Connect models to your QML user interface

Connect models from C++ backend to QML frontend

As shown from the previous tutorial, you can connect C++ code to QML by creating a class that will be treated as just another component in QML. However, you may want to represent more complicated data, such as data that needs to act as a custom ListModel or in some way needs to be delegated from a Repeater.

We can create our own models from the C++ side, and declare how the data from that model should be represented on the QML frontend.

Preparing the Class

In this tutorial, we will create a class that contains a QMap, where a QString is used as a key and QStringList objects are used as values. The frontend will be able to read and display the keys and values and be simple to use just like a one-dimensional array. It should look similar to a QML ListModel.

To do this, we need to declare a class that inherits from QAbstractListModel. Let's also add in some add data to the QMap. These declarations will be located in model.h.

Note

If you are following along, please remember to update your CMakeLists.txt file!

#pragma once

#include <QAbstractListModel>

class Model : public QAbstractListModel {
private:
    QMap<QString, QStringList> m_list = {
            {"Feline", {"Tigress",   "Waai Fuu"}},
            {"Fox",    {"Carmelita", "Diane", "Krystal"}},
            {"Goat",   {"Sybil",     "Toriel"}}
    };
};

Of course, we can't just display this class as is. We also need to tell QML on how to represent this data in the class. We can do this by overriding three virtual functions that are essential at doing this, all of which do their own tasks.

• rowCount() - Think of this function as a way to tell QML how many items are in the model to represent.

• roleNames() - You can think of role names as property names attached to data in QML. This function allows you to create those roles.

• data() - This function is called when you want to retrieve the data that corresponds to the role names from the model.

Note

The custom role names created by roleNames() are only usable when a model is being delegated, and are not usable outside of it. See Models and Views.

Note

Technically, models in Qt are represented as tables, with rows and columns. So, what overriding rowCount() does is tell Qt how many rows are in a model. Since we are just dealing with a one-dimensional array in this tutorial, you can just think of "rows" as "number of elements."

Overriding and Implementing rowCount()

Let's override the function in the header file. The rowCount() comes with its own parameter, but will not be used in this example and is excluded.

class Model : public QAbstractListModel {
...
public:
    int rowCount(const QModelIndex &) const override;
};

Then, let's declare how many rows are in this model in model.cpp.

#include "model.h"

int Model::rowCount(const QModelIndex &) const {
    return m_list.count();
}

Overriding and Implementing roleNames()

Before we override roleNames(), we need to declare what the roles are in the C++ side using an public enum variable. The reason for this is because these values from the enum variable are passed into data() every time QML accesses a corresponding role, and as such we can make data() return what we want.

Let's begin with creating the enum variable for roles, where each value is a role for the C++ side.

class Model : public QAbstractListModel {
...
public:
    enum Roles {
        SpeciesRole = Qt::UserRole,
        CharactersRole
    };

    ...
    QHash<int, QByteArray> roleNames() const override;
};

Once we have that settled, we can finally create what these roles are in the QML side using a QHash where the keys are the enumerated values paired with QByteArrays. The text in the QByteArray is what's used in the actual QML code.

QHash<int, QByteArray> Model::roleNames() const {
    return {
        {SpeciesRole,   "species"},
        {CharactersRole, "characters"}
    };
}

In our example model, the role "species" can be used to retrieve the QString key "Feline", "Fox", "Goat", each in a separate delegate. The same can be done with the QStringList values for the character names list.

Overriding and Implementing data()

There are two parameters that are passed in data(): index and role. index is the location of where the data is when being delegated. As previously stated, role is used by QML to get specific data returned when it's accessing a role.

In data(), we can use a switch statement to return the appropriate data and data type depending on the role, which is possible as data() returns a QVariant. We still need to make sure we get the appropriate location of the data, though. In this example below, you can see that a new iterator variable is being declared, which is set from the beginning of the list plus the row of the index and the data that the iterator is pointing to is what is being returned.

We can't just return whatever data we want though. We may be trying to bind data to a property with an incompatible data type, such as a QStringList to a QString. You may have to do data conversion in order for the data to be displayed properly.

