Qt 5.11 | Qt XML Patterns | C++ Classes | QAbstractXmlNodeModel

Qt 5.11.3 Reference Documentation

# QAbstractXmlNodeModel Class

The QAbstractXmlNodeModel class is an abstract base class for modeling non-XML data to look like XML for QXmlQuery. More...

Header: | #include <QAbstractXmlNodeModel>
qmake: | QT += xmlpatterns
Since: | Qt 4.4
Inherited By: | QSimpleXmlNodeModel

- List of all members, including inherited members

Note: All functions in this class are thread-safe.

## Public Types

typedef | List
typedef | Ptr
enum | SimpleAxis { Parent, FirstChild, PreviousSibling, NextSibling }

## Public Functions

| QAbstractXmlNodeModel()
virtual | ~QAbstractXmlNodeModel()
virtual QUrl | baseUri(const QXmlNodeModelIndex &ni) const = 0
virtual QXmlNodeModelIndex::DocumentOrder | compareOrder(const QXmlNodeModelIndex &ni1, const QXmlNodeModelIndex &ni2) const = 0
virtual QUrl | documentUri(const QXmlNodeModelIndex &ni) const = 0
virtual QXmlNodeModelIndex | elementById(const QXmlName &NCName) const = 0
virtual QXmlNodeModelIndex::NodeKind | kind(const QXmlNodeModelIndex &ni) const = 0
virtual QXmlName | name(const QXmlNodeModelIndex &ni) const = 0
virtual QVector<QXmlName> | namespaceBindings(const QXmlNodeModelIndex &n) const = 0
virtual QVector<QXmlNodeModelIndex> | nodesByIdref(const QXmlName &NCName) const = 0
virtual QXmlNodeModelIndex | root(const QXmlNodeModelIndex &n) const = 0
QSourceLocation | sourceLocation(const QXmlNodeModelIndex &index) const
virtual QString | stringValue(const QXmlNodeModelIndex &n) const = 0
virtual QVariant | typedValue(const QXmlNodeModelIndex &n) const = 0

## Protected Functions

virtual QVector<QXmlNodeModelIndex> | attributes(const QXmlNodeModelIndex &element) const = 0
QXmlNodeModelIndex | createIndex(qint64 data) const
QXmlNodeModelIndex | createIndex(void *pointer, qint64 additionalData = 0) const
QXmlNodeModelIndex | createIndex(qint64 data, qint64 additionalData) const
virtual QXmlNodeModelIndex | nextFromSimpleAxis(QAbstractXmlNodeModel::SimpleAxis axis, const QXmlNodeModelIndex &origin) const = 0

## Detailed Description

The QAbstractXmlNodeModel class is an abstract base class for modeling non-XML data to look like XML for QXmlQuery.

The QAbstractXmlNodeModel specifies the interface that a node model must implement for that node model be accessible to the query engine for processing XQuery queries. A node model represents data as a structure that can be queried as if the data were XML.

The node model represented by a subclass of QAbstractXmlNodeModel is meant to be accessed by the Qt XML Patterns query engine. If the API seems a little strange in a few places, it is because the member functions are called by the query engine as it evaluates an XQuery. They aren't meant to be used programatically.

### Usage

QAbstractXmlNodeModel bridges the gap between the arbitrary structure of the non-XML data to be queried and the well-defined structure of XML data understood by QXmlQuery.

Consider a chemistry application that reads the file `chemistryData`, which contains non-XML data that represents a chemical structure composed of molecules and atoms. The application will query this chemistry data with an XQuery it reads from file `queryFile`. We write a custom subclass of QAbstractXmlNodeModel (`ChemistryNodeModel`) that reads `chemistryData` and builds a data structure, perhaps composed of objects of our own classes `molecule` and `atom`. Clearly, this data structure is not XML. Our custom subclass will know how to traverse this non-XML structure and present it through the XPath Data Model interface.

      QFile queryFile(argv[1]);
      QFile chemistryData(argv[2]);
      QString moleculeName = argv[3];

      QXmlQuery query;
      query.setQuery(&queryFile, QUrl::fromLocalFile(queryFile.fileName()));

      ChemistryNodeModel myNodeModel(query.namePool(), chemistryData);
      QXmlNodeModelIndex startNode = myNodeModel.nodeFor(moleculeName);
      query.bindVariable("queryRoot", startNode);

      QFile out;
      out.open(stdout, QIODevice::WriteOnly);

      QXmlSerializer serializer(query, &out);
      query.evaluateTo(&serializer);

The application first creates an instance of QXmlQuery and calls setQuery() to read `queryFile` containing the XQuery we want to run. Then it creates an instance of our custom node model class, `ChemistryNodeModel`, which is a subclass of QAbstractXmlNodeModel. Its constructor is called with the name pool obtained from our QXmlQuery, and with the `chemistryFile` containing the structure of molecules and atoms to be queried. The name pool is required because our custom node model has the member function name(), which returns the name of any node in the model. The query and the custom node model must use the same name pool for constructing these names. The constructor would then read `chemistryFile` and build the custom node model structure.

To connect the `query` to the custom node model, we must bind a variable name used in the query to a node in the model. The variable can then be used in the query as a starting node. First, an index for the desired starting node is retrieved by calling QAbstractXmlNodeModel::createIndex(). Then the index is bound to a variable name, in this case `queryRoot`, by passing the name and the index to QXmlQuery::bindVariable(). The query can then use a variable reference `$queryRoot` to refer to the starting node. Note that if the query uses multiple variable references, a call to QXmlQuery::bindVariable() is required to bind each different variable name to a node in the model.

