doc/src/filetree.qdoc

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/*!
  \example filetree
  \title File System Example
  \ingroup xmlpattern_examples
  \brief 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.

  \tableofcontents

  \section1 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 \c{cpp} files in the subtree beginning at
  \c{/filetree}:

  \image filetree_1-example.png

  \section2 The User Interface

  The example is shown below. First, we use \c{File->Open Directory}
  (not shown) to select the \c{/filetree} directory. Then we use the
  combobox on the right to select the XQuery that searches for \c{cpp}
  files (\c{listCPPFiles.xq}).  Selecting an XQuery runs the query,
  which in this case traverses the model looking for all the \c{cpp}
  files. The XQuery text and the query results are shown on the right:

  \image filetree_2-example.png

  Don't be mislead by the XML representation of the \c{/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.

  \section2 Running your own XQueries

  You can write your own XQuery files and run them in the example
  program. The file \c{xmlpatterns/filetree/queries.qrc} is the \l{The
  Qt Resource System} {resource file} for this example. It is used in
  \c{main.cpp} (\c{Q_INIT_RESOURCE(queries);}). It lists the XQuery
  files (\c{.xq}) that can be selected in the combobox.

  \quotefromfile filetree/queries.qrc
  \printuntil

  To add your own queries to the example's combobox, store your
  \c{.xq} files in the \c{examples/xmlpatterns/filetree/queries}
  directory and add them to \c{queries.qrc} as shown above.

  \section1 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 subclassing QAbstractXmlNodeModel 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 provides QSimpleXmlNodeModel,
  which subclasses QAbstractXmlNodeModel and provides implementations
  for the callback functions that you can ignore. By subclassing
  QSimpleXmlNodeModel instead of QAbstractXmlNodeModel, you can reduce
  development time.

  \section2 The Custom Node Model Class: FileTree

  The custom node model for this example is class \c{FileTree}, which
  is derived from QSimpleXmlNodeModel. \c{FileTree} implements all the
  callback functions that don't have standard implementations in
  QSimpleXmlNodeModel. When you implement your own custom node model,
  you must provide implementations for these callback functions:

  \snippet filetree/filetree.h 0
  \snippet filetree/filetree.h 1

  The \c{FileTree} class declares four data members:

  \snippet filetree/filetree.h 2

  The QVector \c{m_fileInfos} will contain the node model. Each
  QFileInfo 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 (\c{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
  existing QObject 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 of QFileInfo, and we use that analog as the custom
  node model.

  The two sets of flags, \c{m_filterAllowAll} and \c{m_sortFlags},
  contain OR'ed flags from QDir::Filters and QDir::SortFlags
  respectively. They are set by the \c{FileTree} constructor and used
  in calls to QDir::entryInfoList() for getting the child list for a
  directory node, i.e. a QFileInfoList containing the file and
  directory nodes for all the immediate children of a directory.

  The QVector \c{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. \c{m_names}
  is indexed by the enum \c{FileTree::Type}, which specifies the node
  types:

  \target Node_Type
  \snippet filetree/filetree.h 4

  \c{Directory} and \c{File} 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. The \c{FileTree} constructor
  initializes \c{m_names} with an appropriate QXmlName for each
  element and attribute type:

  \snippet filetree/filetree.cpp 2

  Note that the constructor does \e{not} pre-build the entire node
  model. Instead, the node model is built \e{incrementally} as the
  query engine evaluates a query. To see how the query engine causes
  the node model to be built incrementally, see \l{Building And
  Traversing The Node Model}. To see how the query engine accesses the
  node model, see \l{Accessing the node model}. See also: \l{Node
  Model Building Strategy}.

  \section3 Accessing The Node Model

  Since the node model is stored outside the query engine in the
  \c{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 a QXmlNodeModelIndex to identify the node in
  the node model that it wants to access.  Hence all the virtual
  functions in the callback interface use a QXmlNodeModelIndex to
  uniquely identify a node in the model.

