XML Schema Validation Example¶
Using Qt XML Patterns to validate XML with a W3C XML Schema.
The XML Schema Validation example shows how to use Qt XML Patterns to validate XML with a W3C XML Schema.
Introduction¶
The example application shows different XML schema definitions and for every definition two XML instance documents, one that is valid according to the schema and one that is not. The user can select the valid or invalid instance document, change it and validate it again.
The User Interface¶
The UI for this example was created using Qt Designer:
The UI consists of three parts, at the top the XML schema
selection
and the schemaviewer
, below the XML instanceselection
and the instanceeditor
and at the bottom the validation statuslabel
next to the validationbutton
.
Validating XML Instance Documents¶
You can select one of the three predefined XML schemas and for each schema an valid or invalid instance document. A click on the ‘Validate’ button will validate the content of the XML instance editor against the schema from the XML schema viewer. As you can modify the content of the instance editor, different instances can be tested and validation error messages analysed.
Code Walk-Through¶
The example’s main() function creates the standard instance of
QApplication
. Then it creates an instance of the mainwindow class, shows it, and starts the Qt event loop:int main(int argc, char* argv[]) { Q_INIT_RESOURCE(schema); QApplication app(argc, argv); MainWindow* const window = new MainWindow; window->show(); return app.exec(); }
The UI Class: MainWindow¶
The example’s UI is a conventional Qt GUI application inheriting
QMainWindow
and the class generated by Qt Designer:class MainWindow : public QMainWindow, private Ui::SchemaMainWindow { Q_OBJECT public: MainWindow(); private Q_SLOTS: void schemaSelected(int index); void instanceSelected(int index); void validate(); void textChanged(); private: void moveCursor(int line, int column); };The constructor fills the schema and instance
QComboBox
selections with the predefined schemas and instances and connects theircurrentIndexChanged()
signals to the window’sschemaSelected()
resp.instanceSelected()
slot. Furthermore the signal-slot connections for the validationbutton
and the instanceeditor
are set up.The call to
schemaSelected(0)
andinstanceSelected(0)
will trigger the validation of the initial Contact Schema example.MainWindow::MainWindow() { setupUi(this); new XmlSyntaxHighlighter(schemaView->document()); new XmlSyntaxHighlighter(instanceEdit->document()); schemaSelection->addItem(tr("Contact Schema")); schemaSelection->addItem(tr("Recipe Schema")); schemaSelection->addItem(tr("Order Schema")); instanceSelection->addItem(tr("Valid Contact Instance")); instanceSelection->addItem(tr("Invalid Contact Instance")); connect(schemaSelection, SIGNAL(currentIndexChanged(int)), SLOT(schemaSelected(int))); connect(instanceSelection, SIGNAL(currentIndexChanged(int)), SLOT(instanceSelected(int))); connect(validateButton, SIGNAL(clicked()), SLOT(validate())); connect(instanceEdit, SIGNAL(textChanged()), SLOT(textChanged())); validationStatus->setAlignment(Qt::AlignCenter | Qt::AlignVCenter); schemaSelected(0); instanceSelected(0); }In the
schemaSelected()
slot the content of the instanceselection
is adapted to the selected schema and the corresponding schema is loaded from the resource file and displayed in the schemaviewer
. At the end of the method a revalidation is triggered.void MainWindow::schemaSelected(int index) { instanceSelection->clear(); if (index == 0) { instanceSelection->addItem(tr("Valid Contact Instance")); instanceSelection->addItem(tr("Invalid Contact Instance")); } else if (index == 1) { instanceSelection->addItem(tr("Valid Recipe Instance")); instanceSelection->addItem(tr("Invalid Recipe Instance")); } else if (index == 2) { instanceSelection->addItem(tr("Valid Order Instance")); instanceSelection->addItem(tr("Invalid Order Instance")); } textChanged(); const QString fileName = QStringLiteral(":/schema_") + QString::number(index) + QStringLiteral(".xsd"); QFile schemaFile(fileName); if (!schemaFile.open(QIODevice::ReadOnly)) { qWarning() << "Cannot open" << QDir::toNativeSeparators(fileName) << ':' << schemaFile.errorString(); return; } const QString schemaText(QString::fromUtf8(schemaFile.readAll())); schemaView->setPlainText(schemaText); validate(); }In the
