【转】
Text Model 存储文本以及相应属性的模型，主要有两种不同的实现策略：树形结构和平面结构。树形结构类似 Dom，可以很好的表现结构化文档，但是比较复杂。平面结构相对简单的多，大多数的编辑器（e.g. Emacs），字处理器（e.g. AbiWord, OpenOffice）都用的是平面结构。
Text Model Coords 用来在 Text Model 中定位的坐标系。对于树形结构的 Text Model 其定位方式为节点链接（或指向节点的路径 e.g. XPath）以及节点内偏移值。对于平面结构则仅仅是一个索引值。
Text Presentation 展示文本的模型，一般分别为 Block, Line, Run …
Text Presentation Coords 用来在 Text Presentation 中定位的坐标系，其值通常为 (x,y)。Text Editor 必须能够在 Text Model Coords 和 Text Presentation Coords 之间变换。例如将鼠标点击（x,y）对应到 Text Model 中的某个字符/元素。由fp_Page::mapXYToPosition成员函数实现
FL_DocLayout 类：
page fmt_overview Formatter
FL_DocLayout is a formatted representation of a specific PD_Document,
formatted for a specific GR_Graphics context.
A %FL_DocLayout encapsulates two related hierarchies of objects.
The logical (or content) hierarchy corresponds
to the logical structure of the document.
- each FL_DocLayout contains a list of fl_SectionLayout objects
- each fl_SectionLayout are composed of fl_BlockLayout objects
Where each fl_BlockLayout corresponds to a logical element in
the PD_Document (i.e., usually a paragraph of text).
The physical (or layout) hierarchy, by contrast,
encapsulates the subdivision of physical space into objects of
successively finer granularity.
- each FL_DocLayout contains a list of fp_Page objects, each
of which was constructed during the process of formatting
the document
- each fp_Page is a list of fp_Columns (fp_Column)
- each fp_Column is a list of fp_Lines (fp_Line)
- each fp_Line is a list of fp_Runs (fp_Run)
Where each fp_Run contains some fragment of content from the original
document, usually text.