  We use the index of a QFileInfo in \c{m_fileInfos} to uniquely
  identify a node in the node model. To get the QXmlNodeModelIndex for
  a QFileInfo, the class uses the private function \c{toNodeIndex()}:

  \target main toNodeIndex
  \snippet filetree/filetree.cpp 1

  It searches the \c{m_fileInfos} vector for a QFileInfo that matches
  \c{fileInfo}. If a match is found, its array index is passed to
  QAbstractXmlNodeModel::createIndex() as the \c data value for the
  QXmlNodeIndex. If no match is found, the unmatched QFileInfo is
  appended to the vector, so this function is also doing the actual
  incremental model building (see \l{Building And Traversing The Node
  Model}).

  Note that \c{toNodeIndex()} gets a \l{Node_Type} {node type} as the
  second parameter, which it just passes on to
  \l{QAbstractXmlNodeModel::createIndex()} {createIndex()} as the
  \c{additionalData} value. Logically, this second parameter
  represents a second dimension in the node model, where the first
  dimension represents the \e element nodes, and the second dimension
  represents each element's attribute nodes. The meaning is that each
  QFileInfo in the \c{m_fileInfos} vector can represent an \e{element}
  node \e{and} one or more \e{attribute} nodes. In particular, the
  QFileInfo for a file will contain the values for the attribute nodes
  path, name, suffix, size, and mime type (see
  \c{FileTree::attributes()}). Since the attributes are contained in
  the QFileInfo of the file element, there aren't actually any
  attribute nodes in the node model. Hence, we can use a QVector for
  \c{m_fileInfos}.

  A convenience overloading of \l{toNodeIndex of convenience}
  {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 the QFileInfo parameter and calls the other \l{main toNodeIndex}
  {toNodeIndex()}, after obtaining either the Directory or File node
  type directly from the QFileInfo:

  \target toNodeIndex of convenience
  \snippet filetree/filetree.cpp 0

  Note that the auxiliary vector \c{m_names} is accessed using the
  \l{Node_Type} {node type}, for example:

  \snippet filetree/filetree.cpp 3

  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.

  \section3 Building And Traversing The Node Model

  The node model in \c{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 \l{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 \l{next node on axis} {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.

  \l{next node on axis} {nextFromSimpleAxis()} takes an
  \l{QAbstractXmlNodeModel::SimpleAxis} {axis identifier} and a
  \l{QXmlNodeModelIndex} {node identifier} as parameters. The
  \l{QXmlNodeModelIndex} {node identifier} represents the \e{context
  node} (i.e. the query engine's current location in the model), and
  the \l{QAbstractXmlNodeModel::SimpleAxis} {axis identifier}
  represents the direction we want to move from the context node. The
  function finds the appropriate next node and returns its
  QXmlNodeModelIndex.

  \l{next node on axis} {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 \l{QAbstractXmlNodeModel::SimpleAxis} {axis}.

  \target next node on axis
  \snippet filetree/filetree.cpp 4

  The first thing this function does is call \l{to file info}
  {toFileInfo()} to get the QFileInfo of the context node. The use of
  QVector::at() here is guaranteed to succeed because the context node
  must already be in the node model, and hence must have a QFileInfo
  in \c{m_fileInfos}.

  \target to file info
  \snippet filetree/filetree.cpp 6

  The \l{QAbstractXmlNodeModel::Parent} {Parent} case looks up the
  context node's parent by constructing a QFileInfo from the context
  node's \l{QFileInfo::absoluteFilePath()} {path} and passing it to
  \l{main toNodeIndex} {toNodeIndex()} to find the QFileInfo in
  \c{m_fileInfos}.

  The \l{QAbstractXmlNodeModel::FirstChild} {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
  constructed QXmlNodeModelIndex is returned. Otherwise,
  QDir::entryInfoList() constructs a QFileInfoList of the context
  node's children. The first QFileInfo in the list is passed to
  \l{toNodeIndex of convenience} {toNodeIndex()} to get its
  QXmlNodeModelIndex. Note that this will add the child to the node
  model, if it isn't in the model yet.

