qml Tutorial => Creating a simple model for TreeView

Example

Since Qt 5.5 we have a new wonderful TreeView, a control we've all been waiting for. A TreeView implements a tree representation of items from a model. In general it looks like other QML views - ListView or TableView. But data structure of TreeView is more complex.

A data in ListView or TableView is represented by one-dimensional array of nodes. In TreeView each node can contain its own array of nodes. Therefore, unlike the others views in TreeView to get specified node we must know parent node, not only row or column of element.

Another major difference is that TreeView doesn't support ListModel. To provide a data we must subclass QAbstractItemModel. In Qt there are ready to use model classes like QFileSystemModel which provides access to local file system, or QSqlTableModel which provides access to a data base.

In following example we will create such model derived from QAbstractItemModel. But to make the example more realistic I suggest to make the model like ListModel but specified for trees so we can add nodes from QML. It's necessary to clarify that model itself doesn't contain any data but only provide access to it. So providing and organization of data is entirely our responsibility.

Since model data is organized in a tree the simplest node structure is seen as follows, in pseudo code:

Node {
    var data;
    Node parent;
    list<Node> children;
}

In C++ the node declaration should be as following:

class MyTreeNode : public QObject
{
    Q_OBJECT
public:
    Q_PROPERTY(QQmlListProperty<MyTreeNode> nodes READ nodes)
    Q_CLASSINFO("DefaultProperty", "nodes")
    MyTreeNode(QObject *parent = Q_NULLPTR);

    void setParentNode(MyTreeNode *parent);
    Q_INVOKABLE MyTreeNode *parentNode() const;
    bool insertNode(MyTreeNode *node, int pos = (-1));
    QQmlListProperty<MyTreeNode> nodes();
    
    MyTreeNode *childNode(int index) const;
    void clear();

    Q_INVOKABLE int pos() const;
    Q_INVOKABLE int count() const;

private:
    QList<MyTreeNode *> m_nodes;
    MyTreeNode *m_parentNode;
};

We derive our class from QObject to be able to create a node in QML. All the children nodes will be added to nodes property so next 2 part of code are the same:

TreeNode {
    nodes:[
        TreeNode {}
        TreeNode {}
    ]
}

TreeNode {
    TreeNode {}
    TreeNode {}
}

See this acticle to know more about default property.

Node class implementation:

MyTreeNode::MyTreeNode(QObject *parent) :
    QObject(parent),
    m_parentNode(nullptr) {}

void MyTreeNode::setParentNode(MyTreeNode *parent)
{
    m_parentNode = parent;
}

MyTreeNode *MyTreeNode::parentNode() const
{
    return m_parentNode;
}

QQmlListProperty<MyTreeNode> MyTreeNode::nodes()
{
    QQmlListProperty<MyTreeNode> list(this,
                                      0,
                                      &append_element,
                                      &count_element,
                                      &at_element,
                                      &clear_element);
    return list;
}

MyTreeNode *MyTreeNode::childNode(int index) const
{
    if(index < 0 || index >= m_nodes.length())
        return nullptr;
    return m_nodes.at(index);
}

void MyTreeNode::clear()
{
    qDeleteAll(m_nodes);
    m_nodes.clear();
}

bool MyTreeNode::insertNode(MyTreeNode *node, int pos)
{
    if(pos > m_nodes.count())
        return false;
    if(pos < 0)
        pos = m_nodes.count();
    m_nodes.insert(pos, node);
    return true;
}

int MyTreeNode::pos() const
{
    MyTreeNode *parent = parentNode();
    if(parent)
        return parent->m_nodes.indexOf(const_cast<MyTreeNode *>(this));
    return 0;
}

int MyTreeNode::count() const
{
    return m_nodes.size();
}

MyTreeNode *MyTreeModel::getNodeByIndex(const QModelIndex &index)
{
    if(!index.isValid())
        return nullptr;
    return static_cast<MyTreeNode *>(index.internalPointer());
}

QModelIndex MyTreeModel::getIndexByNode(MyTreeNode *node)
{
    QVector<int> positions;
    QModelIndex result;
    if(node) {
        do
        {
            int pos = node->pos();
            positions.append(pos);
            node = node->parentNode();
        } while(node != nullptr);


        for (int i = positions.size() - 2; i >= 0 ; i--)
        {
            result = index(positions[i], 0, result);
        }
    }
    return result;
}

bool MyTreeModel::insertNode(MyTreeNode *childNode, const QModelIndex &parent, int pos)
{
    MyTreeNode *parentElement = getNode(parent);
    if(pos >= parentElement->count())
        return false;
    if(pos < 0)
        pos = parentElement->count();

    childNode->setParentNode(parentElement);
    beginInsertRows(parent, pos, pos);
    bool retValue = parentElement->insertNode(childNode, pos);
    endInsertRows();
    return retValue;
}

MyTreeNode *MyTreeModel::getNode(const QModelIndex &index) const
{
    if(index.isValid())
        return static_cast<MyTreeNode *>(index.internalPointer());
    return m_rootNode;
}

