This chapter describes the optional DOM Level 2 Traversal
feature. Its TreeWalker, NodeIterator, and
NodeFilter interfaces provide easy-to-use, robust, selective
traversal of a document's contents. A DOM application can use the
hasFeature method of the DOMImplementation interface
to determine whether this feature is supported or not. The feature string for
all the interfaces listed in this section is "Traversal" and
the version is "2.0".
NodeIterators and TreeWalkers are two
different ways of representing the nodes of a document subtree and a position
within the nodes they present. A NodeIterator presents a flattened
view of the subtree as an ordered sequence of nodes, presented in document
order. Because this view is presented without respect to hierarchy, iterators
have methods to move forward and backward, but not to move up and down.
Conversely, a TreeWalker maintains the hierarchical relationships
of the subtree, allowing navigation of this hierarchy. In general,
TreeWalkers are better for tasks in which the structure of the
document around selected nodes will be manipulated, while
NodeIterators are better for tasks that focus on the content of
each selected node.
NodeIterators and TreeWalkers each present a
view of a document subtree that may not contain all nodes found in the subtree.
In this specification, we refer to this as the
logical view to distinguish it from
the physical view, which corresponds to the document subtree per
se. When an iterator or TreeWalker is created, it may be
associated with a NodeFilter, which examines each node and
determines whether it should appear in the logical view. In addition, flags may
be used to specify which node types should occur in the logical view.
NodeIterators and TreeWalkers are dynamic -
the logical view changes to reflect changes made to the underlying document.
However, they differ in how they respond to those changes.
NodeIterators, which present the nodes sequentially, attempt to
maintain their location relative to a position in that sequence when the
sequence's contents change. TreeWalkers, which present the nodes
as a filtered tree, maintain their location relative to their current node and
remain attached to that node if it is moved to a new context. We will discuss
these behaviors in greater detail below.
NodeIteratorsA NodeIterator allows the members of a list of nodes to
be returned sequentially. In the current DOM interfaces, this list will always
consist of the nodes of a subtree, presented in document order. When an
iterator is first created, calling its nextNode() method returns
the first node in the logical view of the subtree; in most cases, this is the
root of the subtree. Each successive call advances the
NodeIterator through the list, returning the next
node available in the logical view. When no more nodes are visible,
nextNode() returns null.
NodeIterators are created using the
createNodeIterator method found in the
DocumentTraversal interface. When a NodeIterator is
created, flags can be used to determine which node types will be "visible" and
which nodes will be "invisible" while traversing the tree; these flags can be
combined using the OR operator. Nodes that are "invisible" are
skipped over by the iterator as though they did not exist.
The following code creates an iterator, then calls a function to print the name of each element:
NodeIterator iter=
((DocumentTraversal)document).createNodeIterator(
root, NodeFilter.SHOW_ELEMENT, null);
while (Node n = iter.nextNode())
printMe(n);
NodeIterators present nodes as an ordered list, and
move forward and backward within this list. The iterator's position is always
either between two nodes, before the first node, or after the last node. When
an iterator is first created, the position is set before the first item. The
following diagram shows the list view that an iterator might provide for a
particular subtree, with the position indicated by an asterisk '*' :
* A B C D E F G H I
Each call to nextNode() returns the next node and
advances the position. For instance, if we start with the above position, the
first call to nextNode() returns "A" and advances the
iterator:
[A] * B C D E F G H I
The position of a NodeIterator can best be described
with respect to the last node returned, which we will call the reference
node. When an iterator is created, the first node is the reference node,
and the iterator is positioned before the reference node. In these diagrams, we
use square brackets to indicate the reference node.
A call to previousNode() returns the previous node and
moves the position backward. For instance, if we start with the
NodeIterator between "A" and "B", it would
return "A" and move to the position shown below:
* [A] B C D E F G H I
If nextNode() is called at the end of a list, or
previousNode() is called at the beginning of a list, it returns
null and does not change the position of the iterator. When a
NodeIterator is first created, the reference node is the first
node:
* [A] B C D E F G H I
A NodeIterator may be active while the data structure it
navigates is being edited, so an iterator must behave gracefully in the face of
change. Additions and removals in the underlying data structure do not
invalidate a NodeIterator; in fact, a NodeIterator is
never invalidated unless its detach() method is invoked. To make
this possible, the iterator uses the reference node to maintain its position.