QVariant Model::data(const QModelIndex &index, int role) const {
    const auto it = m_list.begin() + index.row();
    switch (role) {
        case SpeciesRole:
            return it.key();
        case CharactersRole:
            return formatList(it.value());
        default:
            return {};
    }
}

QString Model::formatList(const QStringList& list) {
    QString result;
    for (const QString& character : list) {
        result += character;
        if (list.last() != character) {
            result += ", ";
        }
    }
    return result;
}

Allow the Class to be Declared in QML

Let's not forget to make our class usable in QML.

int main(int argc, char *argv[]) {
    ...
    qmlRegisterType<Model>("CustomModel", 1, 0, "CustomModel");
    ...
}

Class Usage in QML

The QML file that is used will just contain three Kirigami.AbstractCard components, where the key is the header and the value is the content. These cards are created by delegating an AbstractCard using a Repeater, where the custom model we created acts as the model. The data is accessed using word model, followed by the roles we declared in roleNames().

import QtQuick
import QtQuick.Layouts
import QtQuick.Controls as Controls
import org.kde.kirigami as Kirigami
import CustomModel 1.0

Kirigami.ApplicationWindow {
    id: root
    title: "Tutorial"

    CustomModel {
        id: customModel
    }

    pageStack.initialPage: Kirigami.ScrollablePage {
        ColumnLayout {
            Repeater {
                model: customModel
                delegate: Kirigami.AbstractCard {
                    header: Kirigami.Heading {
                        text: model.species
                        level: 2
                    }
                    contentItem: Controls.Label {
                        text: model.characters
                    }
                }
            }
        }
    }
}

Data Modification

Editing Data Using dataChanged() and setData()

You may encounter a situation where you want to modify data in the model, and have the changes reflected on the frontend side. Every time we change data in the model, we must emit the dataChanged() signal which will apply those changes on the frontend side at the specific cells specified in its arguments. In this tutorial, we can just use the index argument of setData().

setData() is a virtual function you can override so that attempting to modify the data from the frontend side automatically reflects those changes on the backend side. It requires three parameters:

• index - The location of the data.

• value - The contents of the new data.

• role - In this context, the role here is used to tell views how they should handle data. The role here should be Qt::EditRole.

The role parameter in this case is used to ensure setData() can be edited via user input (Qt::EditRole). Using index, we can use that to determine the location of where the data should be edited with the contents of value.

bool Model::setData(const QModelIndex &index, const QVariant &value, int role) {
    if (!value.canConvert<QString>() && role != Qt::EditRole) {
        return false;
    }

    auto it = m_list.begin() + index.row();
    QString charactersUnformatted = value.toString();
    QStringList characters = charactersUnformatted.split(", ");

    m_list[it.key()] = characters;
    emit dataChanged(index, index);

    return true;
}

Note

setData() does not automatically emit dataChanged() and that still has to be done manually.

Let's update the QML code so that we can open up a prompt that allows us to edit the model using a Controls.Button attached to the cards.

Kirigami.ApplicationWindow {
    ...

    Kirigami.OverlaySheet {
        id: editPrompt

        property var model
        property alias text: editPromptText.text

        title: "Edit Characters"

        Controls.TextField {
            id: editPromptText
        }

        footer: Controls.DialogButtonBox {
            standardButtons: Controls.DialogButtonBox.Ok
            onAccepted: {
                const model = editPrompt.model;
                model.characters = editPromptText.text;
                editPrompt.close();
            }
        }
    }

    pageStack.initialPage: Kirigami.ScrollablePage {
        ColumnLayout {
            Repeater {
                model: customModel
                delegate: Kirigami.AbstractCard {
                    Layout.fillHeight: true
                    header: Kirigami.Heading {
                        text: model.species
                        level: 2
                    }
                    contentItem: Item {
                        implicitWidth: delegateLayout.implicitWidth
                        implicitHeight: delegateLayout.implicitHeight
                        ColumnLayout {
                            id: delegateLayout
                            Controls.Label {
                                text: model.characters
                            }
                            Controls.Button {
                                text: "Edit"
                                onClicked: {
                                    editPrompt.text = model.characters;
                                    editPrompt.model = model;
                                    editPrompt.open();
                                }
                            }
                        }
                    }
                }
            }
        }
    }
}

Now, whenever the values of the model change in the frontend, the changes should automatically update on the backend.

Adding Rows

We added a way to modify the data in existing keys of the QMap, and in the front end, this is reflected as modifying the contents inside the AbstractCards. But what if we need to add a new key entry in the QMap and have that reflected on the QML side? Let's do this by creating a new method that is callable on the QML side to perform this task.

