CSC/ECE 517 Fall 2010/ch6 6g ss

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Document Object Model


Introduction

Browser Scripting Languages are widely used today, one example of which is JavaScript.. It allows the user to interact with the web page by accessing the component parts of a web page. The technique used to access these components is called Document Object Model (DOM). It creates objects corresponding to the components of the web page which scripting languages interact with to manipulate web pages.

What is DOM

Information presented in XML and HTML documents is in a structured format. The technique used to access these documents is called Document Object Model (DOM). DOM is platform language independent interface that allows to create and modify the content, structure and style of documents. These changes can be incorporated in the document again [2][4][5]. DOM is based on the O-O concepts :

Objects are the main part while representing these objects. We can say that Objects are an encapsulation of a set of methods and interfaces. Methods are used to access or change the object’s state and interfaces refer to declaration of a set of methods.

What DOM isn't

1) It does not describe how to persist objects into XML or HTML, but it represents documents as a collection of objects [2].

2) DOM does not describe any internal representation of object or data. It is not a specific set of data structures. For example, even though we have parent-child relationship between the nodes they are logical relationships of objects .

3) In addition, DOM does not describe what information in a document is relevant or how that information in structured. The structuring of the document is stored by the XML Infoset. DOM is just an API for to access this information [3].

History

Before DOM became an official W3C specification[5], the web community had started to develop ways of creating page-based script programs. This early work led to development of Dynamic HTML, which was primarily used for richer user interfaces than plain HTML. At the time, Netscape Navigator was a very popular browser, and also was the first to bring out a programming language that would allow web pages to become interactive. The language, called LiveScript was designed at Netscape Communications and came integrated into the Netscape Navigator browser. LiveScript, now identified as JavaScript is the most widely used browser scripting language today. It has a significant role in development of present DOM. The timeline for DOM evolution is as follows:

  • JavaScript:[6] In December 1995, LiveScript was renamed JavaScript and released as part of Netscape Navigator 2.0.
  • Jscript & vbscript : Internet Explorer was updated to support two integrated languages, vbscript and Jscript. Jscript was very similar to JavaScript. In July 1996, Microsoft released Internet Explorer 3.0 supporting these languages.
  • ECMAScript:[7] Netscape delivered JavaScript to Ecma International for standardization and the work on the specification, ECMA-262, began in November 1996.
  • In June 1997, ECMA adopted a hybrid version of the scripting languages called ECMAScript. However, ECMAScript arrived late for the 4.0 releases of Netscape Navigator and Internet Explorer. Each introduced their own document object model, DHTML and dHTML, that came to be called Dynamic HTML.
  • DOM: At the beginning of 1997, the companies involved in this consortium (including Netscape Communications and Microsoft) decided to find a consensus around their object models to access and manipulate documents. While trying to stay as backward compatible as possible with the original browser object models, the W3C's Document Object Model (DOM) provided a better object representation of HTML documents.
  • DOM on Client:[3] The Internet Explorer 6 and Netscape Navigator 6 browsers have achieved complete support for DOM Level 1. Other browser vendors followed them.
  • DOM on Server:[3] Soon did the companies realized that the benefits of DOM can be applied on server side as well as client side. DOM implementations began appearing for server-side products as well. One well-known is the Xercesparser from Apache Foundation.
  • HTML and XML:[1] In 1996, a new markup language, the Extensible Markup Language (XML), was developed in the W3C as well. Meant to remove the HTML language's extensibility restrictions, the idea of developing an object model for XML quickly became another goal of the DOM effort.
  • In June 2004,[5] Ecma International published ECMA-357 standard, defining an extension to ECMAScript, known as E4X (ECMAScript for XML).

Levels of DOM

Dom has 3 levels each with its own specifications. Generally, developers prefer to use DOM Level2 because of its wide support by Internet Explorer (IE) and Firefox. Here is a brief description of each of these levels.

Level 1

This was the first version of standards published in 1998 and later updated in 2000

Consists of two major parts [3] .

DOM Core : Deals with the internal structure of the nodes of the tree. Also provides, properties and methods to create, edit and manipulate the tree.

DOM HTML : Objects, properties and methods for specific elements associated with HTML documents and tags.

This Level allowed basic document operations, such as creating and deleting nodes, working with their contents, and traversing the document tree. Rest of the operations such as loading and saving documents, etc were left to the developer community. So a lot of this code was implementation dependent.