The state of an iterator also depends on whether the iterator is positioned
before or after the reference node.
If changes to the iterated list do not remove the reference node, they do
not affect the state of the NodeIterator. For instance, the
iterator's state is not affected by inserting new nodes in the vicinity of the
iterator or removing nodes other than the reference node. Suppose we start from
the following position:
A B C [D] * E F G H I
Now let's remove "E". The resulting state is:
A B C [D] * F G H I
If a new node is inserted, the NodeIterator stays close to the
reference node, so if a node is inserted between "D" and "F", it will occur
between the iterator and "F":
A B C [D] * X F G H I
Moving a node is equivalent to a removal followed by an insertion. If we move "I" to the position before "X" the result is:
A B C [D] * I X F G H
If the reference node is removed from the list being iterated over, a
different node is selected as the reference node. If the reference node's
position is before that of the NodeIterator, which is usually the
case after nextNode() has been called, the nearest node before the
iterator is chosen as the new reference node. Suppose we remove the "D" node,
starting from the following state:
A B C [D] * F G H I
The "C" node becomes the new reference node, since it is the nearest node to
the NodeIterator that is before the iterator:
A B [C] * F G H I
If the reference node is after the NodeIterator, which is
usually the case after previousNode() has been called, the nearest
node after the iterator is chosen as the new reference node. Suppose we remove
"E", starting from the following state:
A B C D * [E] F G H I
The "F" node becomes the new reference node, since it is the nearest node to
the NodeIterator that is after the iterator:
A B C D * [F] G H I
As noted above, moving a node is equivalent to a removal followed by an insertion. Suppose we wish to move the "D" node to the end of the list, starting from the following state:
A B C [D] * F G H I C
The resulting state is as follows:
A B [C] * F G H I D
One special case arises when the reference node is the last node in the list and the reference node is removed. Suppose we remove node "C", starting from the following state:
A B * [C]
According to the rules we have given, the new reference node should be the
nearest node after the NodeIterator, but there are no further
nodes after "C". The same situation can arise when previousNode()
has just returned the first node in the list, which is then removed. Hence: If
there is no node in the original direction of the reference node, the nearest
node in the opposite direction is selected as the reference node:
A [B] *
If the NodeIterator is positioned within a block of nodes that
is removed, the above rules clearly indicate what is to be done. For instance,
suppose "C" is the parent node of "D", "E", and "F", and we remove "C",
starting with the following state:
A B C [D] * E F G H I D
The resulting state is as follows:
A [B] * G H I D
Finally, note that removing a NodeIterator's root
node from its parent does not alter the list being iterated over, and thus does
not change the iterator's state.
The underlying data structure that is being iterated may contain nodes that
are not part of the logical view, and therefore will not be returned by the
NodeIterator. If nodes that are to be excluded because of the
value of the whatToShow flag, nextNode() returns the
next visible node, skipping over the excluded "invisible" nodes. If a
NodeFilter is present, it is applied before returning a node; if
the filter does not accept the node, the process is repeated until a node is
accepted by the filter and is returned. If no visible nodes are encountered, a
null is returned and the iterator is positioned at the end of the
list. In this case, the reference node is the last node in the list, whether or
not it is visible. The same approach is taken, in the opposite direction, for
previousNode().
In the following examples, we will use lowercase letters to represent nodes that are in the data structure, but which are not in the logical view. For instance, consider the following list:
A [B] * c d E F G
A call to nextNode() returns E and advances to the following
position:
A B c d [E] * F G
Nodes that are not visible may nevertheless be used as reference nodes if a reference node is removed. Suppose node "E" is removed, started from the state given above. The resulting state is:
A B c [d] * F G
Suppose a new node "X", which is visible, is inserted before "d". The resulting state is:
A B c X [d] * F G
Note that a call to previousNode() now returns node X. It is
important not to skip over invisible nodes when the reference node is removed,
because there are cases, like the one just given above, where the wrong results
will be returned. When "E" was removed, if the new reference node had been "B"
rather than "d", calling previousNode() would not return "X".
