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QXmlStreamReader to parse XML in Qt

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Article Metadata
Code ExampleTested with
Devices(s): 5800 XpressMusic
Compatibility
Platform(s): Qt
Symbian
Article
Keywords: Qt, XML, SAX, QXmlStreamReader
Created: taaidant (15 Jun 2009)
Last edited: hamishwillee (11 Oct 2012)

Contents

Overview

This example shows you how to parse well-formed XML with QXmlStreamReader. Qt documentation states that it is a faster and more convenient replacement for Qt's own SAX parser (QXmlSimpleReader) and it's also faster, as well as in some cases more convenient, than a DOM tree (QDomDocument).


Preconditions

QXmlStreamReader Example

XML file

<?xml version="1.0" encoding="UTF-8" ?>
<persons>
<person id="1">
<firstname>John</firstname>
<surname>Doe</surname>
<email>john.doe@example.com</email>
<website>http://en.wikipedia.org/wiki/John_Doe</website>
</person>
<person id="2">
<firstname>Jane</firstname>
<surname>Doe</surname>
<email>jane.doe@example.com</email>
<website>http://en.wikipedia.org/wiki/John_Doe</website>
</person>
<person id="3">
<firstname>Matti</firstname>
<surname>Meikäläinen</surname>
<email>matti.meikalainen@example.com</email>
<website>http://fi.wikipedia.org/wiki/Matti_Meikäläinen</website>
</person>
</persons>

Header file

#include <QtXml/QXmlStreamReader>
 
class QXSRExample : public QMainWindow {
Q_OBJECT
 
public:
QXSRExample(QWidget *parent = 0);
~QXSRExample();
 
private slots:
void parseXML();
 
private:
QPointer<QVBoxLayout> _layout;
 
void setupUI();
 
QMap<QString, QString> parsePerson(QXmlStreamReader& xml);
void addElementDataToMap(QXmlStreamReader& xml,
QMap<QString, QString>& map) const;
 
void addPersonsToUI(QList< QMap<QString,QString> >& persons);
};

Source file

void QXSRExample::parseXML() {
/* We'll parse the example.xml */
QFile* file = new QFile(":/example.xml");
/* If we can't open it, let's show an error message. */
if (!file->open(QIODevice::ReadOnly | QIODevice::Text)) {
QMessageBox::critical(this,
"QXSRExample::parseXML",
"Couldn't open example.xml",
QMessageBox::Ok);
return;
}
/* QXmlStreamReader takes any QIODevice. */
QXmlStreamReader xml(file);
QList< QMap<QString,QString> > persons;
/* We'll parse the XML until we reach end of it.*/
while(!xml.atEnd() &&
!xml.hasError()) {
/* Read next element.*/
QXmlStreamReader::TokenType token = xml.readNext();
/* If token is just StartDocument, we'll go to next.*/
if(token == QXmlStreamReader::StartDocument) {
continue;
}
/* If token is StartElement, we'll see if we can read it.*/
if(token == QXmlStreamReader::StartElement) {
/* If it's named persons, we'll go to the next.*/
if(xml.name() == "persons") {
continue;
}
/* If it's named person, we'll dig the information from there.*/
if(xml.name() == "person") {
persons.append(this->parsePerson(xml));
}
}
}
/* Error handling. */
if(xml.hasError()) {
QMessageBox::critical(this,
"QXSRExample::parseXML",
xml.errorString(),
QMessageBox::Ok);
}
/* Removes any device() or data from the reader
* and resets its internal state to the initial state. */

xml.clear();
this->addPersonsToUI(persons);
}
 
QMap<QString, QString> QXSRExample::parsePerson(QXmlStreamReader& xml) {
QMap<QString, QString> person;
/* Let's check that we're really getting a person. */
if(xml.tokenType() != QXmlStreamReader::StartElement &&
xml.name() == "person") {
return person;
}
/* Let's get the attributes for person */
QXmlStreamAttributes attributes = xml.attributes();
/* Let's check that person has id attribute. */
if(attributes.hasAttribute("id")) {
/* We'll add it to the map. */
person["id"] = attributes.value("id").toString();
}
/* Next element... */
xml.readNext();
/*
* We're going to loop over the things because the order might change.
* We'll continue the loop until we hit an EndElement named person.
*/

while(!(xml.tokenType() == QXmlStreamReader::EndElement &&
xml.name() == "person")) {
if(xml.tokenType() == QXmlStreamReader::StartElement) {
/* We've found first name. */
if(xml.name() == "firstname") {
this->addElementDataToMap(xml, person);
}
/* We've found surname. */
if(xml.name() == "surname") {
this->addElementDataToMap(xml, person);
}
/* We've found email. */
if(xml.name() == "email") {
this->addElementDataToMap(xml, person);
}
/* We've found website. */
if(xml.name() == "website") {
this->addElementDataToMap(xml, person);
}
}
/* ...and next... */
xml.readNext();
}
return person;
}
 
void QXSRExample::addElementDataToMap(QXmlStreamReader& xml,
QMap<QString, QString>& map) const {
/* We need a start element, like <foo> */
if(xml.tokenType() != QXmlStreamReader::StartElement) {
return;
}
/* Let's read the name... */
QString elementName = xml.name().toString();
/* ...go to the next. */
xml.readNext();
/*
* This elements needs to contain Characters so we know it's
* actually data, if it's not we'll leave.
*/

if(xml.tokenType() != QXmlStreamReader::Characters) {
return;
}
/* Now we can add it to the map.*/
map.insert(elementName, xml.text().toString());
}
 
void QXSRExample::addPersonsToUI(QList< QMap<QString,QString> >& persons) {
while(!persons.isEmpty()) {
QGroupBox* personGB = new QGroupBox("Person");
QFormLayout* layout = new QFormLayout;
QMap<QString,QString> person = persons.takeFirst();
layout->addRow("ID", new QLineEdit(person["id"]));
layout->addRow("First name", new QLineEdit(person["firstname"]));
layout->addRow("Surname", new QLineEdit(person["surname"]));
layout->addRow("E-mail", new QLineEdit(person["email"]));
layout->addRow("Website", new QLineEdit(person["website"]));
personGB->setLayout(layout);
this->_layout->addWidget(personGB);
}
}

Postconditions

You are able to parse the XML and display it in the UI.

QXSRExample.png

Supplementary material

  • You can test QXSRExample with a test application. The application is available for download at Media:QXSRExample.zip.

See also

This page was last modified on 11 October 2012, at 04:18.
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