Simple Tree Model Example

TreeModel Class Implementation

For simplicity, the model does not allow its data to be edited. As a result, the constructor takes an argument containing the data that the model will share with views and delegates:

 TreeModel::TreeModel(const QString &data, QObject *parent)
     : QAbstractItemModel(parent)
 {
     QList<QVariant> rootData;
     rootData << "Title" << "Summary";
     rootItem = new TreeItem(rootData);
     setupModelData(data.split(QString("\n")), rootItem);
 }

It is up to the constructor to create a root item for the model. This item only contains vertical header data for convenience. We also use it to reference the internal data structure that contains the model data, and it is used to represent an imaginary parent of top-level items in the model.

The model's internal data structure is populated with items by the setupModelData() function. We will examine this function separately at the end of this document.

The destructor ensures that the root item and all of its descendants are deleted when the model is destroyed:

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

Since we cannot add data to the model after it is constructed and set up, this simplifies the way that the internal tree of items is managed.

Models must implement an index() function to provide indexes for views and delegates to use when accessing data. Indexes are created for other components when they are referenced by their row and column numbers, and their parent model index. If an invalid model index is specified as the parent, it is up to the model to return an index that corresponds to a top-level item in the model.

When supplied with a model index, we first check whether it is valid. If it is not, we assume that a top-level item is being referred to; otherwise, we obtain the data pointer from the model index with its internalPointer() function and use it to reference a TreeItem object. Note that all the model indexes that we construct will contain a pointer to an existing TreeItem, so we can guarantee that any valid model indexes that we receive will contain a valid data pointer.

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

     TreeItem *parentItem;

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

     TreeItem *childItem = parentItem->child(row);
     if (childItem)
         return createIndex(row, column, childItem);
     else
         return QModelIndex();
 }

Since the row and column arguments to this function refer to a child item of the corresponding parent item, we obtain the item using the TreeItem::child() function. The createIndex() function is used to create a model index to be returned. We specify the row and column numbers, and a pointer to the item itself. The model index can be used later to obtain the item's data.

The way that the TreeItem objects are defined makes writing the parent() function easy:

 QModelIndex TreeModel::parent(const QModelIndex &index) const
 {
     if (!index.isValid())
         return QModelIndex();

     TreeItem *childItem = static_cast<TreeItem*>(index.internalPointer());
     TreeItem *parentItem = childItem->parent();

     if (parentItem == rootItem)
         return QModelIndex();

     return createIndex(parentItem->row(), 0, parentItem);
 }

We only need to ensure that we never return a model index corresponding to the root item. To be consistent with the way that the index() function is implemented, we return an invalid model index for the parent of any top-level items in the model.

When creating a model index to return, we must specify the row and column numbers of the parent item within its own parent. We can easily discover the row number with the TreeItem::row() function, but we follow a convention of specifying 0 as the column number of the parent. The model index is created with createIndex() in the same way as in the index() function.

The rowCount() function simply returns the number of child items for the TreeItem that corresponds to a given model index, or the number of top-level items if an invalid index is specified:

 int TreeModel::rowCount(const QModelIndex &parent) const
 {
     TreeItem *parentItem;
     if (parent.column() > 0)
         return 0;

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

     return parentItem->childCount();
 }

Since each item manages its own column data, the columnCount() function has to call the item's own columnCount() function to determine how many columns are present for a given model index. As with the rowCount() function, if an invalid model index is specified, the number of columns returned is determined from the root item:

 int TreeModel::columnCount(const QModelIndex &parent) const
 {
     if (parent.isValid())
         return static_cast<TreeItem*>(parent.internalPointer())->columnCount();
     else
         return rootItem->columnCount();
 }

Data is obtained from the model via data(). Since the item manages its own columns, we need to use the column number to retrieve the data with the TreeItem::data() function:

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

     if (role != Qt::DisplayRole)
         return QVariant();

     TreeItem *item = static_cast<TreeItem*>(index.internalPointer());

     return item->data(index.column());
 }

Note that we only support the DisplayRole in this implementation, and we also return invalid QVariant objects for invalid model indexes.

We use the flags() function to ensure that views know that the model is read-only:

 Qt::ItemFlags TreeModel::flags(const QModelIndex &index) const
 {
     if (!index.isValid())
         return 0;

     return Qt::ItemIsEnabled | Qt::ItemIsSelectable;
 }

The headerData() function returns data that we conveniently stored in the root item:

 QVariant TreeModel::headerData(int section, Qt::Orientation orientation,
                                int role) const
 {
     if (orientation == Qt::Horizontal && role == Qt::DisplayRole)
         return rootItem->data(section);

     return QVariant();
 }

This information could have been supplied in a different way: either specified in the constructor, or hard coded into the headerData() function.

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