XML Tutorial/es
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Introducción
El Lenguaje de Marcas Extensible (XML) es recomendado por el W3C y fue creado para el intercambio de información entre sistemas diferentes. Utiliza texto para almacenar la información. Lenguajes modernos de intercambio de datos, como XHTML y muchas tecnologías de servicios WEB están basados en XML.
Actualmente hay un conjunto de unidades que dan soporte a XML en Free Pascal. Estas unidades son "XMLRead", "XMLWrite" y "DOM" y son parte de FCL del compilador Free Pascal. las unidades de la FCL están en el ruta de búsqueda por defecto del compilador de Lazarus, por lo que sólo tendrá que añadir las unidades a la cláusula uses para utilizar XML. La FCL no está actualmente documentada completamente (Octubre / 2005), por lo que este breve tutorial tiene por objeto realizar una introducción al acceso a ficheros XML utilizando las unidades mencionadas.
El Modelo de Objeto de Documento (DOM) de XML es un conjunto normalizado de objetos que proporcionan una interfaz similar para el uso de XML en diferentes lenguajes y sistemas. La norma sólo especifica los métodos, propiedades y otras partes de la interfaz del objeto, dejando la implementación libre para los diferentes ilenguajes. El FCL actualmente apoya plenamente DOM XML 1.0.
Ejemplos
Lo que sigue son ejemplos de manipulación de datos XML con una complejidad creciente.
Leyendo un nodo de texto
Para progrmadores Delphi:
Resaltar que cuándo se trabaja con TXMLDocument, el texto en un nodo es considerado un nodo de Texto separado. Por tanto se accede al texto del nodo en un nodo separado. Alternativamente, la propiedad TextContent puede utilizarse para recuperar el contenido de todos los nodos de texto por debajo de uno dado, concatenados todos ellos.
El procedimiento ReadXMLFile crea siempre un nuevo objeto TXMLDocument, por lo que no hay que crearlo previamente de forma manual. Hay que asegurarse de destruir el documento llamando a Free cuando ya no lo necesitemos.
Por ejemplo, veamos el siguinete XML:
<xml>
<?xml version="1.0"?> <solicitud> <tipo_solicitud>PUT_FILE</tipo_solicitud> <usuario>123</usuario> <contrasenya>abc</contrasenya> </solicitud>
</xml>
The following code example shows both the correct and the incorrect ways of getting the value of the text node:
<delphi>
var PassNode: TDOMNode; Doc: TXMLDocument; begin // Read in xml file from disk ReadXMLFile(Doc, 'c:\xmlfiles\test.xml'); // Retrieve the "contrasenya" node PassNode := Doc.DocumentElement.FindNode('contrasenya'); // 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 WriteLn(PassNode.TextContent); // finally, free the document Doc.Free;
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>
<?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> </imageNode> </images>
</xml>
And here the Pascal code to execute the task:
<delphi>
var Documento: TXMLDocument; Child: TDOMNode; j: Integer; begin ReadXMLFile(Documento, 'C:\Programas\teste.xml'); Memo.Lines.Clear; // using FirstChild and NextSibling properties Child := Documento.DocumentElement.FirstChild; while Assigned(Child) do begin Memo.Lines.Add(Child.NodeName + ' ' + Child.Attributes.Item[0].NodeValue); // using ChildNodes method with Child.ChildNodes do try for j := 0 to (Count - 1) do Memo.Lines.Add(Item[j].NodeName + ' ' + Item[j].FirstChild.NodeValue); finally Free; end; Child := Child.NextSibling; end; Documento.Free; end;
</delphi>
This will print:
image Node 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); var cNode: TDOMNode; begin 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 begin ProcessNode(cNode, TreeNode); cNode := cNode.NextSibling; end; end;
begin
iNode := XMLDoc.DocumentElement.FirstChild; while iNode <> nil do begin ProcessNode(iNode, nil); // Recursive iNode := iNode.NextSibling; end;
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:
<delphi>
function CreateElement(const tagName: DOMString): TDOMElement; virtual; function CreateTextNode(const data: DOMString): TDOMText; function CreateCDATASection(const data: DOMString): TDOMCDATASection; virtual; function CreateAttribute(const name: DOMString): TDOMAttr; virtual;
</delphi>
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;
begin
{******************************************************************* * Detects the selected element *******************************************************************} if TreeView1.Selected = nil then Exit;
if TreeView1.Selected.Level = 0 then begin position := TreeView1.Selected.Index;
NovoNo := XMLDoc.CreateElement('item'); TDOMElement(NovoNo).SetAttribute('nome', 'Item'); TDOMElement(NovoNo).SetAttribute('arquivo', 'Arquivo'); with XMLDoc.DocumentElement.ChildNodes do begin Item[position].AppendChild(NovoNo); Free; end;
{******************************************************************* * Updates the TreeView *******************************************************************} TreeView1.Items.Clear; XML2Tree(TreeView1, XMLDoc); end else if TreeView1.Selected.Level >= 1 then begin {******************************************************************* * This function only works on the first level of the tree, * but can easely modifyed to work for any number of levels *******************************************************************} end;
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;
begin
S:= TStringStream.Create(MyXMLString); Try S.Position:=0; XML:=Nil; ReadXMLFile(XML,S); // Complete XML document // Alternatively: ReadXMLFragment(AParentNode,S); // Read only XML fragment. Finally S.Free; end;
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>
<?xml version='1.0'?> <!DOCTYPE root [ <!ELEMENT root (child)+ > <!ELEMENT child (#PCDATA)> ]> <root> <child>This is a first child.</child> <child>And this is the second one.</child> </root>
</xml>
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;
begin
// 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 Src.Free; Parser.Free;
end;
procedure TMyObject.ErrorHandler(E: EXMLReadError); begin
if E.Severity = esError then // we are interested in validation errors only writeln(E.Message);
end; </delphi>
Generating a XML file
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+}
interface
uses
Classes, SysUtils, LResources, Forms, Controls, Graphics, Dialogs, StdCtrls, DOM, XMLWrite;
type
{ TForm1 } TForm1 = class(TForm) Button1: TButton; Label1: TLabel; Label2: TLabel; procedure Button1Click(Sender: TObject); private { private declarations } public { public declarations } end;
var
Form1: TForm1;
implementation
{ TForm1 }
procedure TForm1.Button1Click(Sender: TObject); var
xdoc: TXMLDocument; // variable to document RootNode, parentNode, nofilho: TDOMNode; // variable to nodes
begin
//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
end;
initialization
{$I unit1.lrs}
end. </delphi>
The result will be the XML file below: <xml> <?xml version="1.0"?> <register>
<usuario id="001"> <nome>Fernando</nome> <idade>32</idade> </usuario>
</register> </xml>
- Fernandosinesio 22:28, 24 April 2008 (CEST)fernandosinesio@gmail.com
- Versión en castellano (español) iskraelectrica (jldc) /diciembre de 2008.
Encoding
According to the XML standard, although there may be an encoding attribute in the first line of the XML, there is no need for it. 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.
External Links
- W3Schools Xml Tutorial
- Thomas Zastrow article FPC and XML