Recipes Example¶
Using Qt XML Patterns to query XML data loaded from a file.
The Recipes example shows how to use Qt XML Patterns to query XML data loaded from a file.
Introduction¶
In this case, the XML data represents a cookbook,
cookbook.xml
, which contains<cookbook>
as its document element, which in turn contains a sequence of<recipe>
elements. This XML data is searched using queries stored in XQuery files (*.xq
).
The User Interface¶
The UI for this example was created using Qt Designer:
The UI consists of three
group boxes
arranged vertically. The top one contains atext viewer
that displays the XML text from the cookbook file. The middle group box contains acombo box
for choosing the XQuery to run and atext viewer
for displaying the text of the selected XQuery . The.xq
files in the file list above are shown in the combo box menu. Choosing an XQuery loads, parses, and runs the selected XQuery . The query result is shown in the bottom group box’stext viewer
.
Running your own XQueries¶
You can write your own XQuery files and run them in the example program. The file
xmlpatterns/recipes/recipes.qrc
is the resource file for this example. It is used inmain.cpp
(Q_INIT_RESOURCE(recipes);
). It lists the XQuery files (.xq
) that can be selected in the combobox.To add your own queries to the example’s combobox, store your
.xq
files in theexamples/xmlpatterns/recipes/files
directory and add them torecipes.qrc
as shown above.
Code Walk-Through¶
The example’s main() function creates the standard instance of
QApplication
. Then it creates an instance of the UI class, shows it, and starts the Qt event loop:int main(int argc, char* argv[]) { Q_INIT_RESOURCE(recipes); QApplication app(argc, argv); QueryMainWindow* const queryWindow = new QueryMainWindow; queryWindow->show(); return app.exec(); }
The UI Class: QueryMainWindow¶
The example’s UI is a conventional Qt GUI application inheriting
QMainWindow
and the class generated by Qt Designer:class QueryMainWindow : public QMainWindow, private Ui::QueryWidget { Q_OBJECT public: QueryMainWindow(); public slots: void displayQuery(int index); private: QComboBox *ui_defaultQueries = nullptr; void evaluate(const QString &str); void loadInputFile(); };The constructor finds the window’s
combo box
child widget and connects itscurrentIndexChanged()
signal to the window’sdisplayQuery()
slot. It then callsloadInputFile()
to loadcookbook.xml
and display its contents in the top group box’stext viewer
. Finally, it finds the XQuery files (.xq
) and adds each one to thecombo box
menu.QueryMainWindow::QueryMainWindow() { setupUi(this); new XmlSyntaxHighlighter(findChild<QTextEdit*>("inputTextEdit")->document()); new XmlSyntaxHighlighter(findChild<QTextEdit*>("outputTextEdit")->document()); ui_defaultQueries = findChild<QComboBox*>("defaultQueries"); QMetaObject::connectSlotsByName(this); connect(ui_defaultQueries, QOverload<int>::of(&QComboBox::currentIndexChanged), this, &QueryMainWindow::displayQuery); loadInputFile(); const QStringList queries(QDir(":/files/", "*.xq").entryList()); for (const auto &query : queries) ui_defaultQueries->addItem(query); if (queries.count() > 0) displayQuery(0); }The work is done in the displayQuery() slot and the evaluate() function it calls. displayQuery() loads and displays the selected query file and passes the XQuery text to evaluate() .
void QueryMainWindow::displayQuery(int index) { QFile queryFile(QString(":files/") + ui_defaultQueries->itemText(index)); queryFile.open(QIODevice::ReadOnly); const QString query(QString::fromLatin1(queryFile.readAll())); findChild<QTextEdit*>("queryTextEdit")->setPlainText(query); evaluate(query); }evaluate() demonstrates the standard Qt XML Patterns usage pattern. First, an instance of
QXmlQuery
is created (query
). Thequery's
bindVariable()
function is then called to bind thecookbook.xml
file to the XQuery variableinputDocument
. After the variable is bound,setQuery()
is called to pass the XQuery text to thequery
.Note
setQuery()
must be called afterbindVariable()
.Passing the XQuery to
setQuery()
causes Qt XML Patterns to parse the XQuery .isValid()
is called to ensure that the XQuery was correctly parsed.void QueryMainWindow::evaluate(const QString &str) { QFile sourceDocument; sourceDocument.setFileName(":/files/cookbook.xml"); sourceDocument.open(QIODevice::ReadOnly); QByteArray outArray; QBuffer buffer(&outArray); buffer.open(QIODevice::ReadWrite); QXmlQuery query; query.bindVariable("inputDocument", &sourceDocument); query.setQuery(str); if (!query.isValid()) return; QXmlFormatter formatter(query, &buffer); if (!query.evaluateTo(&formatter)) return; buffer.close(); findChild<QTextEdit*>("outputTextEdit")->setPlainText(QString::fromUtf8(outArray.constData())); }If the XQuery is valid, an instance of
QXmlFormatter
is created to format the query result as XML into aQBuffer
. To evaluate the XQuery , an overload ofevaluateTo()
is called that takes aQAbstractXmlReceiver
for its output (QXmlFormatter
inheritsQAbstractXmlReceiver
). Finally, the formatted XML result is displayed in the UI’s bottom text view.Note
Each XQuery
.xq
file must declare the$inputDocument
variable to represent thecookbook.xml
document:(: All ingredients for Mushroom Soup. :) declare variable $inputDocument external; doc($inputDocument)/cookbook/recipe[@xml:id = "MushroomSoup"]/ingredient/ <p>{@name, @quantity}</p>Note
If you add add your own query.xq files, you must declare the
$inputDocument
and use it as shown above.
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