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Calling Qt class methods from QML

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This code snippet shows how to call Qt class methods directly from QML.
 
This code snippet shows how to call Qt class methods directly from QML.
  
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 <tt>Q_INVOKABLE</tt> 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 <tt>Q_INVOKABLE</tt> macro is able to do this.
+
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 {{Icode|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 {{Icode|Q_INVOKABLE}} macro is able to do this.
  
In the following demonstration we define a new Qt class, <tt>StringHelper</tt>, which has one invokable <tt>echo</tt> method and one public slot, <tt>toggleEcho</tt>. The <tt>echo</tt> method will return the given string as is, or as reversed if the public slot <tt>toggleEcho(true)</tt> has been called.
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In the following demonstration we define a new Qt class, {{Icode|StringHelper}}, which has one invokable {{Icode|echo}} method and one public slot, {{Icode|toggleEcho}}. The {{Icode|echo}} method will return the given string as is, or as reversed if the public slot {{Icode|toggleEcho(true)}} has been called.
  
The <tt>StringHelper</tt> class is instantiated in '''main.cpp'''. The <tt>QObject</tt>-derived object <tt>stringHelper</tt> is then set as a context property to the root element of <tt>QDeclarativeView</tt>. The result is that the root element <tt>rect</tt> in the <tt>ui.qml</tt> document will see <tt>StringHelper</tt> as its child element.
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The {{Icode|StringHelper}} class is instantiated in '''main.cpp'''. The {{Icode|QObject}}-derived object {{Icode|stringHelper}} is then set as a context property to the root element of {{Icode|QDeclarativeView}}. The result is that the root element {{Icode|rect}} in the {{Icode|ui.qml}} document will see {{Icode|StringHelper}} as its child element.
  
 
==Preconditions==
 
==Preconditions==
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===Postconditions===
 
===Postconditions===
  
The code snippet has demonstrated how to call Qt class methods from QML. This required setting the <tt>QObject</tt>-derived object as a context property to the root object of <tt>QDeclarativeView</tt>. The methods of the Qt-derived class had to be either public slots or regular methods declared with the <tt>Q_INVOKABLE</tt> macro.
+
The code snippet has demonstrated how to call Qt class methods from QML. This required setting the {{Icode|QObject}}-derived object as a context property to the root object of {{Icode|QDeclarativeView}}. The methods of the Qt-derived class had to be either public slots or regular methods declared with the {{Icode|Q_INVOKABLE}} macro.
  
[[Category:Qt]][[Category: Qt Quick]][[Category:Code Examples]][[Category:Code Snippet]][[Category:MeeGo]] [[Category:Symbian]]
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[[Category:Qt]][[Category: Qt Quick]][[Category:Code Snippet]][[Category:Code Snippet]][[Category:MeeGo]] [[Category:Symbian]]

Revision as of 09:37, 11 May 2012

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: kratsan (24 Jun 2010)
Last edited: hamishwillee (11 May 2012)

Contents

Overview

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

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.

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.

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