File System Example¶
Using Qt XML Patterns for querying non-XML data that is modeled to look like XML.
This example shows how to use Qt XML Patterns for querying non-XML data that is modeled to look like XML.
Introduction¶
The example models your computer’s file system to look like XML and allows you to query the file system with XQuery . Suppose we want to find all the
cpp
files in the subtree beginning at/filetree
:
The User Interface¶
The example is shown below. First, we use
File->Open Directory
(not shown) to select the/filetree
directory. Then we use the combobox on the right to select the XQuery that searches forcpp
files (listCPPFiles.xq
). Selecting an XQuery runs the query, which in this case traverses the model looking for all thecpp
files. The XQuery text and the query results are shown on the right:Don’t be mislead by the XML representation of the
/filetree
directory shown on the left. This is not the node model itself but the XML obtained by traversing the node model and outputting it as XML. Constructing and using the custom node model is explained in the code walk-through.
Running your own XQueries¶
You can write your own XQuery files and run them in the example program. The file
xmlpatterns/filetree/queries.qrc
is the resource file for this example. It is used inmain.cpp
(Q_INIT_RESOURCE(queries);
). 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/filetree/queries
directory and add them toqueries.qrc
as shown above.
Code Walk-Through¶
The strategy is to create a custom node model that represents the directory tree of the computer’s file system. That tree structure is non-XML data. The custom node model must have the same callback interface as the XML node models that the Qt XML Patterns query engine uses to execute queries. The query engine can then traverse the custom node model as if it were traversing the node model built from an XML document.
The required callback interface is in
QAbstractXmlNodeModel
, so we create a custom node model by subclassingQAbstractXmlNodeModel
and providing implementations for its pure virtual functions. For many cases, the implementations of several of the virtual functions are always the same, so Qt XML Patterns also providesQSimpleXmlNodeModel
, which subclassesQAbstractXmlNodeModel
and provides implementations for the callback functions that you can ignore. By subclassingQSimpleXmlNodeModel
instead ofQAbstractXmlNodeModel
, you can reduce development time.
The Custom Node Model Class: FileTree¶
The custom node model for this example is class
FileTree
, which is derived fromQSimpleXmlNodeModel
.FileTree
implements all the callback functions that don’t have standard implementations inQSimpleXmlNodeModel
. When you implement your own custom node model, you must provide implementations for these callback functions:virtual QXmlNodeModelIndex::DocumentOrder compareOrder(const QXmlNodeModelIndex&, const QXmlNodeModelIndex&) const; virtual QXmlName name(const QXmlNodeModelIndex &node) const; virtual QUrl documentUri(const QXmlNodeModelIndex &node) const; virtual QXmlNodeModelIndex::NodeKind kind(const QXmlNodeModelIndex &node) const; virtual QXmlNodeModelIndex root(const QXmlNodeModelIndex &node) const; virtual QVariant typedValue(const QXmlNodeModelIndex &node) const; virtual QVector<QXmlNodeModelIndex> attributes(const QXmlNodeModelIndex &element) const; virtual QXmlNodeModelIndex nextFromSimpleAxis(SimpleAxis, const QXmlNodeModelIndex&) const;The
FileTree
class declares four data members:mutable QVector<QFileInfo> m_fileInfos; const QDir::Filters m_filterAllowAll; const QDir::SortFlags m_sortFlags; QVector<QXmlName> m_names;The
QVector
m_fileInfos
will contain the node model. EachQFileInfo
in the vector will represent a file or a directory in the file system. At this point it is instructive to note that although the node model class for this example (FileTree
) actually builds and contains the custom node model, building the custom node model isn’t always required. For example, it is possible to use an already existingQObject
tree as a node model and just implement the callback interface for that already existing data structure. In this file system example, however, although we have an already existing data structure, i.e. the file system, that data structure is not in memory and is not in a form we can use. So we must build an analog of the file system in memory from instances ofQFileInfo
, and we use that analog as the custom node model.The two sets of flags,
m_filterAllowAll
andm_sortFlags
, contain OR’ed flags fromFilters
andSortFlags
respectively. They are set by theFileTree
constructor and used in calls toentryInfoList()
for getting the child list for a directory node, i.e. aQFileInfoList
containing the file and directory nodes for all the immediate children of a directory.The
QVector
m_names
