XML IV

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XML IV
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• The Document Object Model
• The Document Object model is a hierarchical
  structure of an XML document.
• It provides a means for accessing, and
  manipulating XML documents.
• It allows access to all the parts of XML
  documents, i.e. the document, its root element,
  child elements, attributes, etc.
• The DOM also provides methods that allow
  manipulations, additions, and deletions to the
  original document.
• Without DOM, XML would be nothing more than a
  storage system for data.
• Generally, DOMs can follow one of three patterns
  Linear, Tree, or Object Models.

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• Linear Models
• Simplest model, detailing the document in a
  linear fashion.
• Illustration Consider if I said Go to page 100
  of a book, go to line 10, and read the 1st word.
• The problem with this is that if I altered the
  earlier section of the book, it would invalidate
  all previous references to the latter sections of
  the book.
• Tree Models
• Describes the document in terms of a tree, and
  each item on the tree is referred to as a node.
  Terminal items are regarded as leaves.
• It describes a root element, and child nodes.
• A parent node refers to an immediately preceding
  node, and sibling nodes have the same parent

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• An advantage of the tree is that any part of the
  document can be reached by walking the tree
• Tree nodes are not as sensitive to change like
  the linear model, but are still somewhat
  sensitive in the sense that if a complete node
  was removed, the relationship between remaining
  nodes might be altered.
• Using the book analogy, I would perhaps say Go
  to the 1st item of the 2nd paragraph of the 4th
  chapter.
• Object Models
• Least sensitive to change.
• Each section of a document has a name property,
  hence even when the document changes, the model
  would still be valid.
• The W3C DOM is a combination of the Tree and
  Object models.

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• Example of a Document tree
• Simple XML Document
• lt?xml version1.0?gt
• lt!--demonstration of a parse tree -- gt
• ltcontact typepersonalgt
• ltnamegtJames Blogglt/namegt
• lttelephonegt02089443243lt/telephonegt
• lt/contactgt
• Document tree

Document
Version
Comment
Element
Element
Element
Text
Text
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• The W3C DOM
• The specification for the W3C DOM can be found at
  http//www.w3.org/TR/REC-DOM-Level-1
• It implements the language of the OMG IDL (Object
  Management Group Interface Definition Language).
• OMG IDL is a language that allows applications to
  communicate with each other even if they are
  written in different programming languages.
• Under the IDL, objects expose various interfaces.
• Each interface has a series of attributes that
  describe the properties of the object behind that
  interface.
• The object can also be operated on by methods
  that activate functions behind the interface.
  These methods will return some kind of value to
  the requesting application.
• Together, the attributes and methods constitute
  the API of that object.

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• DOM Objects
• An object that supports DOM must be capable of
  loading an XML document into itself.
• It must also be able to expose all the interfaces
  with the appropriate attributes and methods, as
  laid out in the DOM specification.
• DOM Interfaces
• DOMImplementation This is a query to the object
  itself, and is independent of the document loaded
  into it.
• Document This interface provides information
  about the document that has been loaded into the
  object.
• Node Everything in a document can be considered
  as a node of the document, i.e. elements,
  comments, etc. It contains several attributes and
  methods for manipulating any kind of node.

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• NodeList This is an ordered collection of nodes
  that can be accessed using an index.
• NamedNodeMap Collection of nodes that can be
  accessed by name.
• CharacterData Deals with the text of the
  document.
• Attr Deals with the XML attributes of a node.
  However, it is not represented in the tree
  structure as it is considered to be a property of
  its element.
• Element Most nodes of an XML document will be
  elements. The Element interface has properties
  and methods for dealing with elements and their
  XML attributes
• Text Deals with the text content of an element
• CDATASection, Notation, Entity, EntityReference
  and Processing instructions interfaces all deal
  with their namesake nodes.

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• Before considering DOM interfaces in detail, Ill
  introduce the concept of XML Data Source Objects
  (DSO), as well use this in the examples on DOM
  interfaces.
• Consider the following HTML file
• lthtmlgtltheadgtlttitlegtXML DSOlt/titlegtlt/headgt
• ltbodygt
• ltxml idislegt
• ltaddressbookgt...........lt/ addressbook gt
• lt/xmlgt
• ltscript languageJavaScriptgt
• var MyDocisle
• var rootElMyDoc.documentElement
• document.write(rootEl.nodeName)
• lt/scriptgt
• lt/bodygt lt/htmlgt

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• The example demonstrates the creation of an XML
  DSO or data island within an HTML file, with the
  use of the xml element.
• Note that the xml element is a proprietary
  element of IE5, and will not work on other
  browsers.
• The same xml file can be referenced with the src
  attribute of the xml element, i.e.
• lthtmlgtltheadgtlttitlegtXML DSOlt/titlegtlt/headgt
• ltbodygt
• ltxml idisle srcaddressbook.xmlgt
• lt/xmlgt
• ltscript languageJavaScriptgt
• var MyDocisle
• var rootElMyDoc.documentElement
• document.write(rootEl.nodeName)
• lt/scriptgt
• lt/bodygt lt/htmlgt

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• The Document Interface
• Returns information about the document.
• It has the following read-only attributes
• doctype Returns the lt!DOCTYPE information in the
  document, with the exception of information on
  the DTD.
• implementation Returns the implementation node.
  Has a boolean method called hasFeatures( ) which
  takes 2 parameters type of document (HTML or
  XML) and version.
• documentElement Most important attribute of the
  Document interface because it returns the root
  element of the document. Its an ideal starting
  point if youre walking the tree to access a
  node.
• Document Interface Methods
• createElement( )
• createDocumentFragment( )
• createTextNode( )

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• createComment( )
• createCDATASection( )
• createProcessingInstruction( )
• createAttribute( )
• createEntityReference( )
• getElementsByTagName( ) Gets a list of all the
  elements of that name, passed to it as an
  argument. It returns a NodeList object. Widely
  used in search routines.
• Accessing Nodes in DOM
• There are 2 ways of accessing nodes in DOM.
• Walking the Tree Start anywhere in the DOM, and
  use methods of the DOM node interface such as
  parentNode( ), firstChild, nextSibling and
  previousSibling.
• Accessing nodes by name This approach uses the
  getElementsByTagName( ) method to get a list of
  elements of that name.

