path: root/doc/src/examples/simpledommodel.qdoc

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/*!
    \example itemviews/simpledommodel
    \title Simple DOM Model Example

    \brief The Simple DOM Model example shows how an existing class can be adapted for use with
    the model/view framework.

    \image simpledommodel-example.png

    Qt provides two complementary sets of classes for reading XML files: The classes based
    around QXmlReader provide a SAX-style API for incremental reading of large files, and
    the classes based around QDomDocument enable developers to access the contents of XML
    files using a Document Object Model (DOM) API.

    In this example, we create a model that uses the DOM API to expose the structure and
    contents of XML documents to views via the standard QAbstractModel interface.

    \section1 Design and Concepts

    Reading an XML document with Qt's DOM classes is a straightforward process. Typically,
    the contents of a file are supplied to QDomDocument, and nodes are accessed using the
    functions provided by QDomNode and its subclasses.

    \omit
    For example, the following code
    snippet reads the contents of a file into a QDomDocument object and traverses the
    document, reading all the plain text that can be found:

    \snippet doc/src/snippets/code/doc_src_examples_simpledommodel.cpp 0

    In principle, the functions provided by QDomNode can be used to navigate from any
    given starting point in a document to the piece of data requested by another component.
    Since QDomDocument maintains information about the structure of a document, we can
    use this to implement the required virtual functions in a QAbstractItemModel subclass.
    \endomit

    The aim is to use the structure provided by QDomDocument by wrapping QDomNode objects
    in item objects similar to the \c TreeItem objects used in the
    \l{Simple Tree Model Example}{Simple Tree Model} example.

    \section1 DomModel Class Definition

    Let us begin by examining the \c DomModel class:

    \snippet examples/itemviews/simpledommodel/dommodel.h 0

    The class definition contains all the basic functions that are needed for a
    read-only model. Only the constructor and \c document() function are specific to
    this model. The private \c domDocument variable is used to hold the document
    that is exposed by the model; the \c rootItem variable contains a pointer to
    the root item in the model.

    \section1 DomItem Class Definition

    The \c DomItem class is used to hold information about a specific QDomNode in
    the document:

    \snippet examples/itemviews/simpledommodel/domitem.h 0

    Each \c DomItem provides a wrapper for a QDomNode obtained from the underlying
    document which contains a reference to the node, it's location in the parent node's
    list of child nodes, and a pointer to a parent wrapper item.

    The \c parent(), \c child(), and \c row() functions are convenience functions for
    the \c DomModel to use that provide basic information about the item to be discovered
    quickly. The node() function provides access to the underlying QDomNode object.

    As well as the information supplied in the constructor, the class maintains a cache
    of information about any child items. This is used to provide a collection of
    persistent item objects that the model can identify consistently and improve the
    performance of the model when accessing child items.

    \section1 DomItem Class Implementation

    Since the \c DomItem class is only a thin wrapper around QDomNode objects, with a
    few additional features to help improve performance and memory usage, we can provide
    a brief outline of the class before discussing the model itself.

    The constructor simply records details of the QDomNode that needs to be wrapped:

    \snippet examples/itemviews/simpledommodel/domitem.cpp 0
    \snippet examples/itemviews/simpledommodel/domitem.cpp 1

    As a result, functions to provide the parent wrapper, the row number occupied by
    the item in its parent's list of children, and the underlying QDomNode for each item
    are straightforward to write:

    \snippet examples/itemviews/simpledommodel/domitem.cpp 4
    \codeline
    \snippet examples/itemviews/simpledommodel/domitem.cpp 6
    \codeline
    \snippet examples/itemviews/simpledommodel/domitem.cpp 3

    It is necessary to maintain a collection of items which can be consistently identified
    by the model. For that reason, we maintain a hash of child wrapper items that, to
    minimize memory usage, is initially empty. The model uses the item's \c child()
    function to help create model indexes, and this constructs wrappers for the children
    of the item's QDomNode, relating the row number of each child to the newly-constructed
    wrapper:

    \snippet examples/itemviews/simpledommodel/domitem.cpp 5

    If a QDomNode was previously wrapped, the cached wrapper is returned; otherwise, a
    new wrapper is constructed and stored for valid children, and zero is returned for
    invalid ones.

    The class's destructor deletes all the child items of the wrapper:

    \snippet examples/itemviews/simpledommodel/domitem.cpp 2

    These, in turn, will delete their children and free any QDomNode objects in use.

    \section1 DomModel Class Implementation

    The structure provided by the \c DomItem class makes the implementation of \c DomModel
    similar to the \c TreeModel shown in the
    \l{Simple Tree Model Example}{Simple Tree Model} example.

    The constructor accepts an existing document and a parent object for the model:

    \snippet examples/itemviews/simpledommodel/dommodel.cpp 0

    A shallow copy of the document is stored for future reference, and a root item is
    created to provide a wrapper around the document. We assign the root item a row
    number of zero only to be consistent since the root item will have no siblings.

