QAbstractXmlNodeModel Class | Qt XML Patterns 5.14.1

Qt 5.14 | Qt XML Patterns | C++ Classes | QAbstractXmlNodeModel

Qt 5.14.1 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
Inherits: | QSharedData
Inherited By: | QSimpleXmlNodeModel

This class was introduced in Qt 4.4.

- 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 &n) const = 0
virtual QXmlNodeModelIndex::DocumentOrder | compareOrder(const QXmlNodeModelIndex &ni1, const QXmlNodeModelIndex &ni2) const = 0
virtual QUrl | documentUri(const QXmlNodeModelIndex &n) const = 0
virtual QXmlNodeModelIndex | elementById(const QXmlName &id) 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 &idref) 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 &node) 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 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

Returns the attributes of element. The caller guarantees that element is an element in this node model.

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

Returns the base URI for the node whose index is n. The caller guarantees that n is not `null` and that it belongs to a node in this node model.

The base URI of a node can be extracted using the `fn:base-uri()` function. The base URI is typically used for resolving relative URIs that appear in the node or its children. It is conformant to just return the document URI, although that might not properly reflect the underlying data.

This function maps to the `dm:base-uri` accessor, which returns a base URI according to the following:

- For document nodes, the base URI and the document URI are the same.
- For elements, the base URI is the URI appearing in the element's `xml:base` attribute, if present, or it is resolved to the parent element's base URI.
- Namespace nodes have no base URI.
- The base URI for a processing instruction, comment, attribute, or text node is the base URI of the node's parent element.

The implementation guarantees to return a valid QUrl, or a default constructed QUrl. If a node has no base URI, as in the case where a comment has no parent, a default constructed QUrl is returned.

See also XQuery 1.0 and XPath 2.0 Data Model (XDM), 5.2 base-uri Accessor.

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

This function returns the relative document order for the nodes indexed by ni1 and ni2. It is used for the `Is` operator and for sorting nodes in document order.

The caller guarantees that ni1 and ni2 are not `null` and that both identify nodes in this node model.

If ni1 is identical to ni2, QXmlNodeModelIndex::Is is returned. If ni1 precedes ni2 in document order, QXmlNodeModelIndex::Precedes is returned. If ni1 follows ni2 in document order, QXmlNodeModelIndex::Follows is returned.

See also XQuery 1.0 and XPath 2.0 Data Model (XDM), 2.4 Document Order.

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

Creates a node index with data as its internal data. data is not constrained.

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

Creates a node index with pointer and additionalData as its internal data.

What pointer and additionalData is, is not constrained.

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

This is an overloaded function.

Creates a QXmlNodeModelIndex containing data and additionalData.

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

Returns the document URI of n. The document URI identifies the resource which is the document. For example, the document could be a regular file, e.g., `file:/`, or it could be the `http://` URL of the location of a file. The document URI is used for resolving URIs and to simply know where the document is.

If the node model maps to a URI in a natural way, return that URI. Otherwise, return the company or product URI. The document URI can be any URI as long as its valid and absolute.

The caller guarantees that n is not `null` and that it belongs to this QAbstractXmlNodeModel.

This function maps to the `dm:document-uri` accessor, which returns a document URI according to the following:

- If n is a document node, return an absolute QUrl containing the document URI, or a default constructed QUrl. The latter signals that no document URI is available for the document node.
- For all other nodes, return a default constructed QUrl.

See also XQuery 1.0 and XPath 2.0 Data Model (XDM), 5.4 document-uri Accessor, QUrl::isValid(), and QUrl::isRelative().

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

Returns the index of the element identified as id. XQuery's `id()` function calls this function.

The node index returned will be the element node whose value is of type `ID` and equals id, or it will be the element node that has an attribute whose typed value is of type `ID` and equals id. If there is no such element, a default constructed QXmlNodeModelIndex instance is returned. The implementor guarantees that if the returned node index is not null, it identifies an element.

It is not sufficient for an attribute or element to merely be called `id`. Its value type must also be `ID`. However, the reserved name `xml:id` is sufficient.

In id, the `namespace URI` and the `prefix` are undefined, and the `local name` is the ID that should be looked up.

See also XQuery 1.0 and XPath 2.0 Functions and Operators, 15.5.2 fn:id.