Level 2

This level of DOM provides additional interfaces to the existing set of functionality of DOM1. It was first proposed in 2000 and the HTML specification was updated in 2003. It has six different specifications [1][3]

DOM2Core : Adds name-space specific methods to DOM Core and gives control of structure of DOM elements

DOM2 HTML : Similar to DOM HTML

DOM2 Events : Control of mouse related events such as targeting, capturing,canceling. This does not handle keyboard-related events

DOM2 STYLE : Gives access to CSS related styling and rules. Also known as DOM2 CSS

DOM2 Traversal and Range : Iterative access to DOM so that navigating and manipulating the document is easy.

DOM2 VIEWS : Ability to access and update a representation of a document.

This level added support for more types of document operations and content, such as events and styles. As result of this, developers had standard ways to handle user events and also work with Cascading Style Sheets (CSS) information in the documents.

Level 3

This level was drafted in 2003 and 2004. It consists of five different specifications [3]

DOM3 Core : Adds more methods and changes to existing core

DOM3 Load and Save : Two major functions provided include 1) content of an XML document can be loaded into your DOM document 2) serialize your DOM document into an XML document

DOM3 Validation : check id dynamic document is valid and conforms to the DOCTYPE.

DOM3 Events : handles keyboard and mouse related events.

DOM3 XPATH : Using XPath features to traverse the document

This level focused on the functionality left out in Level 2 such as Load and Save, etc. Also, support for XPath provides easier navigation through the document. DOM Level 3 introduced two new data types 1) DOMObject : Represents an arbitrary memory object 2) DOMUserData : Represents an interface to an arbitrary block of data in a DOM application.

It includes abstract schema modules which support Document Type Definition (DTD) files, XML Schemas, and other nonspecific schema types. Another feature of Level3 DOM is error handling interfaces.

Navigation

DOM represents a document using a tree structure. This tree can be thought as a collection of individual sub trees. Following example shows a simple HTML page and corresponding DOM.


The <BODY> branch can be thought as its own tree within the larger document tree. Each box in Figure 1 is called a node in DOM terminology. The DOM API provides several interfaces in the Core module for manipulating nodes and inserting/extracting information to and from nodes. There are also methods present for determining and changing the relationships between nodes and discovering the type of a particular mode. Lets first have a look at different object types supported by DOM.

Core Node Object

A node is an object representation of a particular element in the document's context. Nodes have special names, depending on where they are located in the document tree and what their position is relative to other nodes.

All documents have a root node.

Each element in Figure 1 extends from Node object. There are few properties of Node object which are available to all DOM objects are extended from Node object [3][8].

Types of Node Objects

Node Type Description nodeName nodeValue attributes
Element Represents an element in an HTML or XML document, which is represented by tags. In Figure 1, <BODY> node is an element type Tag name null NamedNodeMap
Attribute Represents an element's attribute. A tag with the syntax <img src = "picture.jpg"> has an Attribute node src. Attribute name Attribute value null
Text Represents the textual content of an element. In Figure 1, the <B> node has text child node which represents the text "Bold Text" #text Comment text null
CDATA Selection Represents a Character Data selection in an XML document #cdata-section CDATASection content null
Comment Represents a comment. Comments are of the form <!-- commented text --> #comment Comment text null
Document Represents the root node of a document #document null null
DocumentType Each Document node has a DocumentType node that provides a list of entities defined in the document Name of document type null null
DocumentFragment They are lightweight or minimal Document nodes. They help when we want to extract just a portion of a document for processing #document-fragment null null
ProcessingInstruction Represents instructions to be used by the document processor Target name Content excluding the target null
Entity Represents an entity in XML document Entity name null null
EntityReference Represents an entity reference in the document Name of referenced entity null null
Notation Represents a notation in an XML document. Notations have no parent nodes Notation name null null

Node name, values, types The above table gives a brief idea about different types of node names, their values and a short description about nodes.

Node parents, children, siblings

To easily navigate the document tree, each Node object has number of predefined properties that reference various parts of the tree. Each of these properties references an actual DOM object, with exception of childNodes which references a NodeList of DOM objects (like an array) [2][3].

  • parentNode: references a single direct parent of the specific node.
  • childNodes: references all child elements of a node.
  • firstChild: references first child in child nodes.
  • lastChild: references last child in child nodes.
  • previousSiblings: Represents sibling node immediately before the selected node.
  • nextSiblings: Represents sibling node immediately after the selected node.


Node Attributes

Just like rest of DOM document, attributes are also based on the Node object, but they are not part of the general parent/child relationship tree. Attributes are instances of Attr pbject are contained in a NamedNodeMap in node's attributes property.

