Calling Qt class methods from QML

This code snippet shows how to call Qt class methods directly from QML.

Article Metadata

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Tested with

Devices(s): Nokia 900

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Compatibility

Platform(s): S60 5th Edition
Maemo

Device(s): Qt 4.7 and later

Dependencies: Qt 4.7 and later

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Article

Keywords: QML, QDeclarativeContext, Q_INVOKABLE

Created: kratsan (24 Jun 2010)

Last edited: hamishwillee (18 Oct 2012)

Overview

In order to enable the calling of Qt class methods from QML code, the methods must be declared as public slots or conventional methods with the Q_INVOKABLE macro. In both cases, Qt methods are available to the Qt Meta-Object system and the methods are callable from QML. One drawback in the public slot method is that the slot cannot return a value. Conversely, the method with the Q_INVOKABLE macro is able to do this.

In the following demonstration we define a new Qt class, StringHelper, which has one invokable echo method and one public slot, toggleEcho. The echo method will return the given string as is, or as reversed if the public slot toggleEcho(true) has been called.

The StringHelper class is instantiated in main.cpp. The QObject-derived object stringHelper is then set as a context property to the root element of QDeclarativeView. The result is that the root element rect in the ui.qml document will see StringHelper as its child element.

Preconditions

• Qt 4.7 or higher is installed on your platform.

Qt Project File

#To make sure we use declarative
QT += declarative
 
#To get files deployed on device / emulator
files.sources += ui.qml
DEPLOYMENT += files

Source

stringhelper.h

#ifndef STRINGHELPER_H
#define STRINGHELPER_H
 
#include <QObject>
#include <QString>
 
class StringHelper : public QObject
{
    Q_OBJECT
public:
    StringHelper(QObject *parent = 0) : QObject(parent), reverse(false) { }
 
    Q_INVOKABLE QString echo(const QString &text) const {
        if(reverse == false) { return text; }
 
        QString reversed;
        for(QString::const_iterator it = text.begin(); it != text.end(); it++) {
            reversed.push_front(*it);
        }
 
        return reversed;
    }
 
public slots:
    void toggleEcho(bool reverse) { this->reverse = reverse; }
 
protected:
    bool reverse;
};
 
#endif // STRINGHELPER_H

ui.qml

import Qt 4.7
 
Rectangle {
    id: rect
 
    property string text: "Using Qt class to echo this"
 
    function updateUI() {
        StringHelper.toggleEcho(button.pressed); // calling StringHelper::toggleEcho
        text.text = StringHelper.echo(rect.text) // calling StringHelper::echo
    }
 
    anchors.fill: parent
    color: "black"
 
    Component.onCompleted: updateUI()
 
    Text {
        id: text
        anchors.centerIn: parent
        color: "white"
    }
 
    Rectangle {
        id: button
 
        property bool pressed: false
 
        width: 100; height: 40
        anchors.right: parent.right; anchors.rightMargin: 20
        anchors.bottom: parent.bottom; anchors.bottomMargin: 20
        radius: 6
        color: pressed ? "gray" : "white"
 
        Text {
            anchors.centerIn: parent
            text: "Reverse"
        }
 
        MouseArea {
            anchors.fill: parent
            onClicked: { button.pressed = !button.pressed; updateUI() }
        }
    }
}

main.cpp

#include <QApplication>
#include <QDeclarativeView>
#include <QDeclarativeContext>
#include "stringhelper.h"
 
int main(int argc, char *argv[])
{
    QApplication a(argc, argv);
 
    StringHelper stringHelper;
 
    QDeclarativeView view;
    view.setResizeMode(QDeclarativeView::SizeRootObjectToView);
    view.rootContext()->setContextProperty("StringHelper", &stringHelper);
    view.setSource(QUrl("./ui.qml"));
 
#if defined(Q_WS_S60) || defined(Q_WS_MAEMO)
    view.showMaximized();
#else
    view.setGeometry(100, 100, 800, 480);
    view.show();
#endif
 
    return a.exec();
}

Postconditions

The code snippet has demonstrated how to call Qt class methods from QML. This required setting the QObject-derived object as a context property to the root object of QDeclarativeView. The methods of the Qt-derived class had to be either public slots or regular methods declared with the Q_INVOKABLE macro.