C++ model for QML TreeView

Question

Due to lack of any other Qt demo, I am making use of Qt widgets's SimpleTreeModeldemo to implement C++ model for my QML TreeView. I have defined roles so QML can use it but I am having trouble connecting them with actual model data.

~~What I also find interesting is that widgets (C++) demo works fine but TreeModeldoesn't seem to store the data as its member variable..leaves me scratching my head.~~ I figured this out, every TreeItem store all its child items and TreeModel has only one rootItem which in turn stores all data as its child.

The TreeItem class

class TreeItem
{
public:
    explicit TreeItem(const QList<QVariant> &data, TreeItem *parentItem = 0);
    ~TreeItem();

    void appendChild(TreeItem *child);

    TreeItem *child(int row);
    int childCount() const;
    int columnCount() const;
    QVariant data(int column) const;
    int row() const;
    TreeItem *parentItem();

private:
    QList<TreeItem*> m_childItems;
    QList<QVariant> m_itemData;
    TreeItem *m_parentItem;
};

The TreeModel class

class TreeModel : public QAbstractItemModel
{
    Q_OBJECT

public:
    enum DisplayRoles {
        TitleRole = Qt::UserRole + 1,
        SummaryRole
    };

    explicit TreeModel(const QString &data, QObject *parent = 0);
    ~TreeModel();

    QVariant data(const QModelIndex &index, int role) const Q_DECL_OVERRIDE;
    Qt::ItemFlags flags(const QModelIndex &index) const Q_DECL_OVERRIDE;
    QVariant headerData(int section, Qt::Orientation orientation,
                        int role = Qt::DisplayRole) const Q_DECL_OVERRIDE;
    QModelIndex index(int row, int column,
                      const QModelIndex &parent = QModelIndex()) const Q_DECL_OVERRIDE;
    QModelIndex parent(const QModelIndex &index) const Q_DECL_OVERRIDE;
    int rowCount(const QModelIndex &parent = QModelIndex()) const Q_DECL_OVERRIDE;
    int columnCount(const QModelIndex &parent = QModelIndex()) const Q_DECL_OVERRIDE;

    QHash<int, QByteArray> TreeModel::roleNames() const {
        QHash<int, QByteArray> roles;
        roles[TitleRole] = "title";
        roles[SummaryRole] = "summary";
        return roles;
    }


private:
    void setupModelData(const QStringList &lines, TreeItem *parent);

    TreeItem *rootItem;
};

The model loads the data from a default.txt

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);
}

void TreeModel::setupModelData(const QStringList &lines, TreeItem *parent)
{
    QList<TreeItem*> parents;
    QList<int> indentations;
    parents << parent;
    indentations << 0;

    int number = 0;

    while (number < lines.count()) {
        int position = 0;
        while (position < lines[number].length()) {
            if (lines[number].mid(position, 1) != " ")
                break;
            position++;
        }

        QString lineData = lines[number].mid(position).trimmed();

        if (!lineData.isEmpty()) {
            // Read the column data from the rest of the line.
            QStringList columnStrings = lineData.split("\t", QString::SkipEmptyParts);
            QList<QVariant> columnData;
            for (int column = 0; column < columnStrings.count(); ++column)
                columnData << columnStrings[column];

            if (position > indentations.last()) {
                // The last child of the current parent is now the new parent
                // unless the current parent has no children.

                if (parents.last()->childCount() > 0) {
                    parents << parents.last()->child(parents.last()->childCount()-1);
                    indentations << position;
                }
            } else {
                while (position < indentations.last() && parents.count() > 0) {
                    parents.pop_back();
                    indentations.pop_back();
                }
            }

            // Append a new item to the current parent's list of children.
            parents.last()->appendChild(new TreeItem(columnData, parents.last()));
        }

        ++number;
    }
}

My problem is in this function, how do I connect the roles with the data which is stored in rootItem?. Note titleString and summaryStringare possible proposed functions (if needed) but I don't know what to write in them get access the data!

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

    if (index.isValid() && role >= TitleRole) {
        switch (role) {
        case TitleRole:
            return QVariant(titleString(rootItem(index))); // get title through rootItem?
        case SummaryRole:
            return QVariant(summaryString(rootItem(index))); // get summary through rootItem?
        }
    }

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

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

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

The default.txthas the following data but the same is available in the Qt Creator demo itself as well.

Getting Started             How to familiarize yourself with Qt Designer
    Launching Designer          Running the Qt Designer application
    The User Interface          How to interact with Qt Designer

Designing a Component           Creating a GUI for your application
    Creating a Dialog           How to create a dialog
    Composing the Dialog        Putting widgets into the dialog example
    Creating a Layout           Arranging widgets on a form
    Signal and Slot Connections     Making widget communicate with each other

Using a Component in Your Application   Generating code from forms
    The Direct Approach         Using a form without any adjustments
    The Single Inheritance Approach Subclassing a form's base class
    The Multiple Inheritance Approach   Subclassing the form itself
    Automatic Connections       Connecting widgets using a naming scheme
        A Dialog Without Auto-Connect   How to connect widgets without a naming scheme
        A Dialog With Auto-Connect  Using automatic connections

Form Editing Mode           How to edit a form in Qt Designer
    Managing Forms          Loading and saving forms
    Editing a Form          Basic editing techniques
    The Property Editor         Changing widget properties
    The Object Inspector        Examining the hierarchy of objects on a form
    Layouts             Objects that arrange widgets on a form
        Applying and Breaking Layouts   Managing widgets in layouts 
        Horizontal and Vertical Layouts Standard row and column layouts
        The Grid Layout         Arranging widgets in a matrix
    Previewing Forms            Checking that the design works

Using Containers            How to group widgets together
    General Features            Common container features
    Frames              QFrame
    Group Boxes             QGroupBox
    Stacked Widgets         QStackedWidget
    Tab Widgets             QTabWidget
    Toolbox Widgets         QToolBox

Connection Editing Mode         Connecting widgets together with signals and slots
    Connecting Objects          Making connections in Qt Designer
    Editing Connections         Changing existing connections

My output shows the same number of lines as in widgets demo except that there is no text. It seems like it is just not connected to the roles properly or roles are not connected to day. I am attaching the screen shot of my output.

filesystembrowser seems to be a better fit - look for treeview in QtCreator samples screen — CapelliC
@CapelliC That is what I started with, the data function there works with standard QFileSystemModel and only calls fileInfo(index) to retrieve the file in question. Here the data model is custom data. It really is a question how how to traverse this data. I need this structure to work because my actual project will store the data in same way. — zar
Here you have a good example to start with. TreeItem should be modified to take into account your new roles. — Tarod

zar zar · Accepted Answer · 2016-01-25T20:45:35

Just so if anyone is trying to do the same thing, I figured it out and here is the answer. The data method of the model should read the following.

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

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

    if (index.isValid() && role >= TitleRole) {
        switch (role) {
        case TitleRole:
            return item->data(0);
        case SummaryRole:
            return item->data(1);
        }
    }

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

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

TreeItem could be changed in real application to store specific data in which case the above method will point to that data instead of column number approach.

C++ model for QML TreeView

1 Answers