XML Tutorial

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Revision as of 17:56, 8 October 2007 by Gorelkin (talk | contribs) (Added validation example)

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The Extensible Markup Language is a W3C recommended language created to interchange information between different systems. It is a text based way to store information. Modern data interchange languages such as XHTML, as well as most WebServices technologies, are based on XML.

Currently there is a set of units that provides support for XML on Free Pascal. These units are called "XMLRead", "XMLWrite" and "DOM" and they are part of the Free Component Library (FCL) from the Free Pascal Compiler. The FCL is already on the default search path for the compiler on Lazarus, so you only need to add the units to your uses clause in order to get XML support. The FCL is not documented currently (October / 2005), so this short tutorial aims at introducing XML access using those units.

The XML DOM (Document Object Model) is a set of standarized objects that provide a similar interface for the use of XML on different languages and systems. The standard only specifies the methods, properties and other interface parts of the object, leaving the implementation free for different languages. The FCL currently supports fully the XML DOM 1.0.


Bellow there is a list of XML data manipulation examples with growing complexity.

Reading a text node

For Delphi Programmers: Note that when working with TXMLDocument, the text within a Node is considered a separate TEXT Node. As a result, you must access a node's text value as a separate node. Alternatively, the TextContent property may be used to retrieve content of all text nodes beneath the given one, concatenated together.

The ReadXMLFile procedure always creates a new TXMLDocument, so you don't have to create it beforehand. However, be sure to destroy the document by calling Free when you are done.

For instance, consider the following XML:


<?xml version="1.0" encoding="utf-8"?>


The following code example shows both the correct and the incorrect ways of getting the value of the text node:


 PassNode: TDOMNode;
 Doc:      TXMLDocument;
 // Read in xml file from disk
 ReadXMLFile(Doc, 'c:\xmlfiles\test.xml');
 // Retrieve the "password" node
 PassNode := Doc.DocumentElement.FindNode('password');
 // Write out value of the selected node
 WriteLn(PassNode.NodeValue); // will be blank
 // The text of the node is actually a separate child node
 WriteLn(PassNode.FirstChild.NodeValue); // correctly prints "abc"
 // alternatively
 // finally, free the document

end; </delphi>

Printing the names of nodes

A quick note on navigating the DOM tree: When you need to access nodes in sequence, it is best to use FirstChild and NextSibling properties (to iterate forward), or LastChild and PreviousSibling (to iterate backward). For random access it is possible to use ChildNodes or GetElementsByTagName methods, but these will create a TDOMNodeList object which eventually must be freed. This differs from other DOM implementations like MSXML, because FCL implementation is object-based, not interface-based.

The following example shows how to print the names of nodes to a TMemo placed on a form.

Bellow is the XML file called 'C:\Programas\teste.xml':


<?xml version="1.0" encoding="ISO-8859-1"?>
<images directory="mydir">
 <imageNode URL="graphic.jpg" title="">
   <Peca DestinoX="0" DestinoY="0">Pecacastelo.jpg1.swf</Peca>
   <Peca DestinoX="0" DestinoY="86">Pecacastelo.jpg2.swf</Peca>


And here the Pascal code to execute the task:


  Documento: TXMLDocument;
  Child: TDOMNode;
  j: Integer;
  ReadXMLFile(Documento, 'C:\Programas\teste.xml');
  // using FirstChild and NextSibling properties
  Child := Documento.DocumentElement.FirstChild;
  while Assigned(Child) do
    Memo.Lines.Add(Child.NodeName + ' ' + Child.Attributes.Item[0].NodeValue);
    // using ChildNodes method
    with Child.ChildNodes do
      for j := 0 to (Count - 1) do
        Memo.Lines.Add(Item[j].NodeName + ' ' + Item[j].FirstChild.NodeValue);
    Child := Child.NextSibling;


This will print:

imageNode graphic.jpg
Peca Pecacastelo.jpg1.swf
Peca Pecacastelo.jpg1.swf

Populating a TreeView with XML

One common use of XML files is to parse them and show their contents in a tree like format. You can find the TTreeView component on the "Common Controls" tab on Lazarus.

The function below will take a XML document previously loaded from a file or generated on code, and will populate a TreeView with it´s contents. The caption of each node will be the content of the first attribute of each node.

