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DocumentFragment is a "lightweight" or "minimal" Document
object. It is very common to want to be able to extract a portion
of a document's tree or to create a new fragment of a document. Imagine
implementing a user command like cut or rearranging a document by moving
fragments around. It is desirable to have an object which can hold such
fragments and it is quite natural to use a Node for this purpose. While it
is true that a Document object could fulfil this role, a
Document object can potentially be a heavyweight object,
depending on the underlying implementation. What is really needed for this
is a very lightweight object. DocumentFragment is such an
object.
Furthermore, various operations -- such as inserting nodes as children
of another Node -- may take DocumentFragment
objects as arguments; this results in all the child nodes of the
DocumentFragment being moved to the child list of this node.
The children of a DocumentFragment node are zero or more
nodes representing the tops of any sub-trees defining the structure of the
document. DocumentFragment nodes do not need to be
well-formed XML documents (although they do need to follow the rules
imposed upon well-formed XML parsed entities, which can have multiple top
nodes). For example, a DocumentFragment might have only one
child and that child node could be a Text node. Such a
structure model represents neither an HTML document nor a well-formed XML
document.
When a DocumentFragment is inserted into a Document
(or indeed any other Node that may take children) the
children of the DocumentFragment and not the
DocumentFragment itself are inserted into the Node.
This makes the DocumentFragment very useful when the user
wishes to create nodes that are siblings; the DocumentFragment
acts as the parent of these nodes so that the user can use the standard
methods from the Node interface, such as insertBefore()
and appendChild().
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