The query is executed when the application calls one of the QXmlQuery evaluation functions. The application uses QXmlQuery::evaluateTo(QAbstractXmlReceiver *), because it then uses a serializer to out the query result as XML to `stdout`. We could have used QXmlQuery::evaluateTo(QXmlResultItems *) to get a list of result items, or QXmlQuery::evaluateTo(QStringList *) if the query evaluated to a sequence of `xs:string` values.

During query execution, the engine iterates over the node model using nextFromSimpleAxis() to get the index of the next node to be visited. The engine can get the name of a node by calling name() with the node's index. stringValue(), baseUri(), documentUri() and kind() are also called as needed with a node index.

The example demonstrates the standard pattern for using a subclass of QAbstractXmlNodeModel in combination with QXmlQuery to perform an XQuery.

1. Instantiate QXmlQuery and give it the XQuery to be run;
2. Instantiate a subclass of QAbstractXmlNodeModel or QSimpleXmlNodeModel;
3. Retrieve a QXmlNodeModelIndex for the node in the model where the QXmlQuery should start the query;
4. Use QXmlQuery::bindVariable() to bind the QXmlNodeModelIndex to `$variable name`;
5. Call one of the QXmlQuery evaluation functions to run the query.

### Subclassing

Because the XPath Data Model interface presented by QAbstractXmlNodeModel allows QXmlQuery to operate on non-XML data as if it were XML, implementing subclasses of QAbstractXmlNodeModel can involve a significant amount of work. The QSimpleXmlNodeModel class is provided to simplify the implementation for many common use cases.

### Thread Safety

Because the node model can be accessed concurrently by threads in the Qt XML Patterns module, subclasses of QAbstractXmlNodeModel must be written to be thread-safe. Classes that simplify implementing thread-safety include QReadLocker and QWriteLocker.

See the example File System Example for a demonstration.

## Member Type Documentation

### typedef QAbstractXmlNodeModel::List

A list of smart pointers to instances of QAbstractXmlNodeModel.

See also QExplicitlySharedDataPointer.

### typedef QAbstractXmlNodeModel::Ptr

A smart pointer to an instance of QAbstractXmlNodeModel.

See also QExplicitlySharedDataPointer.

### enum QAbstractXmlNodeModel::SimpleAxis

Four axes that each contain one node only.

Constant | Value | Description
--- | --- | ---
`QAbstractXmlNodeModel::Parent` | `0` | The parent of the context node
`QAbstractXmlNodeModel::FirstChild` | `1` | The first child of the context node
`QAbstractXmlNodeModel::PreviousSibling` | `2` | The previous child of the context node
`QAbstractXmlNodeModel::NextSibling` | `3` | The next child of the context node

## Member Function Documentation

### QAbstractXmlNodeModel::QAbstractXmlNodeModel()

Default constructor.

### `[virtual]` QAbstractXmlNodeModel::~QAbstractXmlNodeModel()

Destructor.

### `[pure virtual protected]` QVector<QXmlNodeModelIndex> QAbstractXmlNodeModel::attributes(const QXmlNodeModelIndex &element) const

### `[pure virtual]` QUrl QAbstractXmlNodeModel::baseUri(const QXmlNodeModelIndex &ni) const

### `[pure virtual]` QXmlNodeModelIndex::DocumentOrder QAbstractXmlNodeModel::compareOrder(const QXmlNodeModelIndex &ni1, const QXmlNodeModelIndex &ni2) const

### `[protected]` QXmlNodeModelIndex QAbstractXmlNodeModel::createIndex(qint64 data) const

### `[protected]` QXmlNodeModelIndex QAbstractXmlNodeModel::createIndex(void *pointer, qint64 additionalData = 0) const

### `[protected]` QXmlNodeModelIndex QAbstractXmlNodeModel::createIndex(qint64 data, qint64 additionalData) const

### `[pure virtual]` QUrl QAbstractXmlNodeModel::documentUri(const QXmlNodeModelIndex &ni) const

### `[pure virtual]` QXmlNodeModelIndex QAbstractXmlNodeModel::elementById(const QXmlName &NCName) const

### `[pure virtual]` QXmlNodeModelIndex::NodeKind QAbstractXmlNodeModel::kind(const QXmlNodeModelIndex &ni) const

### `[pure virtual]` QXmlName QAbstractXmlNodeModel::name(const QXmlNodeModelIndex &ni) const

### `[pure virtual]` QVector<QXmlName> QAbstractXmlNodeModel::namespaceBindings(const QXmlNodeModelIndex &n) const

### `[pure virtual protected]` QXmlNodeModelIndex QAbstractXmlNodeModel::nextFromSimpleAxis(QAbstractXmlNodeModel::SimpleAxis axis, const QXmlNodeModelIndex &origin) const

### `[pure virtual]` QVector<QXmlNodeModelIndex> QAbstractXmlNodeModel::nodesByIdref(const QXmlName &NCName) const

### `[pure virtual]` QXmlNodeModelIndex QAbstractXmlNodeModel::root(const QXmlNodeModelIndex &n) const

### QSourceLocation QAbstractXmlNodeModel::sourceLocation(const QXmlNodeModelIndex &index) const

Returns the source location for the object with the given index or a default constructed QSourceLocation in case no location information is available.

This function was introduced in Qt 4.6.

### `[pure virtual]` QString QAbstractXmlNodeModel::stringValue(const QXmlNodeModelIndex &n) const

### `[pure virtual]` QVariant QAbstractXmlNodeModel::typedValue(const QXmlNodeModelIndex &n) const