OpenEdge Development: Working with XML - Product ...

Reading and Writing XML with the Document Object Model (DOM)
2–8
Since an X-document represents the tree of an XML document, it is a container for the branches
and leaves of that tree. In XML, the branches and leafs are elements. In DOM, the branches and
leafs are called nodes. There is a relationship between elements and nodes, but it is not a direct
one-for-one correspondence. In DOM, a single XML element may be further decomposed into
many nodes. The relationships between DOM nodes in a tree and the relationship of XML
components can follow slightly different logic. For example, an attribute of an XML element is
thought of as a part of the element. In DOM, that attribute becomes a separate node related to
the node that represents its associated element.
So, how do you keep all this straight? Basically, when you begin a programming task, you need
to ensure that your XML input documents are valid, well-formed XML and that the design of
your output XML documents produces valid, well-formed XML. For the rest of your project,
you will be immersed in ABL and DOM logic. Think in terms of DOM nodes and the rules of
DOM relationships, and you will be successful.
Representing a DOM node in ABL
In essence, X-document represents the collection of nodes, their content of the nodes, and all
the relationships between nodes.
The X-noderef object is an ABL object that is a reference to any node in a DOM tree (except a
DOCUMENT node, which is the X-document). In ABL, you can create a handle to an X-noderef
object, which in turn is a cursor pointing to a DOM node in a DOM tree (represented by an ABL
X-document object).
The life cycle of an X-noderef object is similar to the X-document object’s. For example:
DEFINE VARIABLE hNode AS HANDLE NO-UNDO
.
CREATE X-NODEREF hNode.
. . .
DELETE OBJECT hNode.
There are many types of DOM nodes as identified by the SUBTYPE attribute of both the
X-document and X-noderef objects. Some types of nodes may have child nodes of various
types, and others are leaf nodes that cannot have anything below them in the document structure.
The ABL X-noderef object can be one of the following DOM node types, as indicated by the
SUBTYPE attribute:
• ATTRIBUTE
• CDATA-SECTION
• COMMENT
• DOCUMENT-FRAGMENT
• ELEMENT
• ENTITY-REFERENCE
• PROCESSING-INSTRUCTION
• TEXT
The default subtype is ELEMENT.