Welcome to the understanding series. This will be a series of notes for developers to understand how FOP works. We will attempt to clarify the processes involved to go from xml(fo) to pdf or other formats. Some areas will get more complicated as we proceed.
FOP takes an xml file does its magic and then writes a document to a stream.
xml -> [FOP] -> document
The document could be pdf, ps etc. or directed to a printer or the screen. The principle remains the same. The xml document must be in the XSL:FO format.
For convenience we provide a mechanism to handle XML+XSL as input.
The xml document is always handled internally as SAX. The SAX events are used to read the elements, attributes and text data of the FO document. After the manipulation of the data the renderer writes out the pages in the appropriate format. It may write as it goes, a page at a time or the whole document at once. Once finished the document should contain all the data in the chosen format ready for whatever use.
The fo data goes through a few stages. Each piece of data will generally go through the process in the same way but some information may be used a number of times or in a different order. To reduce memory one stage will start before the previous is completed.
SAX Handler -> FO Tree -> Layout Managers -> Area Tree -> Render -> document
In the case of rtf, mif etc.
SAX Handler -> FO Tree ->
Structure Renderer -> document
The FO Tree is constructed from the xml document. It is an internal representation of the xml document and it is like a DOM with some differences. The Layout Managers use the FO Tree do their layout stuff and create an Area Tree. The Area Tree is a representation of the final result. It is a representation of a set of pages containing the text and other graphics. The Area Tree is then given to a Renderer. The Renderer can read the Area Tree and convert the information into the render format. For example the PDF Renderer creates a PDF Document. For each page in the Area Tree the renderer creates a PDF Page and places the contents of the page into the PDF Page. Once a PDF Page is complete then it can be written to the output stream.
For the structure documents the Structure listener will read directly from the FO Tree and create the document. These documents do not need the layout process or the Area Tree.
Verify Structure Listener concept.