To make the method visible in QML, we must use the Q_OBJECT macro in the class, and begin the method declaration with the Q_INVOKABLE macro. This method will also include a string parameter, which is intended to be the new key in the QMap.

class Model : public QAbstractListModel {
Q_OBJECT;

    ...
public:
    ...
    Q_INVOKABLE void addSpecies(const QString &species);
};

Inside of this method, we need to tell Qt that we want to create more rows in the model. This is done by calling beginInsertRows() to begin our row adding operation, followed by inserting whatever we need, then use endInsertRows() to end the operation. We still need to emit dataChanged() at the end, however. This time, we are going to update all rows, from the first row to the last one as the QMap may alphabetically reorganize itself, and we need to catch that across all rows.

When calling beginInsertRows(), we need to first pass in a QModelIndex class to specify the location of where the new rows should be added, followed by what the new first and last row numbers are going to be. In this tutorial, the first argument will just be QModelIndex() as there is no need to use the parameter here. We can just use the current row size for the first and last row number, as we'll just be adding one row at the end of the model.

void Model::addSpecies(const QString& species) {
    beginInsertRows(QModelIndex(), m_list.size() - 1, m_list.size() - 1);
    m_list.insert(species, {});
    endInsertRows();
    emit dataChanged(index(0), index(m_list.size() - 1));
}

Note

The dataChanged() function uses QModelIndex as the data type for its parameters. However, we can convert integers in QModelIndex data types using the index() function.

Let's update the QML code so we are given the ability to add a new key to the QMap.

Kirigami.ApplicationWindow {
    ...

    Kirigami.OverlaySheet {
        id: addPrompt

        title: "Add New Species"

        Controls.TextField {
            id: addPromptText
        }

        footer: Controls.DialogButtonBox {
            standardButtons: Controls.DialogButtonBox.Ok
            onAccepted: {
                customModel.addSpecies(addPromptText.text);
                addPromptText.text = ""; // Clear TextField every time it's done
                addPrompt.close();
            }
        }
    }

    pageStack.initialPage: Kirigami.ScrollablePage {
        actions: [
            Kirigami.Action {
                icon.name: "add"
                text: "Add New Species"
                onTriggered: {
                    addPrompt.open();
                }
            }
        ]
        ...
    }
}

Now, we should be given a new action at the top of the app that brings up a prompt that allows to add a new element to the model, with our own custom data.

Removing Rows

The way remove rows is similar to adding rows. Let's create another method that we'll call in QML. This time, we will use an additional parameter, and that is an integer that is the row number. The species name is used to delete the key from the QMap, while the row number will be used to delete the row on the front end.

class Model : public QAbstractListModel {
Q_OBJECT;

...
    public:
    ...

    Q_INVOKABLE void deleteSpecies(const QString &speciesName, const int &rowIndex);
}

void Model::deleteSpecies(const QString &speciesName, const int& rowIndex) {
    beginRemoveRows(QModelIndex(), rowIndex, rowIndex);
    m_list.remove(speciesName);
    endRemoveRows();
    emit dataChanged(index(0), index(m_list.size() - 1));
}

Now, let's update the application so a "Delete" button appears alongside the edit button, and hook it up to our delete method.

ColumnLayout {
    Repeater {
        model: customModel
        delegate: Kirigami.AbstractCard {
            ...
            contentItem: Item {
                implicitWidth: delegateLayout.implicitWidth
                implicitHeight: delegateLayout.implicitHeight
                ColumnLayout {
                    id: delegateLayout
                    Controls.Label {
                        text: model.characters
                    }
                    RowLayout {
                        Controls.Button {
                            text: "Edit"
                            onClicked: {
                                editPrompt.text = model.characters;
                                editPrompt.model = model;
                                editPrompt.open();
                            }
                        }
                        Controls.Button {
                            text: "Delete"
                            onClicked: {
                                customModel.deleteSpecies(model.species, index);
                            }
                        }
                    }
                }
            }
        }
    }
}

Full Code

Main.qml

import QtQuick
import QtQuick.Layouts
import QtQuick.Controls as Controls
import org.kde.kirigami as Kirigami
import CustomModel 1.0

Kirigami.ApplicationWindow {
    id: root
    title: "Tutorial"

    CustomModel {
        id: customModel
    }

    Kirigami.OverlaySheet {
        id: editPrompt

        property var model
        property alias text: editPromptText.text

        title: "Edit Characters"

        Controls.TextField {
            id: editPromptText
        }

        footer: Controls.DialogButtonBox {
            standardButtons: Controls.DialogButtonBox.Ok
            onAccepted: {
                const model = editPrompt.model;
                model.characters = editPromptText.text;
                editPrompt.close();
            }
        }
    }