NodeFiltersNodeFilters allow the user to create objects that "filter out"
nodes. Each filter contains a user-written function that looks at a node and
determines whether or not it should be presented as part of the traversal's
logical view of the document. To use a NodeFilter, you create a
NodeIterator or a TreeWalker that uses the filter.
The traversal engine applies the filter to each node, and if the filter does
not accept the node, traversal skips over the node as though it were not
present in the document. NodeFilters need not know how to navigate
the structure that contains the nodes on which they operate.
Filters will be consulted when a traversal operation is performed, or when a
NodeIterator's reference node is removed from the subtree being
iterated over and it must select a new one. However, the exact timing of these
filter calls may vary from one DOM implementation to another. For that reason,
NodeFilters should not attempt to maintain state based on the
history of past invocations; the resulting behavior may not be portable.
Similarly, TreeWalkers and NodeIterators should
behave as if they have no memory of past filter results, and no anticipation of
future results. If the conditions a NodeFilter is examining have
changed (e.g., an attribute which it tests has been added or removed) since the
last time the traversal logic examined this node, this change in visibility
will be discovered only when the next traversal operation is performed. For
example: if the filtering for the current node changes from
FILTER_SHOW to FILTER_SKIP, a TreeWalker
will be able to navigate off that node in any direction, but not back to it
unless the filtering conditions change again. NodeFilters which
change during a traversal can be written, but their behavior may be confusing
and they should be avoided when possible.
NodeFiltersA NodeFilter contains one method named
acceptNode(), which allows a NodeIterator or
TreeWalker to pass a Node to a filter and ask whether
it should be present in the logical view. The acceptNode()
function returns one of three values to state how the Node should
be treated. If acceptNode() returns FILTER_ACCEPT,
the Node will be present in the logical view; if it returns
FILTER_SKIP, the Node will not be present in the
logical view, but the children of the Node may; if it returns
FILTER_REJECT, neither the Node nor its
descendants
will be present in the logical view. Since iterators present nodes as an
ordered list, without hierarchy, FILTER_REJECT and
FILTER_SKIP are synonyms for NodeIterators, skipping
only the single current node.
Consider a filter that accepts the named anchors in an HTML document. In
HTML, an HREF can refer to any A element that has a NAME attribute. Here is a
NodeFilter in Java that looks at a node and determines whether it
is a named anchor:
class NamedAnchorFilter implements NodeFilter
{
short acceptNode(Node n) {
if (n.getNodeType()==Node.ELEMENT_NODE) {
Element e = (Element)n;
if (! e.getNodeName().equals("A"))
return FILTER_SKIP;
if (e.getAttributeNode("NAME") != null)
return FILTER_ACCEPT;
}
return FILTER_SKIP;
}
}
If the above NodeFilter were to be used only with
NodeIterators, it could have used FILTER_REJECT
wherever FILTER_SKIP is used, and the behavior would not change.
For TreeWalker, though, FILTER_REJECT would reject
the children of any element that is not a named anchor, and since named anchors
are always contained within other elements, this would have meant that no named
anchors would be found. FILTER_SKIP rejects the given node, but
continues to examine the children; therefore, the above filter will work with
either a NodeIterator or a TreeWalker.
To use this filter, the user would create an instance of the
NodeFilter and create a NodeIterator using it:
NamedAnchorFilter myFilter = new NamedAnchorFilter();
NodeIterator iter=
((DocumentTraversal)document).createNodeIterator(
node, NodeFilter.SHOW_ELEMENT, myFilter);
Note that the use of the SHOW_ELEMENT flag is not strictly
necessary in this example, since our sample NodeFilter tests the
nodeType. However, some implementations of the Traversal
interfaces may be able to improve whatToShow performance by taking
advantage of knowledge of the document's structure, which makes the use of
SHOW_ELEMENT worthwhile. Conversely, while we could remove the
nodeType test from our filter, that would make it dependent upon
whatToShow to distinguish between Elements,
Attr's, and ProcessingInstructions.
