Model/View Programming

Inserting dropped data into a model

The way that any given model handles dropped data depends on both its type (list, table, or tree) and the way its contents is likely to be presented to the user. Generally, the approach taken to accommodate dropped data should be the one that most suits the model's underlying data store.

Different types of model tend to handle dropped data in different ways. List and table models only provide a flat structure in which items of data are stored. As a result, they may insert new rows (and columns) when data is dropped on an existing item in a view, or they may overwrite the item's contents in the model using some of the data supplied. Tree models are often able to add child items containing new data to their underlying data stores, and will therefore behave more predictably as far as the user is concerned.

Dropped data is handled by a model's reimplementation of QAbstractItemModel::dropMimeData(). For example, a model that handles a simple list of strings can provide an implementation that handles data dropped onto existing items separately to data dropped into the top level of the model (i.e., onto an invalid item).

The model first has to make sure that the operation should be acted on, the data supplied is in a format that can be used, and that its destination within the model is valid:

 bool DragDropListModel::dropMimeData(const QMimeData *data,
     Qt::DropAction action, int row, int column, const QModelIndex &parent)
 {
     if (action == Qt::IgnoreAction)
         return true;

     if (!data->hasFormat("application/vnd.text.list"))
         return false;

     if (column > 0)
         return false;

A simple one column string list model can indicate failure if the data supplied is not plain text, or if the column number given for the drop is invalid.

The data to be inserted into the model is treated differently depending on whether it is dropped onto an existing item or not. In this simple example, we want to allow drops between existing items, before the first item in the list, and after the last item.

When a drop occurs, the model index corresponding to the parent item will either be valid, indicating that the drop occurred on an item, or it will be invalid, indicating that the drop occurred somewhere in the view that corresponds to top level of the model.

     int beginRow;

     if (row != -1)
         beginRow = row;

We initially examine the row number supplied to see if we can use it to insert items into the model, regardless of whether the parent index is valid or not.

     else if (parent.isValid())
         beginRow = parent.row();

If the parent model index is valid, the drop occurred on an item. In this simple list model, we find out the row number of the item and use that value to insert dropped items into the top level of the model.

     else
         beginRow = rowCount(QModelIndex());

When a drop occurs elsewhere in the view, and the row number is unusable, we append items to the top level of the model.

In hierarchical models, when a drop occurs on an item, it would be better to insert new items into the model as children of that item. In the simple example shown here, the model only has one level, so this approach is not appropriate.

     QByteArray encodedData = data->data("application/vnd.text.list");
     QDataStream stream(&encodedData, QIODevice::ReadOnly);
     QStringList newItems;
     int rows = 0;

     while (!stream.atEnd()) {
         QString text;
         stream >> text;
         newItems << text;
         ++rows;
     }

     insertRows(beginRow, rows, QModelIndex());
     foreach (const QString &text, newItems) {
         QModelIndex idx = index(beginRow, 0, QModelIndex());
         setData(idx, text);
         beginRow++;
     }

     return true;
 }

     QSortFilterProxyModel *filterModel = new QSortFilterProxyModel(parent);
     filterModel->setSourceModel(stringListModel);

     QListView *filteredView = new QListView;
     filteredView->setModel(filterModel);