Qt Quick How-tos

This page aims to provide an easily discoverable, useful reference that shows the simplest and best way of performing specific tasks in Qt Quick. Each solution provides QML and/or C++ code snippets where applicable, and every snippet is automatically tested by Qt to ensure they remain functional.

How do I:

• Call a C++ function from QML when a Button is clicked
• See which item has active focus

Call a C++ function from QML when a Button is clicked

Assuming that the C++ type should be globally available to the QML files in the application, the simplest way is to make it a QML singleton with QML_SINGLETON. For example, in the header file, backend.h:

#include <QObject>
#include <QQmlEngine>

class Backend : public QObject
{
    Q_OBJECT
    QML_ELEMENT
    QML_SINGLETON

public:
    Q_INVOKABLE void doStuff();
};

backend.cpp:

#include "backend.h"

#include <QDebug>

void Backend::doStuff()
{
    qDebug() << "Did stuff!";
}

You can then call that function from any QML file:

import QtQuick.Controls

import MyModule

ApplicationWindow {
    width: 400
    height: 400
    title: qsTr("C++ Button example")

    Button {
        text: qsTr("Click me")
        onClicked: Backend.doStuff()
    }
}

If the C++ type only needs to be available to a small set of QML files, consider using QML_ELEMENT. For more ways of exposing C++ types to QML, see Choosing the Correct Integration Method Between C++ and QML.

This example assumes that the Backend type is available in a QML module. With CMake, this is done via qt_add_qml_module. For an example that demonstrates this in detail, see Building a QML application.

See which item has active focus

Write a property change signal handler for the window's activeFocusItem property:

import QtQuick
import QtQuick.Controls

ApplicationWindow {
    width: 400
    height: 400
    visible: true
    title: qsTr("Active focus debugging example")

    onActiveFocusItemChanged: print("activeFocusItem: " + activeFocusItem)

    Row {
        TextField {
            objectName: "textField1"
        }
        TextField {
            objectName: "textField2"
        }
    }
}

This will print the item which currently has active focus to the console. To ensure that the output is useful, give each item a descriptive objectName.