is an auxiliary component of the node model. It holds the XML element and attribute names (QXmlName
) for all the node types that will be found in the node model.m_names
is indexed by the enumFileTree::Type
, which specifies the node types:enum Type { File, Directory, AttributeFileName, AttributeFilePath, AttributeSize, AttributeMIMEType, AttributeSuffix };
Directory
andFile
will represent the XML element nodes for directories and files respectively, and the other enum values will represent the XML attribute nodes for a file’s path, name, suffix, its size in bytes, and its mime type. TheFileTree
constructor initializesm_names
with an appropriateQXmlName
for each element and attribute type:FileTree::FileTree(const QXmlNamePool& pool) : QSimpleXmlNodeModel(pool), m_filterAllowAll(QDir::AllEntries | QDir::AllDirs | QDir::NoDotAndDotDot | QDir::Hidden), m_sortFlags(QDir::Name) { QXmlNamePool np = namePool(); m_names.resize(7); m_names[File] = QXmlName(np, QLatin1String("file")); m_names[Directory] = QXmlName(np, QLatin1String("directory")); m_names[AttributeFileName] = QXmlName(np, QLatin1String("fileName")); m_names[AttributeFilePath] = QXmlName(np, QLatin1String("filePath")); m_names[AttributeSize] = QXmlName(np, QLatin1String("size")); m_names[AttributeMIMEType] = QXmlName(np, QLatin1String("mimeType")); m_names[AttributeSuffix] = QXmlName(np, QLatin1String("suffix")); }Note that the constructor does not pre-build the entire node model. Instead, the node model is built incrementally as the query engine evaluates a query. To see how the query engine causes the node model to be built incrementally, see Building And Traversing The Node Model . To see how the query engine accesses the node model, see Accessing the node model . See also: Node Model Building Strategy .
Accessing The Node Model¶
Since the node model is stored outside the query engine in the
FileTree
class, the query engine knows nothing about it and can only access it by calling functions in the callback interface. When the query engine calls any callback function to access data in the node model, it passes aQXmlNodeModelIndex
to identify the node in the node model that it wants to access. Hence all the virtual functions in the callback interface use aQXmlNodeModelIndex
to uniquely identify a node in the model.We use the index of a
QFileInfo
inm_fileInfos
to uniquely identify a node in the node model. To get theQXmlNodeModelIndex
for aQFileInfo
, the class uses the private functiontoNodeIndex()
:QXmlNodeModelIndex FileTree::toNodeIndex(const QFileInfo &fileInfo, Type attributeName) const { const int indexOf = m_fileInfos.indexOf(fileInfo); if (indexOf == -1) { m_fileInfos.append(fileInfo); return createIndex(m_fileInfos.count()-1, attributeName); } else return createIndex(indexOf, attributeName); }It searches the
m_fileInfos
vector for aQFileInfo
that matchesfileInfo
. If a match is found, its array index is passed tocreateIndex()
as thedata
value for the QXmlNodeIndex. If no match is found, the unmatchedQFileInfo
is appended to the vector, so this function is also doing the actual incremental model building (see Building And Traversing The Node Model ).Note that
toNodeIndex()
gets a node type as the second parameter, which it just passes on tocreateIndex()
as theadditionalData
value. Logically, this second parameter represents a second dimension in the node model, where the first dimension represents the element nodes, and the second dimension represents each element’s attribute nodes. The meaning is that eachQFileInfo
in them_fileInfos
vector can represent an element node and one or more attribute nodes. In particular, theQFileInfo
for a file will contain the values for the attribute nodes path, name, suffix, size, and mime type (seeFileTree::attributes()
). Since the attributes are contained in theQFileInfo
of the file element, there aren’t actually any attribute nodes in the node model. Hence, we can use aQVector
form_fileInfos
.A convenience overloading of toNodeIndex() is also called in several places, wherever it is known that the
QXmlNodeModelIndex
being requested is for a directory or a file and not for an attribute. The convenience function takes only theQFileInfo
parameter and calls the other toNodeIndex() , after obtaining either the Directory or File node type directly from theQFileInfo
:QXmlNodeModelIndex FileTree::toNodeIndex(const QFileInfo &fileInfo) const { return toNodeIndex(fileInfo, fileInfo.isDir() ? Directory : File); }Note that the auxiliary vector
m_names
is accessed using the node type , for example:QXmlName FileTree::name(const QXmlNodeModelIndex &node) const { return m_names.at(node.additionalData()); }Most of the virtual functions in the callback interface are as simple as the ones described so far, but the callback function used for traversing (and building) the node model is more complex.
