Model Classes

Using Model Indexes

To demonstrate how data can be retrieved from a model, using model indexes, we set up a QFileSystemModel without a view and display the names of files and directories in a widget. Although this does not show a normal way of using a model, it demonstrates the conventions used by models when dealing with model indexes.

We construct a file system model in the following way:

     QFileSystemModel *model = new QFileSystemModel;
     QModelIndex parentIndex = model->index(QDir::currentPath());
     int numRows = model->rowCount(parentIndex);

In this case, we set up a default QFileSystemModel, obtain a parent index using a specific implementation of index() provided by that model, and we count the number of rows in the model using the rowCount() function.

For simplicity, we are only interested in the items in the first column of the model. We examine each row in turn, obtaining a model index for the first item in each row, and read the data stored for that item in the model.

     for (int row = 0; row < numRows; ++row) {
         QModelIndex index = model->index(row, 0, parentIndex);

To obtain a model index, we specify the row number, column number (zero for the first column), and the appropriate model index for the parent of all the items that we want. The text stored in each item is retrieved using the model's data() function. We specify the model index and the DisplayRole to obtain data for the item in the form of a string.

         QString text = model->data(index, Qt::DisplayRole).toString();
         // Display the text in a widget.

     }

The above example demonstrates the basic principles used to retrieve data from a model:

• The dimensions of a model can be found using rowCount() and columnCount(). These functions generally require a parent model index to be specified.
• Model indexes are used to access items in the model. The row, column, and parent model index are needed to specify the item.
• To access top-level items in a model, specify a null model index as the parent index with QModelIndex().
• Items contain data for different roles. To obtain the data for a particular role, both the model index and the role must be supplied to the model.