</code></p>
<p><code> RegionBody</code></p>
<p><code> Root</code></p>
<p><code> SequenceSpecification</code></p>
<p><code> SimplePageMaster</code></p>
<p>Package org.apache.fop.fo.flow:</p>
<p><code>BlockContainer</code></p>
<p><code>DisplayGraphic</code></p>
<p><code>DisplayRule</code></p>
<p><code>DisplaySequence</code></p>
<p><code>Flow</code></p>
<p><code>InlineGraphic</code></p>
<p><code>ListBlock</code></p>
<p><code>ListItem</code></p>
<p><code>ListItemBody</code></p>
<p><code>ListItemLabel</code></p>
<p><code>PageNumber</code></p>
<p><code>StaticContent</code></p>
<p><code>Table</code></p>
<p><code>TableBody</code></p>
<p><code>TableCell</code></p>
<p><code>TableColumn</code></p>
<p><code>TableRow</code></p>
<p>FO's extending SequenceSpecifier:</p>
<p>Package org.apache.fop.fo.pagination:</p>
<p><code>SequenceSpecifierAlternating</code></p>
<p><code>SequenceSpecifierRepeating</code></p>
<p><code>SequenceSpecifierSingle</code></p>
<p>FO's extending FObjMixed:</p>
<p>Package org.apache.fop.fo.flow:</p>
<p><code>Block</code></p>
<p><code>Inline</code></p>
<p><code>BasicLink</code></p>
</s2>
<s2 title="FONode">
<p>
The class inheritance described above only describes the nature of the
content. Every FO in FOP also has a parent, and a Vector of children. The
parent attribute (in the Java sense), in particular, is used to enforce
constraints required by the FO hierarchy.
</p>
<p>
FONode, among other things, ensures that FO's have a parent, that they
have children, that they maintain a marker of where the layout was up to
(for FObj's it is the child number, and for FOText's it is the character
number), and that they have a <code>layout()</code> method.
</p>
</s2>
<s2 title="Making FO's">
<p>
Every FO class has code that looks something like this:
</p>
<p><code>public static class Maker extends FObj.Maker {</code></p>
<p><code>   public FObj make(FObj parent, PropertyList propertyList)</code></p>
<p><code>     throws FOPException</code></p>
<p><code>   {</code></p>
<p><code>     return new SimplePageMaster(parent, propertyList);</code></p>
<p><code>   }</code></p>
<p><code>}</code></p>
<p>
The class also has a static method that resembles
</p>
<p><code>public static FObj.Maker maker()</code></p>
<p><code>   {</code></p>
<p><code>     return new PageSequence.Maker();</code></p>
<p><code>   }</code></p>
<p>
A hash 'fobjTable' exists in <em>FOTreeBuilder</em>, and maps the FO names (such as
'fo:table') to object references to the appropriate factories
(such as <em>Table.Maker</em>).
</p>
<p>
Properties (recall that FO's have properties, areas have traits, and XML
nodes have attributes) are also a concern of <em>FOTreeBuilder</em>. It
accomplishes this by using <em>PropertyListBuilder</em>, which contains a hash of
property names and <ref>their</ref> respective makers. The base class for
properties is <em>Property</em>, and the property makers extend
<em>Property.Maker</em>.
</p>
</s2>
<s2 title="FO Formatting">
<p>
<em>FOTreeBuilder</em> calls <code>format()</code> on the root FO, passing
it the <em>AreaTree</em>
reference. In turn, <em>Root</em> calls <code>format()</code> on each
<em>PageSequence</em>, passing <ref>it</ref>
the <em>AreaTree</em> reference.
</p>
<p>
The <em>PageSequence</em> <code>format()</code> method does the following things:
</p>
<ol>
<li>Makes a <em>Page</em>, using <em>PageMasterFactory</em> to produce a
<em>PageMaster</em>, and
using <code>makePage()</code> in the latter class. In the simplest picture,
a <em>Page</em> has
5 areas represented by <em>AreaContainers</em>;</li>
<li>Handles layout for <em>StaticContent</em> objects in the 'before' and 'after'
regions, if set. This uses the <code>layout()</code> method in
<em>StaticContent</em>;</li>
<li>If a page break is not forced, it will continue to layout the flow into
the body area (<em>AreaContainer</em>) of the current page;</li>
<li>It continues with (1) when layout into the current page is done, but
the flow is not empty.</li>
</ol>
</s2>
<s2 title="Area Layout">
<p>
FO's that represent actual areas, starting with <em>Flow</em> and
<em>StaticContent</em>, have
a <code>layout()</code> method, with the following signature:
</p>
<p>
<code>
public Status layout(Area area)
</code>
</p>
<p>
The fundamental role of the <code>layout()</code> method is to manage the layout of
children and/or to generate new areas.
</p>
<p>
<em>Example</em>: the <code>layout()</code> method for <em>Flow</em> generates no new areas - it manages the
layout of the flow children.
</p>
<p>
<em>Example</em>: the <code>layout()</code> method for <em>Block</em>
generates a new <em>BlockArea</em> in and of
itself, and also manages the layout of the block children, which are added
to the <em>BlockArea</em> before that is itself added to its parent
<em>Area</em>.
</p>
<p>
<code>Layout()</code> methods are subject to the general constraint that possibly not
all of their children can be accommodated, and they report back accordingly
with an appropriate <em>Status</em>.
</p>
</s2>
<s2 title="Rendering">
<p>
This is a separate process. The <code>render()</code> method in
<em>Driver</em> is invoked (say,
by <em>CommandLine</em>) with the laid-out <em>AreaTree</em> and a
<em>PrintWriter</em> as arguments.
This actually calls the <code>render()</code> method in a specific implementation of
the <em>Renderer</em> interface, typically <em>PDFRenderer</em> or
<em>AWTRenderer</em>.
</p>
<p>
At the highest level <em>PDFRenderer</em>, for example, begins by rendering each
<em>Page</em>. The <code>render()</code> method in <em>Page</em> (as is the case for other areas),
invokes a particular method in the renderer of choice, e.g.
<code>renderPage()</code>.
<em>NOTE</em>: this system is bypassed for <em>Page</em>, incidentally.
</p>
<p>
Rendering will not be discussed further in this document, as most of our
current effort must concentrate on layout. Section 4.12 in the XSL WD
discusses some issues applicable to rendering.
</p>
</s2>
<s2 title="SVG">
<p>
FOP supports some svg rendering. SVG is supported as an instream-foreign-object
embedded in an FO document. FOP also supports rendering of an external SVG image.
</p>
<p>
Since the intream object that contains the SVG returns a single fo area then
the construction of the SVG document is handled differently. The SVG is created
by calling the <code>createGraphic()</code> on each SVG element. The element is then
responsible for loading the necessary information and child elements and creating
the corresponding SVG DOM element. When the FO tree is being layed out the
SVG tree is turned into the SVG DOM document which is stored for later rendering.
</p>
<p>
The SVG document is then held as a DOM tree which is then rendered by going through
the elements of the tree and rendering then in turn.
</p>
<p>
For more information see the SVG documentation.
</p>
</s2>
</s1>