×
Namespaces

Variants
Actions

Calling Qt class methods from QML

From Nokia Developer Wiki
Jump to: navigation, search

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

Article Metadata
Tested with
Devices(s): Nokia 900
Compatibility
Platform(s): S60 5th Edition
Maemo
Symbian
S60 5th Edition
Device(s): Qt 4.7 and later
Dependencies: Qt 4.7 and later
Article
Keywords: QML, QDeclarativeContext, Q_INVOKABLE
Created: kratsan (24 Jun 2010)
Last edited: hamishwillee (18 Oct 2012)

Contents

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.

This page was last modified on 18 October 2012, at 01:24.
346 page views in the last 30 days.
×