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Calling Qt class methods from QML

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Article Metadata
Tested with
Devices(s): Nokia 900
Compatibility
Platform(s): S60 5th Edition
Maemo
Symbian
S60 5th Edition
Article
Keywords: QML, QDeclarativeContext, Q_INVOKABLE
Created: (24 Jun 2010)
Last edited: kratsan (24 Jun 2010)

Contents

Overview

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

In order to allow calling of the methods of Qt class from QML code, the methods must be declared as public slots or the conventional methods with Q_INVOKABLE macro. In both of these 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 value. Method with Q_INVOKABLE macro is on the contrary able to do this.

In the following demonstration we define new Qt class StringHelper which has one invokable echo method and one public slot toggleEcho. The echo will return the given string as is, or 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 context property to the root element of the QDeclarativeView. What this does is that the root element rect in the ui.qml document will see StringHelper as of its child element.

Preconditions

  • Qt 4.7 or higher is installed on your platform.

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

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

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