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DOM Programming

DOM Programming. The Document Object Model standardises what an application can see of the XML data how it can access it An XML structure is a tree of Nodes elements – text – entities attributes – processing instructions. DOM Nodes and NodeLists. All nodes have type getNodeType()

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DOM Programming

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  1. DOM Programming • The Document Object Model standardises • what an application can see of the XML data • how it can access it • An XML structure is a tree of Nodes • elements – text – entities • attributes – processing instructions

  2. DOM Nodes and NodeLists • All nodes have • type getNodeType() • name getNodeName() • value getNodeValue() • Nodes are arranged in • NodeLists e.g. child elements of <ol> • NamedNodeMaps e.g. attributes of <img>

  3. DOM Node Traversal Methods • Element nodes have • parent Node getParentNode() • children Node getFirstChild()Node getLastChild()NodeList getChildNodes() • siblings Node getNextSibling()Node getPreviousSibling() • attributes NamedNodeMap getAttributes()

  4. DOM NodeLists • NodeLists have • length int getLength() • individual items Node item(n) • NamedNodeMaps have • length int getLength() • individual items Node item(n) • named items Node getNamedItem(str)

  5. DOM Demonstration • JavaScript binding allows Dynamic XML • dom.html contains a demonstration of DOM access of an XML document.

  6. Microsoft Extensions to DOM • New functions combine DOM and XPath • NodeList selectNodes("XPath expression") • Node selectSingleNode("XPath expression") (see later lesson for XPath) • DOM calls renamed as properties e.g.n.getNodeType() becomes n.nodeType and documentElement.getChildNodes() becomes documentElement.childNodes • The property .textapplied to an element represents the concatenation of its textual contents and those of all its subelements.

  7. Link Checking: Sample DOM Use • Often an application needs to • search through the entire document for • a single piece of data • every occurrence of some data • Need functions to • traverse the complete document hierarchy checkAllNodes() • test each node checkThisNode()

  8. Link Checking: Outline Framework function checkAllNodes(n){ checkThisNode(n); if(n.hasChildNodes){ ... } } functioncheckThisNode(n){ if(n==null)return; ... } Iterate around all children (see next page) Perform application-specific test (see sample file)

  9. Link Checking: Code Details • To iterate around all children var children=n.childNodes var i=0; for(i=0; i<children.length; i++) checkAllNodes(children.item(i)) • Useful fragments for app-specific test • n.nodeName is element name / #PCDATA • n.getAttribute(name) returns value of the named attribute

  10. Link Checking: Putting It Together • To start the recursion off, callcheckAllNodes( xmlstuff.XMLDocument.documentElement); • See checkLinks.html

  11. DOM Pros and Cons • Pros • very powerful and flexible • good for rich, complex data and documents • Cons • Must write a complex program! • Highly tedious to specify correct DOM location

  12. XPath: DOM Path Specification • Standard for declarative expression of DOM traversal • XPath navigates around the elements in an XML document • like a URL navigates around documents in the Web • Also used in conjunction with new standards for queries and linking.

  13. XPath Expressions (1) • /book/chapter/title • a title element inside a chapter element inside the top-level book element • /book/*/title • a title element inside any element inside the top-level book element • /book//title • a title element anywhere inside the top-level book element

  14. XPath Expressions (2) • para/quote • a quote element inside a paragraph element inside the current element • ./para/quote • same as above • ../para/quote • a quote element inside a paragraph element inside the parent of the current element

  15. XPath Expressions (3) • title|heading|label • either a title or a heading or a label element • /book/chapter/@number • the number attribute of a chapter element inside a top-level book element

  16. XPath Expressions (4) • chapter[title] • a chapter element with a title element • chapter[title="Gone with the Wind"] • a chapter element whose title element has the contents "Gone with the Wind" • chapter[1] • the first chapter element • para[@security='classified'] • para elements with a security attribute set

  17. XPath Pros and Cons • XPath is like regular expressions for XML • Pros • Simple, expressive • Good for both documents and data • Cons • Can’t DO anything with it – must use in conjunction with DOM or XSLT processing

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