XFA Specification
Chapter 7, Layout for Growable Objects
Flowing Layout for Containers
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The Layout DOM
The relationship between layout objects is embodied in the Layout DOM. This is a tree graph representing
the result of the layout operation. This includes positioning information for all displayable layout objects.
Within the Layout DOM descent stands for relative positioning, so that the child's position on the page is
established relative to its parent. Usually the child's displayed representation is also contained within the
parent's region of the page, however this is not always the case. The parent is often described as the
container
of the child, even though the child may be rendered outside the parent.
A node within the Layout DOM is called a layout node. Layout nodes other than the root have the same
types as nodes in the Form DOM or in the Template DOM, from which they are copied. Sometimes multiple
layout nodes are copied from the same form node or template node; this happens when a single layout
content object is split into fragments which are spread across multiple
contentArea
objects.
Layout is performed by adding layout nodes to the Layout DOM. The top level below the root holds a
single
pageSet
. The next level holds
pageArea
objects. Each new
pageArea
added to the Layout DOM
is another display surface added to the document. The next level holds
contentArea
objects. Lower
levels of the Layout DOM represent layout content. Each new node of layout content represents a layout
object or fragment of a layout object that has been placed upon a display surface. The document order of
the Layout DOM is also the Z-order of the layout content; later in document order is closer to the front. You
can think of this as the later displayable objects being printed over top of the earlier ones. This is
illustrated in the following diagram.
Order in which objects are drawn on page
[pageArea]
[contentArea]
[subform]
[subform]
[subform]
Among sibling
pageArea
objects, the document order determines the page order; the leftmost (eldest)
sibling is the first (lowest page number) in the document. This is illustrated below.