The Interview Framework

Creating a Custom Model

In this example, we display items of data obtained from a custom list model using a standard view. The custom model is a subclass of QAbstractListModel and provides implementations of a core set of functions.

The complete declaration of our model is as follows:

 class StringListModel : public QAbstractListModel
 {
     Q_OBJECT

 public:
     StringListModel(const QStringList &strings, QObject *parent = 0)
         : QAbstractListModel(parent), stringList(strings) {}

     int rowCount(const QModelIndex &parent = QModelIndex()) const;
     QVariant data(const QModelIndex &index, int role) const;
     QVariant headerData(int section, Qt::Orientation orientation,
                         int role = Qt::DisplayRole) const;

 private:
     QStringList stringList;
 };

The model takes a list of strings when constructed, and supplies these to views as required. Since this is only a simple read-only model, we only need to implement a few functions.

The underlying data structure used to hold the strings is a QStringList. Since the model maps each item in the list to a row in the model, the rowCount() function is quite simple:

 int StringListModel::rowCount(const QModelIndex &parent) const
 {
     return stringList.count();
 }

The data() function returns an item of data for each model index supplied by a view:

 QVariant StringListModel::data(const QModelIndex &index, int role) const
 {
     if (!index.isValid())
         return QVariant();

     if (index.row() >= stringList.size())
         return QVariant();

     if (role == Qt::DisplayRole)
         return stringList.at(index.row());
     else
         return QVariant();
 }

The data() function returns a QVariant containing the information referred to by the model index. Items of data are returned to the view, but only if a number of checks are satisfied; for example, if the view specifies an invalid model index, the model indicates this by returning an invalid QVariant.

Vertical and horizontal headers are supplied by the headerData() function. In this model, the value returned for these items is the row or column number, depending on the header:

 QVariant StringListModel::headerData(int section, Qt::Orientation orientation,
                                      int role) const
 {
     if (role != Qt::DisplayRole)
         return QVariant();

     if (orientation == Qt::Horizontal)
         return QString("Column %1").arg(section);
     else
         return QString("Row %1").arg(section);
 }

We only include an excerpt from the main() function for this short example:

 QStringList numbers;
 numbers << "One" << "Two" << "Three" << "Four" << "Five";

 QAbstractItemModel *model = new StringListModel(numbers);
     ...
 QListView *view = new QListView;
 view->setWindowTitle("View onto a string list model");
 view->setModel(model);

We create a string list to use with the model, and we supply it to the model when it is constructed. The information in the string list is made available to the view via the model.

This example shows that it can be easy to populate views with data from a simple model. The standard models and views planned for Qt 4 will make the process even easier, and the convenience widgets supplied provide support for the classic item-based approach.