Model Classes
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Rows and columns The diagram shows a representation of a basic table model in which each item is located by a pair of row and column numbers. By passing the relevant row and column numbers to the model we obtain a model index that refers to an item of data. QModelIndex indexA = model->index(0, 0, QModelIndex()); QModelIndex indexB = model->index(1, 1, QModelIndex()); QModelIndex indexC = model->index(2, 1, QModelIndex()); Top level items in a model are always referenced by specifying QModelIndex() as their parent item. This is discussed in the next section. |
The table-like interface to item data provided by models is ideal when using data in a table or list view; the row and column number system maps exactly to the way the views display items. However, structures such as tree views require the model to expose a more flexible interface to the items within. As a result, each item can also be the parent of another table of items, in much the same way that a top-level item in a tree view can contain another list of items.
When requesting an index for a model item, we must provide some information about the item's parent. Outside the model, the only way to refer to an item is through a model index, so a parent model index must also be given:
QModelIndex index = model->index(row, column, parent);
Parents, rows, and columns The diagram shows a representation of a tree model in which each item is referred to by a parent, a row number, and a column number. Items "A" and "C" are represented as top-level siblings in the model: QModelIndex indexA = model->index(0, 0, QModelIndex()); QModelIndex indexC = model->index(2, 1, QModelIndex()); Item "A" has a number of children. A model index for item "B" is obtained with the following code: QModelIndex indexB = model->index(1, 0, indexA); |
Items in a model can perform various roles for other components, allowing different kinds of data to be supplied for different situations. For example, Qt::DisplayRole is used to access a string that can be displayed as text in a view. Typically, items contain data for a number of different roles, and the standard roles are defined by Qt::ItemDataRole.
We can ask the model for the item's data by passing it the model index corresponding to the item, and by specifying a role to obtain the type of data we want:
QVariant value = model->data(index, role);
Item roles The role indicates to the model which type of data is being referred to. Views can display the roles in different ways, so it is important to supply appropriate information for each role. The Creating New Models section covers some specific uses of roles in more detail. |
Most common uses for item data are covered by the standard roles defined in Qt::ItemDataRole. By supplying appropriate item data for each role, models can provide hints to views and delegates about how items should be presented to the user. Different kinds of views have the freedom to interpret or ignore this information as required. It is also possible to define additional roles for application-specific purposes.
To demonstrate how data can be retrieved from a model, using model indexes, we set up a QDirModel 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 directory model in the following way:
QDirModel *model = new QDirModel; QModelIndex parentIndex = model->index(QDir::currentPath()); int numRows = model->rowCount(parentIndex);
In this case, we set up a default QDirModel, 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:
New models can be created by implementing the standard interface provided by QAbstractItemModel. In the Creating New Models chapter, we will demonstrate this by creating a convenient ready-to-use model for holding lists of strings.
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