<delphi> procedure TForm1.XML2Tree(tree: TTreeView; XMLDoc: TXMLDocument); var

 iNode: TDOMNode;
 procedure ProcessNode(Node: TDOMNode; TreeNode: TTreeNode);
   cNode: TDOMNode;
   if Node = nil then Exit; // Stops if reached a leaf
   // Adds a node to the tree
   TreeNode := tree.Items.AddChild(TreeNode, Node.Attributes[0].NodeValue);
   // Goes to the child node
   cNode := Node.FirstChild;
   // Processes all child nodes
   while cNode <> nil do
     ProcessNode(cNode, TreeNode);
     cNode := cNode.NextSibling;


 iNode := XMLDoc.DocumentElement.FirstChild;
 while iNode <> nil do
   ProcessNode(iNode, nil); // Recursive
   iNode := iNode.NextSibling;

end; </delphi>

Modifying a XML document

The first thing to remember is that TDOMDocument is the "handle" to the DOM. You can get an instance of this class by creating one or by loading a XML document.

Nodes on the other hand cannot be created like a normal object. You *must* use the methods provided by TDOMDocument to create them, and latter use other methods to put them on the correct place on the tree. This is because nodes must be "owned" by a specific document on DOM.

Below are some common methods from TDOMDocument:


  function CreateElement(const tagName: DOMString): TDOMElement; virtual;
  function CreateTextNode(const data: DOMString): TDOMText;
  function CreateCDATASection(const data: DOMString): TDOMCDATASection;
  function CreateAttribute(const name: DOMString): TDOMAttr; virtual;


And here an example method that will located the selected item on a TTreeView and then insert a child node to the XML document it represents. The TreeView must be previously filled with the contents of a XML file using the XML2Tree function.

<delphi> procedure TForm1.actAddChildNode(Sender: TObject); var

 Posicao: Integer;
 NovoNo: TDomNode;


 *  Detects the selected element
 if TreeView1.Selected = nil then Exit;
 if TreeView1.Selected.Level = 0 then
   Posicao := TreeView1.Selected.Index;
   NovoNo := XMLDoc.CreateElement('item');
   TDOMElement(NovoNo).SetAttribute('nome', 'Item');
   TDOMElement(NovoNo).SetAttribute('arquivo', 'Arquivo');
   with XMLDoc.DocumentElement.ChildNodes do
   *  Updates the TreeView
   XML2Tree(TreeView1, XMLDoc);
 else if TreeView1.Selected.Level >= 1 then
   *  This function only works on the first level of the tree,
   *  but can easely modifyed to work for any number of levels

end; </delphi>

Create a TXMLDocument from a string

Given al XML file in MyXmlString, the following code will create it's DOM:

<delphi> Var

 S : TStringStream;
 XML : TXMLDocument;


 S:= TStringStream.Create(MyXMLString);
   ReadXMLFile(XML,S); // Complete XML document
   // Alternatively:
   ReadXMLFragment(AParentNode,S); // Read only XML fragment.

end; </delphi>

Validating a document

Since March 2007, DTD validation facility has been added to the FCL XML parser. Validation is checking that logical structure of the document conforms to the predefined rules, called Document Type Definition (DTD).

Here is an example of XML document with a DTD:


 <?xml version='1.0' encoding='utf-8'?>
 <!DOCTYPE root [
 <!ELEMENT root (child)+ >
 <!ELEMENT child (#PCDATA)>
   <child>This is a first child.</child>
   <child>And this is the second one.</child>


This DTD specifies that 'root' element must have one or more 'child' elements, and that 'child' elements may have only character data inside. If parser detects any violations from these rules, it will report them.

Loading such document is slightly more complicated. Let's assume we have XML data in a TStream object:

<delphi> procedure TMyObject.DOMFromStream(AStream: TStream); var

 Parser: TDOMParser;
 Src: TXMLInputSource;
 TheDoc: TXMLDocument;


 // create a parser object
 Parser := TDOMParser.Create;
 // and the input source
 Src := TXMLInputSource.Create(AStream);
 // we want validation
 Parser.Options.Validate := True;
 // assign a error handler which will receive notifications
 Parser.OnError := @ErrorHandler;
 // now do the job
 Parser.Parse(Src, TheDoc);
 // ...and cleanup


procedure TMyObject.ErrorHandler(E: EXMLReadError); begin

 if E.Severity = esError then  // we are interested in validation errors only

end; </delphi>

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