To expose list-like property to QML through QQmlListProperty we need next 4 function:

void append_element(QQmlListProperty<MyTreeNode> *property, MyTreeNode *value)
{
    MyTreeNode *parent = (qobject_cast<MyTreeNode *>(property->object));
    value->setParentNode(parent);
    parent->insertNode(value, -1);
}

int count_element(QQmlListProperty<MyTreeNode> *property)
{
    MyTreeNode *parent = (qobject_cast<MyTreeNode *>(property->object));
    return parent->count();
}

MyTreeNode *at_element(QQmlListProperty<MyTreeNode> *property, int index)
{
    MyTreeNode *parent = (qobject_cast<MyTreeNode *>(property->object));
    if(index < 0 || index >= parent->count())
        return nullptr;
    return parent->childNode(index);
}

void clear_element(QQmlListProperty<MyTreeNode> *property)
{
    MyTreeNode *parent = (qobject_cast<MyTreeNode *>(property->object));
    parent->clear();
}

Now let's declare the model class:

class MyTreeModel : public QAbstractItemModel
{
    Q_OBJECT
public:
    Q_PROPERTY(QQmlListProperty<MyTreeNode> nodes READ nodes)
    Q_PROPERTY(QVariantList roles READ roles WRITE setRoles NOTIFY rolesChanged)
    Q_CLASSINFO("DefaultProperty", "nodes")

    MyTreeModel(QObject *parent = Q_NULLPTR);
    ~MyTreeModel();

    QHash<int, QByteArray> roleNames() const Q_DECL_OVERRIDE;
    QVariant data(const QModelIndex &index, int role) const Q_DECL_OVERRIDE;
    Qt::ItemFlags flags(const QModelIndex &index) 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;
    QQmlListProperty<MyTreeNode> nodes();

    QVariantList roles() const;
    void setRoles(const QVariantList &roles);

    Q_INVOKABLE MyTreeNode * getNodeByIndex(const QModelIndex &index);
    Q_INVOKABLE QModelIndex getIndexByNode(MyTreeNode *node);
    Q_INVOKABLE bool insertNode(MyTreeNode *childNode, const QModelIndex &parent = QModelIndex(), int pos = (-1));

protected:
    MyTreeNode *getNode(const QModelIndex &index) const;

private:
    MyTreeNode *m_rootNode;
    QHash<int, QByteArray> m_roles;

signals:
    void rolesChanged();
};

Since we derived out model class from abstract QAbstractItemModel we must redefine next function: data(), flags(), index(), parent(), columnCount() and rowCount(). In order our model could work with QML we define roleNames(). Also, as well as in node class we define default property to be able to add nodes to the model in QML. roles property will hold a list of role names.

The implementation:

MyTreeModel::MyTreeModel(QObject *parent) :
    QAbstractItemModel(parent)
{
    m_rootNode = new MyTreeNode(nullptr);
}
MyTreeModel::~MyTreeModel()
{
    delete m_rootNode;
}

QHash<int, QByteArray> MyTreeModel::roleNames() const
{
    return m_roles;
}

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

    MyTreeNode *item = static_cast<MyTreeNode*>(index.internalPointer());
    QByteArray roleName = m_roles[role];
    QVariant name = item->property(roleName.data());
    return name;
}

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

    return QAbstractItemModel::flags(index);
}

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

    MyTreeNode *parentItem = getNode(parent);
    MyTreeNode *childItem = parentItem->childNode(row);
    if (childItem)
        return createIndex(row, column, childItem);
    else
        return QModelIndex();
}

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

    MyTreeNode *childItem = static_cast<MyTreeNode*>(index.internalPointer());
    MyTreeNode *parentItem = static_cast<MyTreeNode *>(childItem->parentNode());

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

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

int MyTreeModel::rowCount(const QModelIndex &parent) const
{
    if (parent.column() > 0)
        return 0;
    MyTreeNode *parentItem = getNode(parent);
    return parentItem->count();
}

int MyTreeModel::columnCount(const QModelIndex &parent) const
{
    Q_UNUSED(parent);
    return 1;
}

QQmlListProperty<MyTreeNode> MyTreeModel::nodes()
{
    return m_rootNode->nodes();
}

QVariantList MyTreeModel::roles() const
{
    QVariantList list;
    QHashIterator<int, QByteArray> i(m_roles);
    while (i.hasNext()) {
        i.next();
        list.append(i.value());
    }

    return list;
}

void MyTreeModel::setRoles(const QVariantList &roles)
{
    static int nextRole = Qt::UserRole + 1;
    foreach(auto role, roles) {
        m_roles.insert(nextRole, role.toByteArray());
        nextRole ++;
    }
}

MyTreeNode *MyTreeModel::getNodeByIndex(const QModelIndex &index)
{
    if(!index.isValid())
        return nullptr;
    return static_cast<MyTreeNode *>(index.internalPointer());
}