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

Returns a value indicating the kind of node identified by ni. The caller guarantees that ni is not null and that it identifies a node in this node model. This function maps to the `dm:node-kind()` accessor.

See also XQuery 1.0 and XPath 2.0 Data Model (XDM), 5.10 node-kind Accessor.

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

Returns the name of ni. The caller guarantees that ni is not `null` and that it belongs to this QAbstractXmlNodeModel.

If a node does not have a name, e.g., comment nodes, a null QXmlName is returned. QXmlNames must be created with the instance of QXmlQuery that is being used for evaluating queries using this QAbstractXmlNodeModel.

This function maps to the `dm:node-name()` accessor.

If ni is a processing instruction, a QXmlName is returned with the local name as the target name and the namespace URI and prefix both empty.

See also XQuery 1.0 and XPath 2.0 Data Model (XDM), 5.11 node-name Accessor and QXmlName.

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

Returns the in-scope namespaces of n. The caller guarantees that n is not `null` and that it belongs to this QAbstractXmlNodeModel.

This function corresponds to the `dm:namespace-nodes` accessor.

The returned vector of namespace declarations includes namespaces of the ancestors of n.

The caller guarantees that n is an Element that belongs to this QAbstractXmlNodeModel.

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

When Qt XML Patterns evaluate path expressions, it emulate them through a combination of calls with QSimpleXmlNodeModel::SimpleAxis values. Therefore, the implementation of this function must return the node, if any, that appears on the axis emanating from the origin.

If no such node is available, a default constructed QXmlNodeModelIndex is returned.

QSimpleXmlNodeModel eliminates the need to handle redundant corner cases by guaranteeing that it will never ask for:

- Children or siblings for attributes.
- Children for comments, processing instructions, and text nodes.
- Siblings or parents for document nodes.

A typical implementation performs a `switch` on the value of axis:

      QXmlNodeModelIndex MyTreeModel::nextFromSimpleAxis(SimpleAxis axis, const QXmlNodeModelIndex &origin) const
      {
        // Convert the QXmlNodeModelIndex to a value that is specific to what we represent.
        const MyValue value = toMyValue(ni);

        switch(axis)
        {
            case Parent:
                return toNodeIndex(value.parent());
            case FirstChild:
            case PreviousSibling:
            case NextSibling:
                // and so on
        }
      }

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

Returns the elements and/or attributes that have an `IDREF` value equal to idref. XQuery's `idref()` function calls this function.

The implementor guarantees that the nodes identified by the returned indexes are elements or attributes.

It is not sufficient for an attribute or element to merely be called `idref`. It must also be of type `IDREF`. Elements must be typed as `xs:IDREF` or `xs:IDREFS`, or, in the case of attributes, as `IDREF` or `IDREFS` in the schema.

In idref, the `namespace URI` and the `prefix` are undefined, and the `local name` is the ID that should be looked up.

See also XQuery 1.0 and XPath 2.0 Functions and Operators, 15.5.3 fn:idref.

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

Returns the root node of the tree that contains the node whose index is n. The caller guarantees that n is not `null` and that it identifies a node in this node model.

If n identifies a node that is a direct child of the root, parent() would return the same QXmlNodeModelIndex returned by this function.

### 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

Returns the string value for node n.

The caller guarantees that n is not `null` and that it belong to this QAbstractXmlNodeModel instance.

This function maps to the `dm:string-value()` accessor, which the specification completely specifies. Here's a summary:

- For processing instructions, the string value is the data section(excluding any whitespace appearing between the name and the data).
- For text nodes, the string value equals the text node.
- For comments, the content of the comment
- For elements, the concatenation of all text nodes that are descendants. Note, this is not only the children, but the childrens' childrens' text nodes, and so forth.
- For document nodes, the concatenation of all text nodes in the document.

See also XQuery 1.0 and XPath 2.0 Data Model (XDM), 5.13 string-value Accessor.

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

Returns the typed value for node node.

The typed value is an atomic value, which an element or attribute contains.

The caller guarantees that node is either an element or an attribute. The implementor guarantees that the returned QVariant has a value which is supported in XQuery. It cannot be an arbitrary QVariant value. The implementor also guarantees that stringValue() returns a lexical representation of typedValue() (this is guaranteed by QSimpleXmlNodeModel::stringValue()).

If the return QVariant is a default constructed variant, it signals that node has no typed value.