    Since the model only contains information about the root item, the destructor only
    needs to delete this one item:

    \snippet examples/itemviews/simpledommodel/dommodel.cpp 1

    All of the child items in the tree will be deleted by the \c DomItem destructor as
    their parent items are deleted.

    \section2 Basic Properties of The Model

    Some aspects of the model do not depend on the structure of the underlying document,
    and these are simple to implement.

    The number of columns exposed by the model is returned by the \c columnCount()
    function:

    \snippet examples/itemviews/simpledommodel/dommodel.cpp 2

    This value is fixed, and does not depend on the location or type of the underlying
    node in the document. We will use these three columns to display different kinds of
    data from the underlying document.

    Since we only implement a read-only model, the \c flags() function is straightforward
    to write:

    \snippet examples/itemviews/simpledommodel/dommodel.cpp 5

    Since the model is intended for use in a tree view, the \c headerData() function only
    provides a horizontal header:

    \snippet examples/itemviews/simpledommodel/dommodel.cpp 6

    The model presents the names of nodes in the first column, element attributes in the
    second, and any node values in the third.

    \section2 Navigating The Document

    The index() function creates a model index for the item with the given row, column,
    and parent in the model:

    \snippet examples/itemviews/simpledommodel/dommodel.cpp 7

    The function first has to relate the parent index to an item that contains a node
    from the underlying document. If the parent index is invalid, it refers to the root
    node in the document, so we retrieve the root item that wraps it; otherwise, we
    obtain a pointer to the relevant item using the QModelIndex::internalPointer()
    function. We are able to extract a pointer in this way because any valid model index
    will have been created by this function, and we store pointers to item objects in
    any new indexes that we create with QAbstractItemModel::createIndex():

    \snippet examples/itemviews/simpledommodel/dommodel.cpp 8

    A child item for the given row is provided by the parent item's \c child() function.
    If a suitable child item was found then we call
    \l{QAbstractItemModel::createIndex()}{createIndex()} to produce a model index for the
    requested row and column, passing a pointer to the child item for it to store
    internally. If no suitable child item is found, an invalid model index is returned.

    Note that the items themselves maintain ownership of their child items. This means
    that the model does not need to keep track of the child items that have been created,
    and can let the items themselves tidy up when they are deleted.

    The number of rows beneath a given item in the model is returned by the \c rowCount()
    function, and is the number of child nodes contained by the node that corresponds to
    the specified model index:

    \snippet examples/itemviews/simpledommodel/dommodel.cpp 10

    To obtain the relevant node in the underlying document, we access the item via the
    internal pointer stored in the model index. If an invalid index is supplied, the
    root item is used instead. We use the item's \c node() function to access the node
    itself, and simply count the number of child nodes it contains.

    Since the model is used to represent a hierarchical data structure, it needs to
    provide an implementation for the \c parent() function. This returns a model index
    that corresponds to the parent of a child model index supplied as its argument:

    \snippet examples/itemviews/simpledommodel/dommodel.cpp 9

    For valid indexes other than the index corresponding to the root item, we obtain
    a pointer to the relevant item using the method described in the \c index() function,
    and use the item's \c parent() function to obtain a pointer to the parent item.

    If no valid parent item exists, or if the parent item is the root item, we can simply
    follow convention and return an invalid model index. For all other parent items, we
    create a model index containing the appropriate row and column numbers, and a pointer
    to the parent item we just obtained.

    Data is provided by the \c data() function. For simplicity, we only provide data for
    the \l{Qt::DisplayRole}{display role}, returning an invalid variant for all other
    requests:

    \snippet examples/itemviews/simpledommodel/dommodel.cpp 3

    As before, we obtain an item pointer for the index supplied, and use it to obtain
    the underlying document node. Depending on the column specified, the data we return
    is obtained in different ways:

    \snippet examples/itemviews/simpledommodel/dommodel.cpp 4

    For the first column, we return the node's name. For the second column, we read any
    attributes that the node may have, and return a string that contains a space-separated
    list of attribute-value assignments. For the third column, we return any value that
    the node may have; this allows the contents of text nodes to be displayed in a view.

    If data from any other column is requested, an invalid variant is returned.

    \section1 Implementation Notes

    Ideally, we would rely on the structure provided by QDomDocument to help us write
    the \l{QAbstractItemModel::parent()}{parent()} and
    \l{QAbstractItemModel::index()}{index()} functions that are required when subclassing
    QAbstractItemModel. However, since Qt's DOM classes use their own system for
    dynamically allocating memory for DOM nodes, we cannot guarantee that the QDomNode
    objects returned for a given piece of information will be the same for subsequent
    accesses to the document.

    We use item wrappers for each QDomNode to provide consistent pointers that the model
    can use to navigate the document structure.
    \omit
    Since these items contain value references to the QDomNode objects themselves, this
    has the side effect that the DOM nodes themselves can be used to reliably navigate
    the document [not sure about this - QDom* may return different QDomNode objects for
    the same piece of information]. However, this advantage is redundant since we need to
    use wrapper items to obtain it. [Possible use of QDomNode cache in the model itself.]
    \endomit
*/