QModelIndex MyTreeModel::getIndexByNode(MyTreeNode *node)
{
    QVector<int> positions;
    QModelIndex result;
    if(node) {
        do
        {
            int pos = node->pos();
            positions.append(pos);
            node = node->parentNode();
        } while(node != nullptr);


        for (int i = positions.size() - 2; i >= 0 ; i--)
        {
            result = index(positions[i], 0, result);
        }
    }
    return result;
}


bool MyTreeModel::insertNode(MyTreeNode *childNode, const QModelIndex &parent, int pos)
{
    MyTreeNode *parentElement = getNode(parent);
    if(pos >= parentElement->count())
        return false;
    if(pos < 0)
        pos = parentElement->count();

    childNode->setParentNode(parentElement);
    beginInsertRows(parent, pos, pos);
    bool retValue = parentElement->insertNode(childNode, pos);
    endInsertRows();
    return retValue;
}

MyTreeNode *MyTreeModel::getNode(const QModelIndex &index) const
{
    if(index.isValid())
        return static_cast<MyTreeNode *>(index.internalPointer());
    return m_rootNode;
}

In general, this code it's not much different from the standard implementation, for example Simple tree example

Instead of defining roles in C++ we provide a way to do that from QML. TreeView events and methods basically work with QModelIndex. I personally don't see much sense to pass that to qml as the only thing you can do with it is to pass it back to the model.

Anyway, our class provides a way to convert index to node and vice versa. To be able to use our classes in QML we need to register it:

qmlRegisterType<MyTreeModel>("qt.test", 1, 0, "TreeModel");
qmlRegisterType<MyTreeNode>("qt.test", 1, 0, "TreeElement");

And finelly, en example of how we can use our model with TreeView in QML:

import QtQuick 2.7
import QtQuick.Window 2.2    
import QtQuick.Dialogs 1.2
import qt.test 1.0


Window {
    visible: true
    width: 800
    height: 800
    title: qsTr("Tree example")

    Component {
        id: fakePlace
        TreeElement {
            property string name: getFakePlaceName()
            property string population: getFakePopulation()
            property string type: "Fake place"
            function getFakePlaceName() {
                var rez = "";
                for(var i = 0;i < Math.round(3 + Math.random() * 7);i ++) {
                    rez += String.fromCharCode(97 + Math.round(Math.random() * 25));
                }
                return rez.charAt(0).toUpperCase() + rez.slice(1);
            }
            function getFakePopulation() {
                var num = Math.round(Math.random() * 100000000);
                num = num.toString().split("").reverse().join("");
                num = num.replace(/(\d{3})/g, '$1,');
                num = num.split("").reverse().join("");
                return num[0] === ',' ? num.slice(1) : num;
            }
        }
    }

    TreeModel {
        id: treemodel
        roles: ["name","population"]

        TreeElement {
            property string name: "Asia"
            property string population: "4,164,252,000"
            property string type: "Continent"
            TreeElement {
                property string name: "China";
                property string population: "1,343,239,923"
                property string type: "Country"
                TreeElement { property string name: "Shanghai"; property string population: "20,217,700"; property string type: "City" }
                TreeElement { property string name: "Beijing"; property string population: "16,446,900"; property string type: "City" }
                TreeElement { property string name: "Chongqing"; property string population: "11,871,200"; property string type: "City" }
            }
            TreeElement {
                property string name: "India";
                property string population: "1,210,193,422"
                property string type: "Country"
                TreeElement { property string name: "Mumbai"; property string population: "12,478,447"; property string type: "City" }
                TreeElement { property string name: "Delhi"; property string population: "11,007,835"; property string type: "City" }
                TreeElement { property string name: "Bengaluru"; property string population: "8,425,970"; property string type: "City" }
            }
            TreeElement {
                property string name: "Indonesia";
                property string population: "248,645,008"
                property string type: "Country"
                TreeElement {property string name: "Jakarta"; property string population: "9,588,198"; property string type: "City" }
                TreeElement {property string name: "Surabaya"; property string population: "2,765,487"; property string type: "City" }
                TreeElement {property string name: "Bandung"; property string population: "2,394,873"; property string type: "City" }
            }
        }
        TreeElement { property string name: "Africa"; property string population: "1,022,234,000"; property string type: "Continent" }
        TreeElement { property string name: "North America"; property string population: "542,056,000"; property string type: "Continent" }
        TreeElement { property string name: "South America"; property string population: "392,555,000"; property string type: "Continent" }
        TreeElement { property string name: "Antarctica"; property string population: "4,490"; property string type: "Continent" }
        TreeElement { property string name: "Europe"; property string population: "738,199,000"; property string type: "Continent" }
        TreeElement { property string name: "Australia"; property string population: "29,127,000"; property string type: "Continent" }
    }

    TreeView {
        anchors.fill: parent
        model: treemodel
        TableViewColumn {
            title: "Name"
            role: "name"
            width: 200
        }
        TableViewColumn {
            title: "Population"
            role: "population"
            width: 200
        }

        onDoubleClicked: {
            var element = fakePlace.createObject(treemodel);
            treemodel.insertNode(element, index, -1);
        }
        onPressAndHold: {
            var element = treemodel.getNodeByIndex(index);
            messageDialog.text = element.type + ": " + element.name + "\nPopulation: " + element.population;
            messageDialog.open();
        }
    }
    MessageDialog {
          id: messageDialog
          title: "Info"
      }
}

Double click for adding a node, press and hold for node info.

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