instanceSelected()
slot the selected instance is loaded from the resource file and loaded into the instanceeditor
an the revalidation is triggered again.void MainWindow::instanceSelected(int index) { if (index < 0) { instanceEdit->setPlainText(QString()); return; } const QString fileName = QStringLiteral(":/instance_") + QString::number(2 * schemaSelection->currentIndex() + index) + QStringLiteral(".xml"); QFile instanceFile(fileName); if (!instanceFile.open(QIODevice::ReadOnly)) { qWarning() << "Cannot open" << QDir::toNativeSeparators(fileName) << ':' << instanceFile.errorString(); return; } const QString instanceText(QString::fromUtf8(instanceFile.readAll())); instanceEdit->setPlainText(instanceText); validate(); }The
validate()
slot does the actual work in this example. At first it stores the content of the schemaviewer
and theeditor
into temporaryvariables
. Then it instanciates aMessageHandler
object which inherits fromQAbstractMessageHandler
and is a convenience class to store error messages from the XmlPatterns system.class MessageHandler : public QAbstractMessageHandler { public: MessageHandler() : QAbstractMessageHandler(0) { } QString statusMessage() const { return m_description; } int line() const { return m_sourceLocation.line(); } int column() const { return m_sourceLocation.column(); } protected: virtual void handleMessage(QtMsgType type, const QString &description, const QUrl &identifier, const QSourceLocation &sourceLocation) { Q_UNUSED(type); Q_UNUSED(identifier); m_description = description; m_sourceLocation = sourceLocation; } private: QString m_description; QSourceLocation m_sourceLocation; };After the
QXmlSchema
is instanciated and the message handler set on it, theload()
method is called with the schema data as argument. If the schema is invalid or a parsing error has occurred,isValid()
returnsfalse
and the error is flagged inerrorOccurred
. If the loading was successful, aQXmlSchemaValidator
is instanciated and the schema passed in the constructor. A call tovalidate()
will validate the passed XML instance data against the XML schema. The return value of that method signals whether the validation was successful. Depending on the success the statuslabel
is set to ‘validation successful’ or the error message stored in theMessageHandler
The rest of the code does only some fancy coloring and eyecandy.
void MainWindow::validate() { const QByteArray schemaData = schemaView->toPlainText().toUtf8(); const QByteArray instanceData = instanceEdit->toPlainText().toUtf8(); MessageHandler messageHandler; QXmlSchema schema; schema.setMessageHandler(&messageHandler); schema.load(schemaData); bool errorOccurred = false; if (!schema.isValid()) { errorOccurred = true; } else { QXmlSchemaValidator validator(schema); if (!validator.validate(instanceData)) errorOccurred = true; } if (errorOccurred) { validationStatus->setText(messageHandler.statusMessage()); moveCursor(messageHandler.line(), messageHandler.column()); } else { validationStatus->setText(tr("validation successful")); } const QString styleSheet = QString("QLabel {background: %1; padding: 3px}") .arg(errorOccurred ? QColor(Qt::red).lighter(160).name() : QColor(Qt::green).lighter(160).name()); validationStatus->setStyleSheet(styleSheet); }
© 2022 The Qt Company Ltd. Documentation contributions included herein are the copyrights of their respective owners. The documentation provided herein is licensed under the terms of the GNU Free Documentation License version 1.3 as published by the Free Software Foundation. Qt and respective logos are trademarks of The Qt Company Ltd. in Finland and/or other countries worldwide. All other trademarks are property of their respective owners.