Building And Traversing The Node Model¶
The node model in
FileTree
is not fully built before the query engine begins evaluating the query. In fact, when the query engine begins evaluating its first query, the only node in the node model is the one representing the root directory for the selected part of the file system. See The UI Class: MainWindow below for details about how the UI triggers creation of the model.The query engine builds the node model incrementally each time it calls the nextFromSimpleAxis() callback function, as it traverses the node model to evaluate a query. Thus the query engine only builds the region of the node model that it needs for evaluating the query.
nextFromSimpleAxis() takes an
axis identifier
and anode identifier
as parameters. Thenode identifier
represents the context node (i.e. the query engine’s current location in the model), and theaxis identifier
represents the direction we want to move from the context node. The function finds the appropriate next node and returns itsQXmlNodeModelIndex
.nextFromSimpleAxis() is where most of the work of implementing a custom node model will be required. The obvious way to do it is to use a switch statement with a case for each
axis
.QXmlNodeModelIndex FileTree::nextFromSimpleAxis(SimpleAxis axis, const QXmlNodeModelIndex &nodeIndex) const { const QFileInfo fi(toFileInfo(nodeIndex)); const Type type = Type(nodeIndex.additionalData()); if (type != File && type != Directory) { Q_ASSERT_X(axis == Parent, Q_FUNC_INFO, "An attribute only has a parent!"); return toNodeIndex(fi, Directory); } switch (axis) { case Parent: return toNodeIndex(QFileInfo(fi.path()), Directory); case FirstChild: { if (type == File) // A file has no children. return QXmlNodeModelIndex(); else { Q_ASSERT(type == Directory); Q_ASSERT_X(fi.isDir(), Q_FUNC_INFO, "It isn't really a directory!"); const QDir dir(fi.absoluteFilePath()); Q_ASSERT(dir.exists()); const QFileInfoList children(dir.entryInfoList(QStringList(), m_filterAllowAll, m_sortFlags)); if (children.isEmpty()) return QXmlNodeModelIndex(); const QFileInfo firstChild(children.first()); return toNodeIndex(firstChild); } } case PreviousSibling: return nextSibling(nodeIndex, fi, -1); case NextSibling: return nextSibling(nodeIndex, fi, 1); } Q_ASSERT_X(false, Q_FUNC_INFO, "Don't ever get here!"); return QXmlNodeModelIndex(); }The first thing this function does is call toFileInfo() to get the
QFileInfo
of the context node. The use ofat()
here is guaranteed to succeed because the context node must already be in the node model, and hence must have aQFileInfo
inm_fileInfos
.const QFileInfo& FileTree::toFileInfo(const QXmlNodeModelIndex &nodeIndex) const { return m_fileInfos.at(nodeIndex.data()); }The
Parent
case looks up the context node’s parent by constructing aQFileInfo
from the context node’spath
and passing it to toNodeIndex() to find theQFileInfo
inm_fileInfos
.The
FirstChild