NodeFilters and ExceptionsWhen writing a NodeFilter, users should avoid writing code that
can throw an exception. However, because a DOM implementation can not prevent
exceptions from being thrown, it is important that the behavior of filters that
throw an exception be well-defined. A TreeWalker or
NodeIterator does not catch or alter an exception thrown by a
filter, but lets it propagate up to the user's code. The following functions
may invoke a NodeFilter, and may therefore propagate an exception
if one is thrown by a filter:
NodeIterator.nextNode() NodeIterator.previousNode() TreeWalker.firstChild() TreeWalker.lastChild() TreeWalker.nextSibling() TreeWalker.previousSibling() TreeWalker.nextNode() TreeWalker.previousNode() TreeWalker.parentNode() NodeFilters and Document MutationWell-designed NodeFilters should not have to modify the
underlying structure of the document. But a DOM implementation can not prevent
a user from writing filter code that does alter the document structure.
Traversal does not provide any special processing to handle this case. For
instance, if a NodeFilter removes a node from a document, it can
still accept the node, which means that the node may be returned by the
NodeIterator or TreeWalker even though it is no
longer in the subtree being traversed. In general, this may lead to
inconsistent, confusing results, so we encourage users to write
NodeFilters that make no changes to document structures. Instead,
do your editing in the loop controlled by the traversal object.
NodeFilters and whatToShow flags NodeIterator and TreeWalker apply their
whatToShow flags before applying filters. If a node is
skipped by the active whatToShow flags, a NodeFilter
will not be called to evaluate that node. Please note that this behavior is
similar to that of FILTER_SKIP; children of that node will be
considered, and filters may be called to evaluate them. Also note that it will
in fact be a "skip" even if the NodeFilter would have preferred to
reject the entire subtree; if this would cause a problem in your application,
consider setting whatToShow to SHOW_ALL and
performing the nodeType test inside your filter.
TreeWalkerThe TreeWalker interface provides many of the same benefits as
the NodeIterator interface. The main difference between these two
interfaces is that the TreeWalker presents a tree-oriented view of
the nodes in a subtree, rather than the iterator's list-oriented view. In other
words, an iterator allows you to move forward or back, but a
TreeWalker allows you to also move to the parent of a node, to one
of its children, or to a sibling.
Using a TreeWalker is quite similar to navigation using the
Node directly, and the navigation methods for the two interfaces are analogous.
For instance, here is a function that recursively walks over a tree of nodes in
document order, taking separate actions when first entering a node and after
processing any children:
processMe(Node n) {
nodeStartActions(n);
for (Node child=n.firstChild();
child != null;
child=child.nextSibling()) {
processMe(child);
}
nodeEndActions(n);
}
Doing the same thing using a TreeWalker is quite similar. There
is one difference: since navigation on the TreeWalker changes the
current position, the position at the end of the function has changed. A
read/write attribute named currentNode allows the current node for
a TreeWalker to be both queried and set. We will use this to
ensure that the position of the TreeWalker is restored when this
function is completed:
processMe(TreeWalker tw) {
Node n = tw.getCurrentNode();
nodeStartActions(tw);
for (Node child=tw.firstChild();
child!=null;
child=tw.nextSibling()) {
processMe(tw);
}
tw.setCurrentNode(n);
nodeEndActions(tw);
}
The advantage of using a TreeWalker instead of direct
Node navigation is that the TreeWalker allows the
user to choose an appropriate view of the tree. Flags may be used to show or
hide Comments or ProcessingInstructions; entities may
be expanded or shown as EntityReference nodes. In addition,
NodeFilters may be used to present a custom view of the tree.
Suppose a program needs a view of a document that shows which tables occur in
each chapter, listed by chapter. In this view, only the chapter elements and
the tables that they contain are seen. The first step is to write an
appropriate filter:
class TablesInChapters implements NodeFilter {
short acceptNode(Node n) {
if (n.getNodeType()==Node.ELEMENT_NODE) {
if (n.getNodeName().equals("CHAPTER"))
return FILTER_ACCEPT;
if (n.getNodeName().equals("TABLE"))
return FILTER_ACCEPT;
if (n.getNodeName().equals("SECT1")
|| n.getNodeName().equals("SECT2")
|| n.getNodeName().equals("SECT3")
|| n.getNodeName().equals("SECT4")
|| n.getNodeName().equals("SECT5")
|| n.getNodeName().equals("SECT6")
|| n.getNodeName().equals("SECT7"))
return FILTER_SKIP;
}
return FILTER_REJECT;
}
}
This filter assumes that TABLE elements are contained directly in CHAPTER or SECTn elements. If another kind of element is encountered, it and its children are rejected. If a SECTn element is encountered, it is skipped, but its children are explored to see if they contain any TABLE elements.