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• Example
• lthtmlgt
• ltheadgtlttitlegtXML Examplelt/titlegt
• lt/headgt
• ltbodygt
• ltxml id'isle'gt
• ltaddressbookgt
• ltcontactgt
• ltnamegtTony Bennlt/namegt ltaddressgt210 Temple
  roadlt/addressgt
• ltcitygtLondonlt/citygt
• ltpostcodegtNW9 0RTlt/postcodegt
• ltphonegt02082049565lt/phonegt
• lt/contactgt
• ltcontactgt
• ltnamegtMary Blairlt/namegt
• ltaddressgt20 St James roadlt/addressgt
• ltcitygtLondonlt/citygt
• ltpostcodegtSE4 0RTlt/postcodegt
• ltphonegt02072049565lt/phonegt

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• lt/contactgt
• lt/addressbookgt
• lt/xmlgt
• ltscriptgt
• var myDocisle
• var contactListmyDoc.getElementsByTagName('contac
  t')
• for (var i0 iltcontactList.length i)
• document.write(contactList.item(i).firstChild.fi
  rstChild.nodeValue)
• document.write('ltbr\/gt')
• document.write(contactList.item(i).firstChild.next
  Sibling.firstChild.nodeValue)
• document.write('ltbr\/gt ltbr\/gt ')
• lt/scriptgt
• lt/bodygt
• lt/htmlgt

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• The above listing would output
• Tony Benn210 Temple roadMary Blair20 St James
  road
• The Document Fragment Interface
• An utility interface that creates a subsection of
  the document, which can subsequently be inserted
  into the main document.
• Particularly useful when youre creating a lot of
  new elements.
• If you created the new elements in the main
  document directly, the node list would have to be
  updated each time.
• With a DFI, the node list only has to be updated
  once.

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• The Node Interface
• This is the key interface of DOM, as everything
  in a document can be considered as a node.
• Has the following read-only attributes
• nodeName E.g the name of the tag
• nodeValue Null for elements, Content of a text
  node
• nodeType Element, Attr, Text, etc
• parentNode
• childNodes Returns a node list of all child
  nodes of the element
• firstChild
• lastChild
• previousSibling
• nextSibling
• attributes Works only with element type nodes.
  Returns a NamedNodeMap of all the attributes.
• ownerDocument

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• Methods of the Node Interface
• All except the hasChildNodes( ) and cloneNode( )
  take one or more node objects as parameters.
• Prior to using the insertBefore( ), replaceChild(
  ), or appendChild( ) methods, you would have
  created a new node with one of the document
  methods or by the cloneNode( ) method.
• The insertBefore method( ) Inserts the new child
  node before the reference node.
• var dummy
• dummynode_object.insertBefore(new_node,
  reference_node)
• The replaceChild method( ) Replaces the
  reference node, and returns the replaced node.

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• The appendChild( ) method Appends the new node
  to the end of the reference node
• The hasChildNodes method( ) This is a Boolean
  that checks whether a node has child nodes
• The cloneNode method( ) Makes a duplicate of the
  cloned node.
• Example of using node methods
• ltscriptgt
• var dummy
• var myDocisle
• var new_elementmyDoc.createElement('email')
• var rootElmyDoc.documentElement
• dummyrootEl.appendChild(new_element)
• alert('I have just created a new element called
  'dummy.nodeName)
• lt/scriptgt

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• The CharacterData Interface
• Contains attributes and methods for accessing and
  editing strings in nodes that take a string
  value, e.g text and comment nodes.
• Contains the following attributes
• The data attribute Returns all the text in the
  node as a Unicode string.
• Length Returns the number of characters in the
  string.
• It has the following methods (all
  self-explanatory)
• subStringData( ) method
• appendData( ) method
• insertData( ) method
• deleteData( ) method
• replaceData( ) method

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• The Element Interface
• Most of the nodes in a document belong to the
  element or text interfaces.
• However, the node interface deals with most of
  the operations on elements, and the CharacterData
  interface handles most operations on text nodes.
• The majority of the element interface attributes
  and methods are therefore concerned with managing
  its XML attributes properties.
• Methods
• getAttribute( ) Retrieves an attributes value
  by name
• setAttribute( ) Creates an attribute, and sets
  its value at once
• removeAttribute( )
• getAttributeNode( ) When passed a name, it
  retrieves the Attr node.

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• setAttributeNode( )
• removeAttributeNode( )
• getElementsByTagName( ) Similar to when used in
  the document interface.
• The Attr Interface
• Represents an attribute of an element object.
• They are not considered as part of the document
  tree, but as properties of their elements.
• As such, they are accessed by methods of the
  element interface
• Has the following attributes
• name The name of the attribute
• value Its value
• specified A Boolean value that returns true if
  the attribute has been assigned a value, either
  in the original document, or by code.

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• References
• XML Unleashed, Morrison, et al. Chapter 15
• Internet World wide web How to Program, Dietel,
  Dietel and Nieto. Chapter 20.