XML Tutorial/it

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L'Extensible Markup Language un linguaggio raccomandato dal W3C creato per l'interscambio di informazioni tra sistemi differenti. Si tratta di un sistema di memorizzazione di informazioni basato su file di testo. Linguaggi moderni di interscambio di dati come XHTML, così come la maggior parte delle tecnologie WebService, sono basati su XML.

Al momento esistono una serie di unit che forniscono supporto per XML su Free Pascal. Queste unit sono chiamate "XMLRead", "XMLWrite" e "DOM" e sono parte della Free Component Library (FCL) del Free Pascal Compiler. La FCL si trova già nei percorsi di ricerca di default per il compilatore in Lazarus, così bisogna soltanto aggiungere queste unit alla clausola uses per avere il supporto di XML. La FCL al momento (Ottobre / 2005) non è documentata, quindi questo breve tutorial introdurrà XML tramite l'utilizzo di queste unit.

XML DOM (Document Object Model) sono una serie di oggetti standardizzati che forniscono un'interfaccia simile per l'utilizzo di XML con differenti linguaggi e su differenti sistemi. Lo standard specifica soltanto i metodi, le proprietà e altre parti dell'interfaccia dell'oggetto, lasciando l'implementazione libera per i diversi linguaggi. La FCL al momento offre pieno supporto per XML DOM 1.0.


Di seguito c'è una lista di esempi di manipolazione dati tramite XML di complessità crescente.

Leggere un nodo di testo

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"?>


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>

Stampare i nomi dei nodi

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"?>
<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

Popolare un TreeView tramite 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>

Modificare un documento XML

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 locate 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

 position: Integer;
 NovoNo: TDomNode;


 *  Detects the selected element
 if TreeView1.Selected = nil then Exit;
 if TreeView1.Selected.Level = 0 then
   position := 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>

Creare un TXMLDocument da una stringa

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>

Validare un documento

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'?>
 <!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>

Generare un file XML

Below is the complete code to write in a XML file. (This was taken from a tutorial in DeveLazarus blog ) Please, remember DOM and XMLWrite libs in uses clause

<delphi> unit Unit1;

{$mode objfpc}{$H+}



 Classes, SysUtils, LResources, Forms, Controls, Graphics, Dialogs, StdCtrls,
 DOM, XMLWrite;


 { TForm1 }
 TForm1 = class(TForm)
   Button1: TButton;
   Label1: TLabel;
   Label2: TLabel;
   procedure Button1Click(Sender: TObject);
   { private declarations }
   { public declarations }


 Form1: TForm1;


{ TForm1 }

procedure TForm1.Button1Click(Sender: TObject); var

 xdoc: TXMLDocument;                                  // variable to document
 RootNode, parentNode, nofilho: TDOMNode;                    // variable to nodes


 //create a document
 xdoc := TXMLDocument.create;
 //create a root node
 RootNode := xdoc.CreateElement('register');
 Xdoc.Appendchild(RootNode);                           // save root node
 //create a parent node
 RootNode:= xdoc.DocumentElement;
 parentNode := xdoc.CreateElement('usuario');
 TDOMElement(parentNode).SetAttribute('id', '001');       // create atributes to parent node
 RootNode.Appendchild(parentNode);                          // save parent node
 //create a child node
 parentNode := xdoc.CreateElement('nome');                // create a child node
 //TDOMElement(parentNode).SetAttribute('sexo', 'M');     // create atributes
 nofilho := xdoc.CreateTextNode('Fernando');         // insert a value to node
 parentNode.Appendchild(nofilho);                         // save node
 RootNode.ChildNodes.Item[0].AppendChild(parentNode);       // insert child node in respective parent node
 //create a child node
 parentNode := xdoc.CreateElement('idade');               // create a child node
 //TDOMElement(parentNode).SetAttribute('ano', '1976');   // create atributes
 nofilho := xdoc.CreateTextNode('32');               // insert a value to node
 parentNode.Appendchild(nofilho);                         // save node
 .ChildNodes.Item[0].AppendChild(parentNode);       // insert a childnode in respective parent node
 writeXMLFile(xDoc,'teste.xml');                     // write to XML
 Xdoc.free;                                          // free memory



 {$I unit1.lrs}

end. </delphi>

The result will be the XML file below: <xml> <?xml version="1.0"?> <register>

 <usuario id="001">

</register> </xml>

--Fernandosinesio 22:28, 24 April 2008 (CEST)fernandosinesio@gmail.com


Starting from SVN revision 12582, XML reader is able to process data in any encoding by using external decoders. See XML_Decoders for more details.

According to the XML standard, the encoding attribute in the first line of the XML is optional in case the actual encoding is UTF-8 or UTF-16 (which is detected by presence of the BOM). As of version 0.9.26 of Lazarus, there is an encoding property in a TXMLDocument, but it is ignored. writeXMLFile always uses UTF-8 and doesn´t generate an encoding attribute in first line of the XML file.

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