Now the program can create an instance of this NodeFilter,
create a TreeWalker that uses it, and pass this
TreeWalker to our ProcessMe() function:
TablesInChapters tablesInChapters = new TablesInChapters();
TreeWalker tw =
((DocumentTraversal)document).createTreeWalker(
root, NodeFilter.SHOW_ELEMENT, tablesInChapters);
processMe(tw);
(Again, we've chosen to both test the nodeType in the filter's
logic and use SHOW_ELEMENT, for the reasons discussed in the
earlier NodeIterator example.)
Without making any changes to the above ProcessMe() function,
it now processes only the CHAPTER and TABLE elements. The programmer can write
other filters or set other flags to choose different sets of nodes; if
functions use TreeWalker to navigate, they will support any view
of the document defined with a TreeWalker.
Note that the structure of a TreeWalker's filtered view of a
document may differ significantly from that of the document itself. For
example, a TreeWalker with only SHOW_TEXT specified
in its whatToShow parameter would present all the
Text nodes as if they were siblings of each other yet had no
parent.
As with NodeIterators, a TreeWalker may be active
while the data structure it navigates is being edited, and must behave
gracefully in the face of change. Additions and removals in the underlying data
structure do not invalidate a TreeWalker; in fact, a
TreeWalker is never invalidated.
But a TreeWalker's response to these changes is quite different
from that of a NodeIterator. While NodeIterators
respond to editing by maintaining their position within the list that they are
iterating over, TreeWalkers will instead remain attached to their
currentNode. All the TreeWalker's navigation methods
operate in terms of the context of the currentNode at the time
they are invoked, no matter what has happened to, or around, that node since
the last time the TreeWalker was accessed. This remains true even
if the currentNode is moved out of its original subtree.
As an example, consider the following document fragment:
... <subtree> <twRoot> <currentNode/> <anotherNode/> </twRoot> </subtree> ...
Let's say we have created a TreeWalker whose root
node is the <twRoot/> element and whose currentNode is the
<currentNode/> element. For this illustration, we will assume that all
the nodes shown above are accepted by the TreeWalker's
whatToShow and filter settings.
If we use removeChild() to remove the <currentNode/>
element from its parent, that element remains the TreeWalker's
currentNode, even though it is no longer within the
root node's subtree. We can still use the TreeWalker
to navigate through any children that the orphaned currentNode may
have, but are no longer able to navigate outward from the
currentNode since there is no parent available.
If we use insertBefore() or appendChild() to give
the <currentNode/> a new parent, then TreeWalker navigation
will operate from the currentNode's new location. For example, if
we inserted the <currentNode/> immediately after the <anotherNode/>
element, the TreeWalker's previousSibling() operation
would move it back to the <anotherNode/>, and calling
parentNode() would move it up to the <twRoot/>.
If we instead insert the currentNode into the <subtree/>
element, like so:
... <subtree> <currentNode/> <twRoot> <anotherNode/> </twRoot> </subtree> ...
we have moved the currentNode out from under the
TreeWalker's root node. This does not invalidate the
TreeWalker; it may still be used to navigate relative to the
currentNode. Calling its parentNode() operation, for
example, would move it to the <subtree/> element, even though that too is
outside the original root node. However, if the
TreeWalker's navigation should take it back into the original
root node's subtree -- for example, if rather than calling
parentNode() we called nextNode(), moving the
TreeWalker to the <twRoot/> element -- the root
node will "recapture" the TreeWalker, and prevent it from
traversing back out.
This becomes a bit more complicated when filters are in use. Relocation of
the currentNode -- or explicit selection of a new
currentNode, or changes in the conditions that the
NodeFilter is basing its decisions on -- can result in a
TreeWalker having a currentNode which would not
otherwise be visible in the filtered (logical) view of the document. This node
can be thought of as a "transient member" of that view. When you ask the
TreeWalker to navigate off this node the result will be just as if
it had been visible, but you may be unable to navigate back to it unless
conditions change to make it visible again.
