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NodeIterator

From: Arkin <arkin@trendline.co.il>
Date: Thu, 04 Mar 1999 18:07:07 -0500
Message-ID: <36DF121B.6B3D2657@trendline.co.il>
To: "'DOM list'" <www-dom@w3.org>
A few observations regarding the iterators and filters interface, and
recommendations for changes:

1. The factory methods are now defined for the Document class rather
than the Node class, with no clear explanation of reason.

An iterator is constructed for a specific node, a node that might belong
to a document fragment (not a document). The factory method must check
that the supplied node belongs to the document. All these are compelling
reasons for keeping the factory methods in the Node interface.

2. Factory for iterator with a NodeFilter is missing

3. The reset() method has been removed without good explanation.

A Java implementation would require a reference from the iterated node
to the iterator, so the iterator may be notified of relevant changes to
the document tree. Even after the iterator is no longer used by the
application, this reference will keep the iterator alive in memory and
prevent garbage collection. Over time, zombie iterators will eat up
valuable memory.

Consider the use of an XML document for holding configuration
information. Each module uses its own iterator to lookup interesting
elements in the central configuration document. Over time, the same
configuration document might be iterated over and over again, creating
new iterators that are never released.

The solution involves an additional method in NodeIterator, allowing the
freeing up of the notification reference permanently (destroy) or
temporatily( reset). While reset is more versatile and allows for
iterator reuse, destory is simpler to expain:

destory
  Notifies the DOM implementation that the iterator will not be used
anymore. Applications should call this method when the iterator is no
longer used, allowing the DOM implementation to release any references
held up by NodeIterator.

reset
  Usage as before. Applications should call this method when the
iterator is no longer used, or when no longer used in a given context,
allowing the DOM implementation to release any references help up by the
NodeIterator. Applications may utilize this method to reuse iterators,
conserving memory by not recreating often used iterators.

4. Use of mask bits for NodeIterator's whatToShow

The Node interface defines unique values for node types; these values
may be turned into masks by means of bit-shift operations, for use as
the NodeIterator's whatToShow argument. This bit-shift operation is easy
to define and very fast to implement in the filtering of node types:

Definition:
  TW_ELEMENT = 0x0002;     // = 1 << 1 (ELEMENT_NODE)
  TW_TEXT = 0x0018;        // = 1 << 3 (TEXT_NODE)
                           // + 1 << 4 (CDATA_SECTION_NODE)
  TW_ENTITYREF = 0x0020;   // = 1 << 5 (ENTITY_REFERENCE_NODE)
  TW_PI = 0x0080;          // = 1 << 7 (PROCESSING_INSTRUCTION_NODE)
  TW_COMMENT = 0x0100;     //  1 << 4 (COMMENT_NODE)

Test case:
  if ( ( 1 << node.getNodeType() & whatToShow ) != 0 )
      return node;

5. Definition of whatToShow constants

While the NodeIterator interface has not been finalized yet, if some
common functionality is agree upon, it should be placed in its current
state to org.w3c.dom.fi.NodeFi or org.w3c.dom.fi.DocumentFi, so
application developers can start working with it, and by working,
uncover potential problems. The extended interface should include both
factory methods and whatToShow constants.


Arkin
Received on Thursday, 4 March 1999 21:58:01 GMT

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