XML language is increasingly used to store information, exchange it between applications and Web sites. Many applications use this language as the base language for storing data, while others use it for exporting and importing XML data. So it’s time for developers to think about how XML data can be used in their own applications.

In this article, we will look at the XML Document Object Model (DOM) and Microsoft's implementation of the XML DOM.

The XML DOM is an object model that provides the developer with objects for loading and processing XML files. The object model consists of the following core objects: XMLDOMDocument, XMLDOMNodeList, XMLDOMNode, XMLDOMNamedNodeMap, and XMLDOMParseError. Each of these objects (except XMLDOMParseError) contains properties and methods that allow you to get information about the object, manipulate the object's values ​​and structure, and navigate the structure of an XML document.

Let's look at the main XML DOM objects and show some examples of their use in Borland Delphi.

Using XML DOM in Borland Delphi

In order to use Microsoft XML DOM in Delphi applications, you need to connect the appropriate type library to the project. To do this, we execute the command Project | Import Type Library and in the Import Type Library dialog box, select the Microsoft XML version 2.0 (Version 2.0) library, which is usually located in the Windows \ System \ MSXML.DLL file

After clicking the Create Unit button, the MSXML_TLB interface module will be created, which will allow us to use the XML DOM objects: DOMDocument, XMLDocument, XMLHTTPRequest and a number of others, implemented in the MSXML.DLL library. The reference to the MSXML_TLB module must be in the Uses list.

XML DOM device

The Document Object Model represents an XML document in a tree structure of branches. Programming interfaces XML DOMs allow applications to navigate the document tree and manipulate its branches. Each branch can have a specific type (DOMNodeType), according to which the parent and child branches are determined. Most XML documents contain branches of type element, attribute, and text. Attributes are a special kind of branch and are not child branches. Special methods provided by XML DOM objects are used to manipulate attributes.

In addition to implementing World Wide Web Consortium (W3C) recommended interfaces, the Microsoft XML DOM contains methods that support XSL, XSL Patterns, Namespaces, and data types. For example, the SelectNodes method allows you to use XSL Pattern Syntax to find branches in a specific context, and the TransformNode method supports using XSL to perform transformations.

Test XML Document

As an example XML document, let's take a music CD-ROM directory, which has the following structure:

Bob dylan USA Columbia 10.90 1985 Bonnie tylor UK CBS Records 9.90 1988 ... Joe cocker USA EMI 8.20 1987

We are now ready to start looking at the XML DOM object model, starting with the XMLDOMDocument object.

XML Document - XMLDOMDocument Object

Working with an XML document begins with loading it. To do this, we use the Load method, which has only one parameter that specifies the URL of the loaded document. When loading files from a local disk, only the full file name is specified (the file: /// protocol can be omitted in this case). If the XML document is stored as a string, use the LoadXML method to load the document.

The Async property is used to control how the document is loaded (synchronous or asynchronous). By default, this property is set to True, indicating that the document is loaded asynchronously and control is returned to the application before the document is fully loaded. Otherwise, the document loads synchronously, and then you have to check the value of the ReadyState property to see if the document has loaded or not. You can also create an event handler for the OnReadyStateChange event that will take control when the value of the ReadyState property changes.

The following shows how to load an XML document using the Load method:

Uses ... MSXML_TLB ... procedure TForm1.Button1Click (Sender: TObject); var XMLDoc: IXMLDOMDocument; begin XMLDoc: = CoDOMDocument.Create; XMLDoc.Async: = False; XMLDoc.Load (‘C: \ DATA \ DATA.xml’); // // This is where the code that manipulates // the XML document and its branches // XMLDoc: = Nil; end;

After the document is loaded, we can access its properties. So, the NodeName property will contain the #document value, the NodeTypeString property will contain the document value, and the URL property will contain the file: /// C: /DATA/DATA.xml value.

Error handling

Of particular interest are properties related to document processing upon loading. For example, the ParseError property returns an XMLDOMParseError object containing information about an error that occurred while processing the document.

To write an error handler, you can add the following code:

Var XMLError: IXMLDOMParseError; ... XMLDoc.Load (‘C: \ DATA \ DATA.xml’); XMLError: = XMLDoc.ParseError; If XMLError.ErrorCode<>0 Then // // Here we handle the error // Else Memo1.Lines.Add (XMLDoc.XML); ... XMLDoc: = Nil;

To find out what information is returned in case of an error, change the following directory entry:

Bob dylan USA Columbia 10.90 1985

removing the closing element .

Now let's add a new branch. Below is the code showing how to add a new music CD-ROM entry:

Var NewNode: IXMLDOMNode; Child: IXMLDOMNode; ... // Create a new branch - NewNode: = XMLDoc.CreateNode (1, ‘CD’, ‘’); // Add an element