In particular: If the currentNode becomes part of a subtree
that would otherwise have been Rejected by the filter, that entire subtree may
be added as transient members of the logical view. You will be able to navigate
within that subtree (subject to all the usual filtering) until you move upward
past the Rejected ancestor. The behavior is as if the Rejected node had only
been Skipped (since we somehow wound up inside its subtree) until we leave it;
thereafter, standard filtering applies.
Iterators are used to step through a set of nodes, e.g. the set
of nodes in a NodeList, the document subtree governed by a
particular Node, the results of a query, or any other set of
nodes. The set of nodes to be iterated is determined by the implementation of
the NodeIterator. DOM Level 2 specifies a single
NodeIterator implementation for document-order traversal of a
document subtree. Instances of these iterators are created by calling
DocumentTraversal.createNodeIterator().
// Introduced in DOM Level 2:
interface NodeIterator {
readonly attribute Node root;
readonly attribute unsigned long whatToShow;
readonly attribute NodeFilter filter;
readonly attribute boolean expandEntityReferences;
Node nextNode()
raises(DOMException);
Node previousNode()
raises(DOMException);
void detach();
};
expandEntityReferences of type boolean, readonlywhatToShow and the filter. Also note that this is
currently the only situation where NodeIterators may reject a
complete subtree rather than skipping individual nodes.
whatToShow flags to hide the entity reference node and set
expandEntityReferences to true when creating the iterator. To
produce a view of the document that has entity reference nodes but no entity
expansion, use the whatToShow flags to show the entity reference
node and set expandEntityReferences to false.
filter of type NodeFilter, readonlyNodeFilter used to screen nodes.
root of type Node, readonlyNodeIterator, as specified when it was
created.
whatToShow of type unsigned long, readonlyNodeFilter
interface.
Nodes not accepted by whatToShow will be skipped, but their
children may still be considered. Note that this skip takes precedence over the
filter, if any.
detachNodeIterator from the set which it iterated over,
releasing any computational resources and placing the iterator in the INVALID
state. After detach has been invoked, calls to
nextNode or previousNode will raise the exception
INVALID_STATE_ERR.
nextNodeNodeIterator is created, the first call to
nextNode() returns the first node in the set.
|
The next |
|
INVALID_STATE_ERR: Raised if this method is called after the
|
previousNodeNodeIterator backwards in the set.
|
The previous |
|
INVALID_STATE_ERR: Raised if this method is called after the
|
Filters are objects that know how to "filter out" nodes. If a
NodeIterator or TreeWalker is given a
NodeFilter, it applies the filter before it returns the next node.
If the filter says to accept the node, the traversal logic returns it;
otherwise, traversal looks for the next node and pretends that the node that
was rejected was not there.
The DOM does not provide any filters. NodeFilter is just an
interface that users can implement to provide their own filters.
NodeFilters do not need to know how to traverse from node to
node, nor do they need to know anything about the data structure that is being
traversed. This makes it very easy to write filters, since the only thing they
have to know how to do is evaluate a single node. One filter may be used with a
number of different kinds of traversals, encouraging code reuse.
// Introduced in DOM Level 2:
interface NodeFilter {
// Constants returned by acceptNode
const short FILTER_ACCEPT = 1;
const short FILTER_REJECT = 2;
const short FILTER_SKIP = 3;
// Constants for whatToShow
const unsigned long SHOW_ALL = 0xFFFFFFFF;
const unsigned long SHOW_ELEMENT = 0x00000001;
const unsigned long SHOW_ATTRIBUTE = 0x00000002;
const unsigned long SHOW_TEXT = 0x00000004;
const unsigned long SHOW_CDATA_SECTION = 0x00000008;
const unsigned long SHOW_ENTITY_REFERENCE = 0x00000010;
const unsigned long SHOW_ENTITY = 0x00000020;
const unsigned long SHOW_PROCESSING_INSTRUCTION = 0x00000040;
const unsigned long SHOW_COMMENT = 0x00000080;
const unsigned long SHOW_DOCUMENT = 0x00000100;
const unsigned long SHOW_DOCUMENT_TYPE = 0x00000200;
const unsigned long SHOW_DOCUMENT_FRAGMENT = 0x00000400;
const unsigned long SHOW_NOTATION = 0x00000800;
short acceptNode(in Node n);
};
The following constants are returned by the acceptNode() method:
FILTER_ACCEPTNodeIterator or
TreeWalker will return this node.FILTER_REJECTNodeIterator or
TreeWalker will not return this node. For TreeWalker,
the children of this node will also be rejected. NodeIterators
treat this as a synonym for FILTER_SKIP.FILTER_SKIPNodeIterator or TreeWalker will not return this node.