Accessing Node Attributes [2][8]: 

x=element.attributeName

if attributeName is a W3C defined attribute and an Attribute Node for the element, (eg id) x gets assigned the value of that Attribute Node if attributeName isn't a W3C defined attribute or an attribute node for the element, x gets assigned the value of the attributeName property for element (JavaScript object) 

x=element.attributes.attributeName.value   or  x= element.attributes['attributeName'].value

if attributeName is an Attribute Node for the element, x gets assigned that node's value. If attributeName isn't an Attribute Node, produce EXCEPTION 

x = element.attributes[indexNumber].value

if attributeName is an Attribute Node for the element, x gets assigned that node's value. If attributeName isn't an Attribute Node, produce EXCEPTION 

x= element.getAttribute('attributeName')

if attributeName is an Attribute Node for the element, x gets assigned its value. If AttributeName isn't an Attribute Node for the element, x gets assigned the null value


Node OwnerDocument Property

This property can be used to reference the root document to which a node belongs.

Core Element Object

Elements inherit from Node object. All element objects have properties and methods of the Node object as well as a few others that facilitate manipulating the attributes of node and locating child element objects.


Methods for manipulating 'attributes' property of base node [2][3]:

  • getAttribute(name) allows to retrieve an attribute based on the name of the attribute as a string.
  • setAttribute(name, value) allows you to set the value of an attribute based on the name of the attribute as a string.
  • removeAttribute(name) allows to remove the value of an attribute based on the name of the attribute as a string.


Methods for manipulating attributes bsed on actual DOM Attr node object [2][3]:

  • getAttributeNode(name): Allows to retrieve the Attr node of the specified attribute.
  • setAttributeNode(newAttr): Allows to set attributes based on new instances of the Attr object
  • removeAttributeNode(oldAttr): Allows to remove the attribute node the same way one can remove a child node using removeChild() method.


Locating Element objects within Element objects [1]:

  • The getElementByTagName() method returns the NodeList object referencing all ancestors with the given tag name.The resulting list is the list of all the elements in the order they appear in DOM Document if read left to right and top to bottom.

Core Document Object

The Document interface represents the entire document. IT serves as the primary gateway to accessing the document's data. It represents root of the document tree. It also contains methods necessary for creating new document objects such as elements,attributes, Text nodes, comments.

The document.documentElement property is a shortcut to the root element of the document.


Creating nodes with document methods [2][3]

  • createAttribute(name): Creates Attr nodes of type Node.ATTRIBUTE_NODE
  • createComment(data): Crates Comment nodes of type Node.COMMENT_NODE
  • crateElement(tagName): Creates element nodes of type Node.ELEMENT_NODE
  • createTextNode(date): Creates Text nodes of type Node.TEXT_NODE


Locating Elements with Document methods [2][3]:

  • The getElementById('ID') method returns one element corresponding to the ID. This method is singular and only returns one element.
  • The getElementsByTagName('TAG') method: Returns a NodeList containing all the elements in the document with the same tag name as TAG. The NodeList is ordered by the order in which the Elements were encountered in a preorder traversal of the document tree. If TAG is *, all tags are matched.
  • The importNode() method: Imports a node into this document from another document. The returned node has no parent node. A copy of source node is created, thus the source document is not affected in any way. The Documents and DocumentTypes nodes cannot be imported.

Example DOM

The diagram [1] below shows DOM representation of a HTML page. The examples below are some of the ways in which elements can be accessed/modified using DOM [8].

To list all the nodes that are children of the body element[3], 

 ADS.addEvent(window, 'load', function() {
   ADS.log.header('List child nodes of the document body')
   for( var i=0 ; i< document..body.childNodes.length ; i++)  {
       ADS.log.write(document.body.childNodes.item(i).nodeName);
   }
});

To elements of 'ListItem' can be accessed using anchor's attributes property[3],: 

 ADS.addEvent(window, 'load', function() {
   ADS.log.header('Attributes')
   var anchor = document.getElementById('ListItem');
   for( var i=0 ; i< anchor.attributes.length; i++)  {
       ADS.log.write(anchor.attributes.item(i) + '=' + anchor.attributes.item(i).nodeValue);
   }
});

Algorithms

There are certain types of document processing algorithms [2] that you can use to traverse DOM nodes. These algorithms help to

  • Determine whether a node is contained within another node
  • Determine whether a node has a sibling of a certain type
  • Finding a node based on the value of an attribute
  • Processing the children of a node


There are two main types of DOM algorithms: position based and content based.

