Calling C++ function from QML code

Question

I want to call a c++ function from my QML code.

For example in the following code i have a window with 2 inputs : quantity and price I want to call a c++ function which evaluates the subtotal and adds 5% tax to it.

I have tried searching many places but couldn't get complete working code with this latest version of QT5. Please tell me how to call a C++ function from QML.

main.qml :

import QtQuick 2.2
import QtQuick.Controls 1.1

ApplicationWindow {
    visible: true
    width: 640
    height: 480
    title: qsTr("Hello World")

    menuBar: MenuBar {
        Menu {
            title: qsTr("File")
            MenuItem {
                text: qsTr("Exit")
                onTriggered: Qt.quit();
            }
        }
    }

    Column{
        Label {
            text: qsTr("Enter the number of items purchased: ")
        }
        TextField {
            id: in1
            objectName: "in1"
        }
        Label {
            text: qsTr("Enter the price per item ($):")
        }
        TextField {
            id: in2
            objectName: "in2"
        }
        Button {
            id: button
            objectName: "button"
            text: "Compute"
            onClicked: {
                total.text = "Final bill, including 5% tax, is $" + clickedButton(in1.text, in2.text); // here i'm calling the c++ function
            }
        }
        Label {
            id: total
            objectName: "total"
            text: "Final bill, including 5% tax, is $____"
        }
    }
}

main.cpp

#include <QApplication>
#include <QQmlApplicationEngine>

int main(int argc, char *argv[])
{
    QApplication app(argc, argv);

    QQmlApplicationEngine engine;
    engine.load(QUrl(QStringLiteral("qrc:///main.qml")));

    return app.exec();
}

double clickedButton(int number, int price){
    const double TAX_rate = 0.05;
    double subtotal;
    subtotal = price*number;
    return (subtotal + subtotal*TAX_rate);
}

cobalt · Accepted Answer · 2014-10-16 13:33:16Z

2

Create a class like :

class BillCalculator : public QObject
{
   Q_OBJECT
   Q_PROPERTY(double totalPrice READ totalPrice WRITE setTotalPrice NOTIFY totalPriceChanged)
public:
   BillCalculator(QObject *parent = 0) :
     QObject(parent),
    mTotalPrice(0.0)
   {
   }

   double totalPrice() const { return mTotalPrice; }
signals:
   void totalPriceChanged();
public slots: 
   void setTotalPrice(const double &arg) 
   {
     if(mTotalPrice != arg)
     {
       mTotalPrice = arg;
       emit totalPriceChanged();
     }
   }
   void calculateTotalPrice(int number, int price)
   {
    const double TAX_rate = 0.05;
    double subtotal;
    subtotal = price*number;
    setTotalPrice(subtotal + subtotal*TAX_rate);
   }
protected:
   double mTotalPrice;
};

in your main.cpp, include <QQmlContext> and modify as below

  QQmlApplicationEngine engine;
  engine.rootContext()->setContextProperty("billCalculator", new BillCalculator);
  engine.load(QUrl(QStringLiteral("qrc:///main.qml")));

modify your main.qml file as below

   Button {
            id: button
            objectName: "button"
            text: "Compute"
            onClicked: {
                billCalculator.calculateTotalPrice(parseInt(in1.text), parseInt(in2.text));
            }
        }
        Label {
            id: total
            objectName: "total"
            text: "Final bill, including 5% tax, is $" + (billCalculator.totalPrice > 0 ? billCalculator.totalPrice.toFixed(2) : "____")
        }

edited Oct 16, 2014 at 13:33

answered Oct 16, 2014 at 8:36

cobalt

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2 Comments

allstraws Over a year ago

I think you forgot to add declaration for mTotalPrice in the class. But when i added the declaration, it is giving errors : :-1: error: symbol(s) not found for architecture x86_64 & :-1: error: linker command failed with exit code 1 (use -v to see invocation)

cobalt Over a year ago

Yes, you're right, fixed it; for your linker errors; create a seperate billcalculator.h and define the BillCalculator class there

Albi · Accepted Answer · 2014-10-16 08:17:19Z

1

You need to declare clickedButton as Q_INVOKABLE like this:

 public:
 Q_INVOKABLE void cppMethod(const QString &msg) {
     qDebug() << "Called the C++ method with" << msg;
 }

See this sample: http://qt-project.org/doc/qt-4.8/qtbinding.html

answered Oct 16, 2014 at 8:17

Albi

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Comments

Andreas · Accepted Answer · 2014-10-16 14:21:30Z

1

Simply use:

engine.rootContext()->setContextProperty("yourName", new yourClass());

In your qml you can call the function with yourname.yourfunction() Additional in your class you have to make your Function Q_INVOKABEL

edited Oct 16, 2014 at 14:21

answered Oct 16, 2014 at 8:16

Andreas

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