For both NodeIterator and TreeWalker, the children of
this node will still be considered. These are the available values for the whatToShow parameter
used in TreeWalkers and NodeIterators. They are the
same as the set of possible types for Node, and their values are
derived by using a bit position corresponding to the value of
nodeType for the equivalent node type.
If a bit in whatToShow is set false, that will be taken as a
request to skip over this type of node; the behavior in that case is similar to
that of FILTER_SKIP.
Note that if node types greater than 32 are ever introduced, they may not
be individually testable via whatToShow. If that need should
arise, it can be handled by selecting SHOW_ALL together with an
appropriate NodeFilter.
SHOW_ALLNodes.SHOW_ATTRIBUTEAttr nodes. This is meaningful only when creating an
iterator or tree-walker with an attribute node as its root; in
this case, it means that the attribute node will appear in the first position
of the iteration or traversal. Since attributes are never children of other
nodes, they do not appear when traversing over the document tree.SHOW_CDATA_SECTIONCDATASection nodes.SHOW_COMMENTComment nodes.SHOW_DOCUMENTDocument nodes.SHOW_DOCUMENT_FRAGMENTDocumentFragment nodes.SHOW_DOCUMENT_TYPEDocumentType nodes.SHOW_ELEMENTElement nodes.SHOW_ENTITYEntity nodes. This is meaningful only when creating an
iterator or tree-walker with an Entity node as its
root; in this case, it means that the Entity node
will appear in the first position of the traversal. Since entities are not part
of the document tree, they do not appear when traversing over the document
tree.SHOW_ENTITY_REFERENCEEntityReference nodes.SHOW_NOTATIONNotation nodes. This is meaningful only when creating an
iterator or tree-walker with a Notation node as its
root; in this case, it means that the Notation node
will appear in the first position of the traversal. Since notations are not
part of the document tree, they do not appear when traversing over the document
tree.SHOW_PROCESSING_INSTRUCTIONProcessingInstruction nodes.SHOW_TEXTText nodes.acceptNodeTreeWalker or NodeIterator. This function will be
called by the implementation of TreeWalker and
NodeIterator; it is not normally called directly from user code.
(Though you could do so if you wanted to use the same filter to guide your own
application logic.)n of type
Node
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a constant to determine whether the node is accepted, rejected, or skipped, as defined above. |
TreeWalker objects are used to navigate a document tree or
subtree using the view of the document defined by their whatToShow
flags and filter (if any). Any function which performs navigation using a
TreeWalker will automatically support any view defined by a
TreeWalker.
Omitting nodes from the logical view of a subtree can result in a structure
that is substantially different from the same subtree in the complete,
unfiltered document. Nodes that are siblings in the TreeWalker
view may be children of different, widely separated nodes in the original view.
For instance, consider a NodeFilter that skips all nodes except
for Text nodes and the root node of a document. In the logical view that
results, all text nodes will be siblings and appear as direct children of the
root node, no matter how deeply nested the structure of the original
document.
// Introduced in DOM Level 2:
interface TreeWalker {
readonly attribute Node root;
readonly attribute unsigned long whatToShow;
readonly attribute NodeFilter filter;
readonly attribute boolean expandEntityReferences;
attribute Node currentNode;
// raises(DOMException) on setting
Node parentNode();
Node firstChild();
Node lastChild();
Node previousSibling();
Node nextSibling();
Node previousNode();
Node nextNode();
};
currentNode of type NodeTreeWalker is currently positioned.TreeWalker's associated filter.
currentNode may also be explicitly set to any node, whether or not
it is within the subtree specified by the root node or would be
accepted by the filter and whatToShow flags. Further traversal
occurs relative to currentNode even if it is not part of the
current view, by applying the filters in the requested direction; if no
traversal is possible, currentNode is not changed.