  The \l{QAbstractXmlNodeModel::PreviousSibling} {PreviousSibling} and
  \l{QAbstractXmlNodeModel::NextSibling} {NextSibling} cases call the
  \l{nextSibling helper} {nextSibling() helper function}. It takes the
  QXmlNodeModelIndex of the context node, the QFileInfo 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 the QFileInfo 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 \l{toNodeIndex of convenience} {toNodeIndex()} to
  get its QXmlNodeModelIndex. Note again that this will add the
  sibling to the node model, if it isn't in the model yet.

  \target nextSibling helper
  \snippet filetree/filetree.cpp 5

  \section2 The UI Class: MainWindow

  The example's UI is a conventional Qt GUI application inheriting
  QMainWindow and the Ui_MainWindow base class generated by
  \l{Qt Designer Manual} {Qt Designer}.

  \snippet filetree/mainwindow.h 0

  It contains the custom node model (\c{m_fileTree}) and an instance
  of QXmlNodeModelIndex (\c{m_fileNode}) used for holding the node
  index for the root of the file system subtree. \c{m_fileNode} will
  be bound to a $variable in the XQuery to be evaluated.

  Two actions of interest are handled by slot functions: \l{Selecting
  A Directory To Model} and \l{Selecting And Running An XQuery}.

  \section3 Selecting A Directory To Model

  The user selects \c{File->Open Directory} to choose a directory to
  be loaded into the custom node model. Choosing a directory signals
  the \c{on_actionOpenDirectory_triggered()} slot:

  \snippet filetree/mainwindow.cpp 1

  The slot function simply calls the private function
  \c{loadDirectory()} with the path of the chosen directory:

  \target the standard code pattern
  \snippet filetree/mainwindow.cpp 4

  \c{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
  of QXmlQuery and calls QXmlQuery::bindVariable() to bind the node
  index to the XQuery variable \c{$fileTree}. It then calls
  QXmlQuery::setQuery() to load the XQuery text.

  \note QXmlQuery::bindVariable() must be called \e before calling
  QXmlQuery::setQuery(), 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. QXmlQuery::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 \l{Selecting And Running An XQuery}
  {evaluateResult()} is called to run the currently selected XQuery
  over the custom node model.

  \note As described in \l{Building And Traversing The Node Model},
  the \c FileTree class wants to build the custom node model
  incrementally as it evaluates the XQuery. But, because the
  \c{loadDirectory()} function runs the \c{wholeTree.xq} XQuery, it
  actually builds the entire node model anyway. See \l{Node Model
  Building Strategy} for a discussion about building your custom node
  model.

  \section3 Selecting And Running An XQuery

  The user chooses an XQuery from the menu in the combobox on the
  right. Choosing an XQuery signals the
  \c{on_queryBox_currentIndexChanged()} slot:

  \snippet filetree/mainwindow.cpp 2

  The slot function opens and loads the query file and then calls the
  private function \c{evaluateResult()} to run the query:

  \snippet filetree/mainwindow.cpp 3

  \c{evaluateResult()} is a second example of the same code pattern
  shown in \l{the standard code pattern} {loadDirectory()}. In this
  case, it runs the XQuery currently selected in the combobox instead
  of \c{qrc:/queries/wholeTree.xq}, and it outputs the query result to
  the panel on the lower right of the UI window.

  \section2 Node Model Building Strategy

  We saw that the \l{The Custom Node Model Class: FileTree} {FileTree}
  tries to build its custom node model incrementally, but we also saw
  that the \l{the standard code pattern} {MainWindow::loadDirectory()}
  function in the UI class immediately subverts the incremental build
  by running the \c{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
  \c{FileTree} class, we can strip the running of \c{wholeTree.xq} out
  of \l{the standard code pattern} {MainWindow::loadDirectory()}:

  \snippet filetree/mainwindow.cpp 5
  \snippet filetree/mainwindow.cpp 6

  Note, however, that \c{FileTree} doesn't have the capability of
  deleting all or part of the node model. The node model, once built,
  is only deleted when the \c{FileTree} 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 of QFileInfo 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.

*/