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.
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 UI for this example was created using Qt Designer :
The UI consists of three
group boxes
arranged vertically. The top one contains a
text viewer
that displays the XML text from the cookbook file. The middle group box contains a
combo box
for choosing the
XQuery
to run and a
text viewer
for displaying the text of the selected
XQuery
。
.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's
text viewer
.
You can write your own
XQuery
files and run them in the example program. The file
xmlpatterns/recipes/recipes.qrc
是
resource file
for this example. It is used in
main.cpp
(
Q_INIT_RESOURCE(recipes);
). It lists the
XQuery
files (
.xq
) that can be selected in the combobox.
<!DOCTYPE RCC><RCC version="1.0"> <qresource> <file>files/cookbook.xml</file> <file>files/allRecipes.xq</file> <file>files/liquidIngredientsInSoup.xq</file> <file>files/mushroomSoup.xq</file> <file>files/preparationLessThan30.xq</file> <file>files/preparationTimes.xq</file> </qresource> </RCC>
To add your own queries to the example's combobox, store your
.xq
files in the
examples/xmlpatterns/recipes/files
directory and add them to
recipes.qrc
as shown above.
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 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 its
currentIndexChanged()
signal to the window's
displayQuery()
slot. It then calls
loadInputFile()
to load
cookbook.xml
and display its contents in the top group box's
text viewer
. Finally, it finds the
XQuery
files (
.xq
) and adds each one to the
combo box
菜单。
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
)。
query's
bindVariable()
function is then called to bind the
cookbook.xml
file to the
XQuery
variable
inputDocument
.
后于
the variable is bound,
setQuery()
is called to pass the
XQuery
text to the
query
.
注意: setQuery() must be called after bindVariable() .
Passing the XQuery to setQuery() causes Qt XML Patterns to parse the XQuery . QXmlQuery::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())); }
若 XQuery is valid, an instance of QXmlFormatter is created to format the query result as XML into a QBuffer . To evaluate the XQuery , an overload of evaluateTo() is called that takes a QAbstractXmlReceiver for its output ( QXmlFormatter 继承 QAbstractXmlReceiver ). Finally, the formatted XML result is displayed in the UI's bottom text view.
注意:
每个
XQuery
.xq
file must declare the
$inputDocument
variable to represent the
cookbook.xml
document:
(: All ingredients for Mushroom Soup. :) declare variable $inputDocument external; doc($inputDocument)/cookbook/recipe[@xml:id = "MushroomSoup"]/ingredient/ <p>{@name, @quantity}</p>
注意:
If you add add your own query.xq files, you must declare the
$inputDocument
and use it as shown above.