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NOT_SUPPORTED_ERR: Raised if an attempt is made to set
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expandEntityReferences of type boolean, readonlyTreeWalker. If false, they
and their descendants will be rejected. Note that this rejection takes
precedence over whatToShow and the filter, if any.
whatToShow flags to hide the entity reference node and set
expandEntityReferences to true when creating the
TreeWalker. To produce a view of the document that has entity
reference nodes but no entity expansion, use the whatToShow flags
to show the entity reference node and set expandEntityReferences
to false.
filter of type NodeFilter, readonly
root of type Node, readonlyroot node of the TreeWalker, as specified when
it was created.
whatToShow of type unsigned long, readonlyTreeWalker. The available set of constants is defined in the
NodeFilter interface.
Nodes not accepted by whatToShow will be skipped, but their
children may still be considered. Note that this skip takes precedence over the
filter, if any.
firstChildTreeWalker to the first visible child of the current
node, and returns the new node. If the current node has no visible children,
returns null, and retains the current node.
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The new node, or |
lastChildTreeWalker to the last visible child of the current
node, and returns the new node. If the current node has no visible children,
returns null, and retains the current node.
|
The new node, or |
nextNodeTreeWalker to the next visible node in document order
relative to the current node, and returns the new node. If the current node has
no next node, or if the search for nextNode attempts to step upward from the
TreeWalker's root node, returns null,
and retains the current node.
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The new node, or |
nextSiblingTreeWalker to the next sibling of the current node,
and returns the new node. If the current node has no visible next sibling,
returns null, and retains the current node.
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The new node, or |
parentNodeparentNode attempts to step upward from the
TreeWalker's root node, or if it fails to find a
visible ancestor node, this method retains the current position and returns
null.
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The new parent node, or |
previousNodeTreeWalker to the previous visible node in document
order relative to the current node, and returns the new node. If the current
node has no previous node,
or if the search for previousNode attempts to step upward from the
TreeWalker's root node,
returns null, and retains the current node.
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The new node, or |
previousSiblingTreeWalker to the previous sibling of the current
node, and returns the new node. If the current node has no visible previous
sibling, returns null, and retains the current node.
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The new node, or |
DocumentTraversal contains methods that create iterators and
tree-walkers to traverse a node and its children in document order (depth
first, pre-order traversal, which is equivalent to the order in which the start
tags occur in the text representation of the document). In DOMs which support
the Traversal feature, DocumentTraversal will be implemented by
the same objects that implement the Document interface.
// Introduced in DOM Level 2:
interface DocumentTraversal {
NodeIterator createNodeIterator(in Node root,
in unsigned long whatToShow,
in NodeFilter filter,
in boolean entityReferenceExpansion)
raises(DOMException);
TreeWalker createTreeWalker(in Node root,
in unsigned long whatToShow,
in NodeFilter filter,
in boolean entityReferenceExpansion)
raises(DOMException);
};
createNodeIteratorNodeIterator over the subtree rooted at the
specified node.root of type
NodewhatToShow flags
and the filter, if any, are not considered when setting this position. The root
must not be null.whatToShow of type
unsigned longNodeFilter
for the set of possible SHOW_ values.OR.filter of type
NodeFilterNodeFilter to be used with this TreeWalker, or
null to indicate no filter.entityReferenceExpansion of type
boolean|
The newly created |
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NOT_SUPPORTED_ERR: Raised if the specified |
createTreeWalkerTreeWalker over the subtree rooted at the
specified node.root of type
Noderoot for the
TreeWalker. The whatToShow flags and the
NodeFilter are not considered when setting this value; any node
type will be accepted as the root. The currentNode of
the TreeWalker is initialized to this node, whether or not it is
visible. The root functions as a stopping point for traversal
methods that look upward in the document structure, such as
parentNode and nextNode. The root must not be
null.whatToShow of type
unsigned longNodeFilter for the set of possible SHOW_ values.OR.filter of type
NodeFilterNodeFilter to be used with this TreeWalker, or
null to indicate no filter.entityReferenceExpansion of type
booleanEntityReference nodes
are not presented in the logical view.|
The newly created |
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NOT_SUPPORTED_ERR: Raised if the specified |