Integrating QML and C++
QML applications often need to handle more advanced and performance-intensive tasks in C++. The most common and quickest way to do this is to expose the C++ class to the QML runtime, provided the C++ implementation is derived from QObject. Assuming that you have Qt 5.7 or later installed, the following step-by-step instructions guide you through the process of using the C++ class, BackEnd, in a QML application:
- Create a new project using the "Qt Quick Application" template in Qt Creator
Note: Uncheck the With ui.qml file option in the Define Project Details section of New Project Wizard.
- Add a new C++ class called
BackEnd
to the project and replace its header file contents with:#ifndef BACKEND_H #define BACKEND_H #include <QObject> #include <QString> #include <qqml.h> class BackEnd : public QObject { Q_OBJECT Q_PROPERTY(QString userName READ userName WRITE setUserName NOTIFY userNameChanged) QML_ELEMENT public: explicit BackEnd(QObject *parent = nullptr); QString userName(); void setUserName(const QString &userName); signals: void userNameChanged(); private: QString m_userName; }; #endif // BACKEND_H
The
Q_PROPERTY
macro declares a property that could be accessed from QML. TheQML_ELEMENT
macro makes the BackEnd class available in QML. - Add the following lines to your project file:
CONFIG += qmltypes QML_IMPORT_NAME = io.qt.examples.backend QML_IMPORT_MAJOR_VERSION = 1
The BackEnd class is automatically registered as a type, which is accessible from QML by importing the URL, "
io.qt.examples.backend 1.0
". - Replace the contents of
backend.cpp
with:#include "backend.h" BackEnd::BackEnd(QObject *parent) : QObject(parent) { } QString BackEnd::userName() { return m_userName; } void BackEnd::setUserName(const QString &userName) { if (userName == m_userName) return; m_userName = userName; emit userNameChanged(); }
The
setUserName
function emits theuserNameChanged
signal every timem_userName
value changes. The signal can be handled from QML using theonUserNameChanged
handler. - Replace the contents of
main.qml
with the following code:import QtQuick 2.6 import QtQuick.Controls 2.0 import io.qt.examples.backend 1.0 ApplicationWindow { id: root width: 300 height: 480 visible: true BackEnd { id: backend } TextField { text: backend.userName placeholderText: qsTr("User name") anchors.centerIn: parent onEditingFinished: backend.userName = text } }
The
BackEnd
instance lets you access theuserName
property, which is updated when the TextField'stext
property changes.
Now the application can be run.
Application running on Ubuntu
Qt offers several methods to integrate C++ with QML, and the method discussed in this tutorial is just one of them. For more details about these methods, refer to Overview - QML and C++ Integration.
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