    Kirigami.OverlaySheet {
        id: addPrompt

        title: "Add New Species"

        Controls.TextField {
            id: addPromptText
        }

        footer: Controls.DialogButtonBox {
            standardButtons: Controls.DialogButtonBox.Ok
            onAccepted: {
                customModel.addSpecies(addPromptText.text);
                addPromptText.text = ""; // Clear TextField every time it's done
                addPrompt.close();
            }
        }
    }

    pageStack.initialPage: Kirigami.ScrollablePage {
        actions: [
            Kirigami.Action {
                icon.name: "add"
                text: "Add New Species"
                onTriggered: {
                    addPrompt.open();
                }
            }
        ]

        ColumnLayout {
            Repeater {
                model: customModel
                delegate: Kirigami.AbstractCard {
                    Layout.fillHeight: true
                    header: Kirigami.Heading {
                        text: model.species
                        level: 2
                    }
                    contentItem: Item {
                        implicitWidth: delegateLayout.implicitWidth
                        implicitHeight: delegateLayout.implicitHeight
                        ColumnLayout {
                            id: delegateLayout
                            Controls.Label {
                                text: model.characters
                            }
                            RowLayout {
                                Controls.Button {
                                    text: "Edit"
                                    onClicked: {
                                        editPrompt.text = model.characters;
                                        editPrompt.model = model;
                                        editPrompt.open();
                                    }
                                }
                                Controls.Button {
                                    text: "Delete"
                                    onClicked: {
                                        customModel.deleteSpecies(model.species, index);
                                    }
                                }
                            }
                        }
                    }
                }
            }
        }
    }
}

model.h

#pragma once

#include <QAbstractListModel>

class Model : public QAbstractListModel {
Q_OBJECT;

private:
    QMap<QString, QStringList> m_list = {
            {"Feline", {"Tigress",   "Waai Fuu"}},
            {"Fox",    {"Carmelita", "Diane", "Krystal"}},
            {"Goat",   {"Sybil",     "Toriel"}}
    };

    static QString formatList(const QStringList &list);

public:
    enum Roles {
        SpeciesRole = Qt::UserRole,
        CharactersRole
    };

    int rowCount(const QModelIndex &) const override;

    QHash<int, QByteArray> roleNames() const override;

    QVariant data(const QModelIndex &index, int role) const override;

    bool setData(const QModelIndex &index, const QVariant &value, int role) override;

    Q_INVOKABLE void addSpecies(const QString &species);

    Q_INVOKABLE void deleteSpecies(const QString &species, const int &rowIndex);
};

model.cpp

#include "model.h"

int Model::rowCount(const QModelIndex &) const {
    return m_list.count();
}

QHash<int, QByteArray> Model::roleNames() const {
    QHash<int, QByteArray> map = {
            {SpeciesRole,   "species"},
            {CharactersRole, "characters"}
    };
    return map;
}

QVariant Model::data(const QModelIndex &index, int role) const {
    const auto it = m_list.begin() + index.row();
    switch (role) {
        case SpeciesRole:
            return it.key();
        case CharactersRole:
            return formatList(it.value());
        default:
            return {};
    }
}

QString Model::formatList(const QStringList& list) {
    QString result;
    for (const QString& character : list) {
        result += character;
        if (list.last() != character) {
            result += ", ";
        }
    }
    return result;
}

bool Model::setData(const QModelIndex &index, const QVariant &value, int role) {
    if (!value.canConvert<QString>() && role != Qt::EditRole) {
        return false;
    }

    auto it = m_list.begin() + index.row();
    QString charactersUnformatted = value.toString();
    QStringList characters = charactersUnformatted.split(", ");

    m_list[it.key()] = characters;
    emit dataChanged(index, index);

    return true;
}

void Model::addSpecies(const QString& species) {
    beginInsertRows(QModelIndex(), m_list.size() - 1, m_list.size() - 1);
    m_list.insert(species, {});
    endInsertRows();
    emit dataChanged(index(0), index(m_list.size() - 1));
}

void Model::deleteSpecies(const QString &species, const int& rowIndex) {
    beginRemoveRows(QModelIndex(), rowIndex, rowIndex);
    m_list.remove(species);
    endRemoveRows();
    emit dataChanged(index(0), index(m_list.size() - 1));
}

More Information

For more information, see Using C++ Models with Qt Quick Views and Model/View Programming.