case requires that the context node must be a directory, because a file doesn’t have children. If the context node is not a directory, a default constructedQXmlNodeModelIndex
is returned. Otherwise,entryInfoList()
constructs aQFileInfoList
of the context node’s children. The firstQFileInfo
in the list is passed to toNodeIndex() to get itsQXmlNodeModelIndex
. Note that this will add the child to the node model, if it isn’t in the model yet.The
PreviousSibling
andNextSibling
cases call the nextSibling() helper function . It takes theQXmlNodeModelIndex
of the context node, theQFileInfo
of the context node, and an offest of +1 or -1. The context node is a child of some parent, so the function gets the parent and then gets the child list for the parent. The child list is searched to find theQFileInfo
of the context node. It must be there. Then the offset is applied, -1 for the previous sibling and +1 for the next sibling. The resulting index is passed to toNodeIndex() to get itsQXmlNodeModelIndex
. Note again that this will add the sibling to the node model, if it isn’t in the model yet.QXmlNodeModelIndex FileTree::nextSibling(const QXmlNodeModelIndex &nodeIndex, const QFileInfo &fileInfo, qint8 offset) const { Q_ASSERT(offset == -1 || offset == 1); // Get the context node's parent. const QXmlNodeModelIndex parent(nextFromSimpleAxis(Parent, nodeIndex)); if (parent.isNull()) return QXmlNodeModelIndex(); // Get the parent's child list. const QFileInfo parentFI(toFileInfo(parent)); Q_ASSERT(Type(parent.additionalData()) == Directory); const QFileInfoList siblings(QDir(parentFI.absoluteFilePath()).entryInfoList(QStringList(), m_filterAllowAll, m_sortFlags)); Q_ASSERT_X(!siblings.isEmpty(), Q_FUNC_INFO, "Can't happen! We started at a child."); // Find the index of the child where we started. const int indexOfMe = siblings.indexOf(fileInfo); // Apply the offset. const int siblingIndex = indexOfMe + offset; if (siblingIndex < 0 || siblingIndex > siblings.count() - 1) return QXmlNodeModelIndex(); else return toNodeIndex(siblings.at(siblingIndex)); }
The UI Class: MainWindow¶
The example’s UI is a conventional Qt GUI application inheriting
QMainWindow
and the Ui_MainWindow base class generated by Qt Designer.#include "filetree.h" #include "ui_mainwindow.h" class MainWindow : public QMainWindow, private Ui_MainWindow { Q_OBJECT public: MainWindow(); private slots: void on_actionOpenDirectory_triggered(); void on_actionAbout_triggered(); void on_queryBox_currentIndexChanged(const QString &); private: void loadDirectory(const QString &directory); void evaluateResult(); const QXmlNamePool m_namePool; const FileTree m_fileTree; QXmlNodeModelIndex m_fileNode; };It contains the custom node model (
m_fileTree
) and an instance ofQXmlNodeModelIndex
(m_fileNode
) used for holding the node index for the root of the file system subtree.m_fileNode
will be bound to a $variable in the XQuery to be evaluated.Two actions of interest are handled by slot functions: Selecting A Directory To Model and Selecting And Running An XQuery .
