Simple DOM Model Example

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.

The aim is to use the structure provided by QDomDocument by wrapping QDomNode objects in item objects similar to the TreeItem objects used in the Simple Tree Model example.

Let us begin by examining the DomModel class:

class DomModel : public QAbstractItemModel
{
    Q_OBJECT

public:
    explicit DomModel(const QDomDocument &document, QObject *parent = nullptr);
    ~DomModel();

    QVariant data(const QModelIndex &index, int role) const override;
    Qt::ItemFlags flags(const QModelIndex &index) const override;
    QVariant headerData(int section, Qt::Orientation orientation,
                        int role = Qt::DisplayRole) const override;
    QModelIndex index(int row, int column,
                      const QModelIndex &parent = QModelIndex()) const override;
    QModelIndex parent(const QModelIndex &child) const override;
    int rowCount(const QModelIndex &parent = QModelIndex()) const override;
    int columnCount(const QModelIndex &parent = QModelIndex()) const override;

private:
    QDomDocument domDocument;
    DomItem *rootItem;
};

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

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

class DomItem
{
public:
    DomItem(const QDomNode &node, int row, DomItem *parent = nullptr);
    ~DomItem();
    DomItem *child(int i);
    DomItem *parent();
    QDomNode node() const;
    int row() const;

private:
    QDomNode domNode;
    QHash<int, DomItem *> childItems;
    DomItem *parentItem;
    int rowNumber;
};

Each 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 parent(), child(), and row() functions are convenience functions for the 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.

Since the 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:

DomItem::DomItem(const QDomNode &node, int row, DomItem *parent)
    : domNode(node),
      parentItem(parent),
      rowNumber(row)
{}

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:

DomItem *DomItem::parent()
{
    return parentItem;
}

int DomItem::row() const
{
    return rowNumber;
}

QDomNode DomItem::node() const
{
    return domNode;
}

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 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:

DomItem *DomItem::child(int i)
{
    DomItem *childItem = childItems.value(i);
    if (childItem)
        return childItem;

    // if child does not yet exist, create it
    if (i >= 0 && i < domNode.childNodes().count()) {
        QDomNode childNode = domNode.childNodes().item(i);
        childItem = new DomItem(childNode, i, this);
        childItems[i] = childItem;
    }
    return childItem;
}

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:

DomItem::~DomItem()
{
    qDeleteAll(childItems);
}

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

The structure provided by the DomItem class makes the implementation of DomModel similar to the TreeModel shown in the Simple Tree Model example.

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

DomModel::DomModel(const QDomDocument &document, QObject *parent)
    : QAbstractItemModel(parent),
      domDocument(document),
      rootItem(new DomItem(domDocument, 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:

DomModel::~DomModel()
{
    delete rootItem;
}

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

Basic Properties of The Model▲

int DomModel::columnCount(const QModelIndex &parent) const
{
    Q_UNUSED(parent);
    return 3;
}

Qt::ItemFlags DomModel::flags(const QModelIndex &index) const
{
    if (!index.isValid())
        return Qt::NoItemFlags;

    return QAbstractItemModel::flags(index);
}

QVariant DomModel::headerData(int section, Qt::Orientation orientation,
                              int role) const
{
    if (orientation == Qt::Horizontal && role == Qt::DisplayRole) {
        switch (section) {
            case 0:
                return tr("Name");
            case 1:
                return tr("Attributes");
            case 2:
                return tr("Value");
            default:
                break;
        }
    }
    return QVariant();
}

Navigating The Document▲

QModelIndex DomModel::index(int row, int column, const QModelIndex &parent) const
{
    if (!hasIndex(row, column, parent))
        return QModelIndex();

    DomItem *parentItem;

    if (!parent.isValid())
        parentItem = rootItem;
    else
        parentItem = static_cast<DomItem*>(parent.internalPointer());