Position based

Position based DOM algorithms are best when you are more interested in position of nodes within a document than concerned with contents of nodes. For example, to determine whether a node occupies particular place in DOM tree is an application o f position based algorithm.

These algorithms work by examining the locations of nodes within a given DOM document. The contents of the node are secondary (if considered at all). Two common position-based algorithms are

  • Determining whether a node has an ancestor node of a particular type
  • Determining whether a node has a sibling of particular type.


Content-Based Algorithms

Content-based algorithms focus on the actual content of the nodes and their attributes. Common content-based algorithms are determining whether a given node contains another particular node, retrieving nodes based upon their type and finding a node with a particular attribute value.

Browser Support

DOM plays an important role in displaying the same chunk of information across multiple browsers. AS a result of this the user is able to see the same information in Internet Explorer, Firefox, Opera, etc. In addition to the standard W3C implementation of DOM, each browser has some proprietary methods and properties. This is the primary reason why the browsers differ in their functionality and implementation. Internet Explorer supports ActiveXObject for handling XML HTTP requests while other browsers support the MxlHttpRequest object. Another example of the differences might be that some versions prior ti IE6 do not support createAttribute(), getAttributeNode(), or setAttributeNode() methods. An alternative to these is to use the getAttribute() and setAttribute() methods. NetScape Navigator stores newline characters as individual text nodes. The developer has to take into account these changes while target the web page across multiple browser platforms.

Applications

When the initial version of DOM specifications were drafted the focus was on the supporting web browsers efficiently. However, the evolution of the Web and the widespread use of XML has forced the use of DOM in other application as well. For example every application stores some amount of data on either the server or client side when it runs on the internet. This information is stored in a structured format and DOM techniques can be used for navigating through these objects. As a result, DOM finds wide applications in client-server type of business setups.

Standalone DOM implementations are not tied to larger applications such as web browsers. Instead they are provided as libraries which your applications can use for extracting data from the structured information. Example of this are the various parsing libraries which can be included int your application for customized parsing of documents. On the other hand, Embedded DOM implementations are tied to larger applications. In these type f applications, DOM is exposed to extensions, such as Macromedia’s Dreamweaver HTML authoring applications.

DOM and SAX

SAX provides a mechanism for reading data from an XML document. SAX is a popular alternative to the DOM. However, SAX is not the right tool for this job. Following table differentiates between SAX and DOM.


SAX is sequential

SAX does not allow random access to XML document. Only the current element is accessible. Suppose, if the second element is being accessed, then it cannot access information from fourth element because the fourth element hasn't been parsed yet. Also, when the fourth element is getting accessed, it cannot "look back" on the second element [9].


Comparing DOM and SAX

DOM SAX
Before processing, stores the entire XML document in memory Sequential and parses node by node
Occupies more memory Doesn't store the XML in memory
We can insert or delete nodes We can't insert or delete a node
Traverse in any direction Top to bottom traversing
Accessing siblings can be done using predefined functions Accessing siblings is difficult due to sequential nature

Conclusion

The Document Object Model (DOM) is a cross-platform and language-independent convention for representing and interacting with objects in HTML, XHTML and XML documents. It allows the programmer create and modify HTML pages and XML documents as full-fledged program objects DOM is changing the way we browse web pages everyday. As newer version of DOM are drafted to support the modern web standards, it provides better interactivity with web pages. DOM4 promises for developing richer, standardized applications. Also with the advent of new DOM features, the browsers will also see a change in the implementation as well as operation giving the end users a better browsing experience. XSLT Support has been provided in the latest version of DOM drafted. the initial draft of DOM level 4 was finalized in 2009. DOM4 build on DOM Core Level 3 and also incorporates features from DOM Leve2 HTML.

References

  1. Jeffrey Sambells, Aaron Gustafson (2007), Advanced DOM scripting: Dynamic Web Design Techniques, ISBN: 1-59059-856-3.
  2. Jeremy Keith (2005), DOM scripting: Web Design with JavaScript and the Document Object Model, ISBN: 1-59059-533-5.
  3. Joe Marini (2002), The Document Object Model: Processing Structured Documents, ISBN: 0-07-222436-3.
  4. Document Object Model (DOM)
  5. Philippe Le Hégaret, The W3C Document Object Model
  6. Stephen Chapman, A Brief History of Javascript
  7. ECMAScript
  8. Node Attributes
  9. The DOM and JavaScript
  10. DOM and SAX

Further Reading

  1. Document Object Model Core
  2. DOM Level 4
  3. ECMAScript Language Specification
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