Selecting A Directory To Model¶
The user selects
File->Open Directory
to choose a directory to be loaded into the custom node model. Choosing a directory signals theon_actionOpenDirectory_triggered()
slot:void MainWindow::on_actionOpenDirectory_triggered() { const QString directoryName = QFileDialog::getExistingDirectory(this); if (!directoryName.isEmpty()) loadDirectory(directoryName); }The slot function simply calls the private function
loadDirectory()
with the path of the chosen directory:void MainWindow::loadDirectory(const QString &directory) { Q_ASSERT(QDir(directory).exists()); m_fileNode = m_fileTree.nodeFor(directory); QXmlQuery query(m_namePool); query.bindVariable("fileTree", m_fileNode); query.setQuery(QUrl("qrc:/queries/wholeTree.xq")); QByteArray output; QBuffer buffer(&output); buffer.open(QIODevice::WriteOnly); QXmlFormatter formatter(query, &buffer); query.evaluateTo(&formatter); treeInfo->setText(tr("Model of %1 output as XML.").arg(directory)); fileTree->setText(QString::fromLatin1(output.constData())); evaluateResult(); }
loadDirectory()
demonstrates a standard code pattern for using Qt XML Patterns programatically. First it gets the node model index for the root of the selected directory. Then it creates an instance ofQXmlQuery
and callsbindVariable()
to bind the node index to the XQuery variable$fileTree
. It then callssetQuery()
to load the XQuery text.Note
bindVariable()
must be called before callingsetQuery()
, which loads and parses the XQuery text and must have access to the variable binding as the text is parsed.The next lines create an output device for outputting the query result, which is then used to create a
QXmlFormatter
to format the query result as XML.evaluateTo()
is called to run the query, and the formatted XML output is displayed in the left panel of the UI window.Finally, the private function evaluateResult() is called to run the currently selected XQuery over the custom node model.
Note
As described in Building And Traversing The Node Model , the
FileTree
class wants to build the custom node model incrementally as it evaluates the XQuery . But, because theloadDirectory()
function runs thewholeTree.xq
XQuery , it actually builds the entire node model anyway. See Node Model Building Strategy for a discussion about building your custom node model.
Selecting And Running An XQuery¶
The user chooses an XQuery from the menu in the combobox on the right. Choosing an XQuery signals the
on_queryBox_currentIndexChanged()
slot:void MainWindow::on_queryBox_currentIndexChanged(const QString ¤tText) { QFile queryFile(":/queries/" + currentText); queryFile.open(QIODevice::ReadOnly); queryEdit->setPlainText(QString::fromLatin1(queryFile.readAll())); evaluateResult(); }The slot function opens and loads the query file and then calls the private function
evaluateResult()
to run the query:void MainWindow::evaluateResult() { if (queryBox->currentText().isEmpty() || m_fileNode.isNull()) return; QXmlQuery query(m_namePool); query.bindVariable("fileTree", m_fileNode); query.setQuery(QUrl("qrc:/queries/" + queryBox->currentText())); QByteArray formatterOutput; QBuffer buffer(&formatterOutput); buffer.open(QIODevice::WriteOnly); QXmlFormatter formatter(query, &buffer); query.evaluateTo(&formatter); output->setText(QString::fromLatin1(formatterOutput.constData())); }
evaluateResult()
is a second example of the same code pattern shown in loadDirectory() . In this case, it runs the XQuery currently selected in the combobox instead ofqrc:/queries/wholeTree.xq
, and it outputs the query result to the panel on the lower right of the UI window.
Node Model Building Strategy¶
We saw that the FileTree tries to build its custom node model incrementally, but we also saw that the MainWindow::loadDirectory() function in the UI class immediately subverts the incremental build by running the
wholeTree.xq
XQuery , which traverses the entire selected directory, thereby causing the entire node model to be built.If we want to preserve the incremental build capability of the
FileTree
class, we can strip the running ofwholeTree.xq
out of MainWindow::loadDirectory() :void MainWindow::loadDirectory(const QString &directory) { Q_ASSERT(QDir(directory).exists()); m_fileNode = m_fileTree.nodeFor(directory); }Note, however, that
FileTree
doesn’t have the capability of deleting all or part of the node model. The node model, once built, is only deleted when theFileTree
instance goes out of scope.In this example, each element node in the node model represents a directory or a file in the computer’s file system, and each node is represented by an instance of
QFileInfo
. An instance ofQFileInfo
is not costly to produce, but you might imagine a node model where building new nodes is very costly. In such cases, the capability to build the node model incrementally is important, because it allows us to only build the region of the model we need for evaluating the query. In other cases, it will be simpler to just build the entire node model.
© 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.