**Course Title:** QML Application Development **Section Title:** Integrating with C++ **Topic:** Signal-slot connections between QML and C++. Signal-slot connections are a fundamental part of Qt and C++ programming, and they play a crucial role in integrating QML with C++ backends. In this topic, we will explore how to establish signal-slot connections between QML and C++, enabling seamless communication between the two. **What are Signal-Slot Connections?** Signal-slot connections are a mechanism for objects to communicate with each other in Qt. When an object emits a signal, it can be connected to a slot function in another object. This allows objects to react to events or changes in other objects. **Connecting QML Signals to C++ Slots** To connect a QML signal to a C++ slot, you need to follow these steps: 1. **Register the C++ type:** Before you can use a C++ type in QML, you need to register it with the QML engine using the `qmlRegisterType` function. This function is usually called in the main function or when initializing the QML engine. ```cpp qmlRegisterType<MyCppType>("com.example.MyCppType", 1, 0, "MyCppType"); ``` 2. **Expose the C++ object to QML:** You can expose the C++ object to QML by setting it as a context property using `setContextProperty` function. ```cpp QQmlContext *ctxt = engine.rootContext(); ctxt->setContextProperty("myCppObject", myCppObject); ``` 3. **Connect the QML signal to the C++ slot:** In your QML file, you can connect the signal to the C++ slot using the `Connections` element. ```qml Connections { target: myCppObject onMyCppSignal: console.log("QML received the signal") } ``` **Connecting C++ Signals to QML Slots** To connect a C++ signal to a QML slot, you can follow these steps: 1. **Register the C++ type:** As mentioned earlier, you need to register the C++ type with the QML engine using `qmlRegisterType`. 2. **Expose the C++ signal to QML:** You can expose the C++ signal to QML by adding a Q_PROPERTY to your C++ class. This will allow you to signal changes to the QML side. ```cpp private: Q_PROPERTY(QString myCppProperty READ myCppProperty NOTIFY myCppSignal) signals: void myCppSignal(QString); public slots: QString myCppProperty(); ``` 3. **Connect the C++ signal to the QML slot:** In your QML file, you can connect the C++ signal to the QML slot using the `Connections` element. ```qml Connections { target: myCppObject onMyCppSignal: console.log("QML received the signal with value:", param1) } ``` **Example: QML and C++ Signal-Slot Connection** Let's consider a simple example where we have a QML button that emits a signal when clicked. We want to connect this signal to a C++ slot that prints a message to the console. MyButton.qml ```qml import QtQuick 2.0 Button { text: "Click me" onClicked: emit buttonClicked(); signal buttonClicked(); } ``` mybutton.h ```cpp #ifndef MYBUTTON_H #define MYBUTTON_H #include <QObject> class MyButton : public QObject { Q_OBJECT public: explicit MyButton(QObject *parent = 0); signals: public slots: void onButtonClicked(); }; #endif // MYBUTTON_H ``` mybutton.cpp ```cpp #include "mybutton.h" #include <QDebug> MyButton::MyButton(QObject *parent) : QObject(parent) { } void MyButton::onButtonClicked() { qDebug() << "Button clicked"; } ``` main.cpp ```cpp #include <QGuiApplication> #include <QQmlApplicationEngine> #include <QQmlContext> #include "mybutton.h" int main(int argc, char *argv[]) { QCoreApplication app(argc, argv); QGuiApplication guiApp(argc, argv); QQmlApplicationEngine engine; MyButton buttonObject; engine.rootContext()->setContextProperty("myButtonObject", &buttonObject); engine.load(QUrl(QStringLiteral("qrc:/main.qml"))); return guiApp.exec(); } ``` main.qml ```qml import QtQuick 2.0 Rectangle { width: 360 height: 360 visible: true MyButton { anchors.centerIn: parent Connections { target: myButtonObject onMySignal: console.log("Button clicked"); } } } ``` This code creates a QML button that emits a signal when clicked. We connect this signal to a C++ slot that prints a message to the console. This demonstrates a basic signal-slot connection between QML and C++. **Practice Exercise** Create a QML application that has a slider. When the slider's value changes, it should emit a signal. Connect this signal to a C++ slot that prints the new value to the console. **External Resources** * [Qt Documentation: Connecting C++ and QML](https://doc.qt.io/qt-5/qtqml-cppintegration-overview.html) * [Qt Documentation: Qt QML](https://doc.qt.io/qt-5/qtqml-index.html) If you have any questions or need further clarification on any of the concepts discussed in this topic, feel free to ask in the comments below. Next topic: **Building a simple C++-QML integrated application**.