#ifndef THEMEHELPER_H_
#define THEMEHELPER_H_

#include <QObject>
#include <QSettings>
#include <bb/cascades/Application>
#include <bb/cascades/ColorTheme>
#include <bb/cascades/Theme>
#include <bb/cascades/ThemeSupport>
#include <bb/cascades/VisualStyle>

using namespace bb::cascades;


class ThemeHelper : public QObject {

Q_OBJECT

    Q_PROPERTY(Themes currentTheme READ currentTheme NOTIFY currentThemeChanged);
    Q_ENUMS(Themes);

public:
    enum Themes {
        Bright = 0, Dark, Unknown
    };
    static ThemeHelper & getInstance()
    {
        static ThemeHelper instance;
        return instance;
    }

    ThemeHelper(QObject *parent = 0);
    virtual ~ThemeHelper();
    static ThemeHelper::Themes currentTheme();

    Q_INVOKABLE static void setLastTheme();
    Q_INVOKABLE static void changeTheme(ThemeHelper::Themes type);

private:
    static QSettings m_settings;

Q_SIGNALS:
    void currentThemeChanged(ThemeHelper::Themes type);
};
Q_DECLARE_METATYPE(ThemeHelper::Themes);

#endif /* THEMEHELPER_H_ */

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#include "ThemeHelper.h"

/* Public */
ThemeHelper::ThemeHelper(QObject *parent) : QObject(parent) {}

ThemeHelper::~ThemeHelper() {}

ThemeHelper::Themes ThemeHelper::currentTheme() {
    VisualStyle::Type style = Application::instance()->themeSupport()->theme()->colorTheme()->style();
    switch (style) {
        case Bright: return Bright;
        case Dark: return Dark;
        default: return Unknown;
    }
}

/* Q_INVOKABLE */
void ThemeHelper::setLastTheme() {
    if(m_settings.contains("theme")) {
        Application::instance()->themeSupport()->setVisualStyle(m_settings.value("theme").toInt());
   }
}

void ThemeHelper::changeTheme(ThemeHelper::Themes type) {
    qDebug() << "ChangeTheme" << type;
    switch (type) {
        case Bright: Application::instance()->themeSupport()->setVisualStyle(VisualStyle::Bright); break;
        case Dark: Application::instance()->themeSupport()->setVisualStyle(VisualStyle::Dark); break;
        default:
            qWarning() << "Unknown theme";
            break;
    }

    m_settings.setValue("theme", QVariant(type));
    emit ThemeHelper::getInstance().currentThemeChanged(type);
}

/* Private */
QSettings ThemeHelper::m_settings(QSettings::UserScope, QApplication::organizationName());


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nel constructor di ApplicationUI:

ApplicationUI::ApplicationUI(bb::cascades::Application *app) :
        QObject(app)
{
    qRegisterMetaType<ThemeHelper::Themes>("themes");

    [...]

    // Create scene document from main.qml asset, the parent is set
    // to ensure the document gets destroyed properly at shut down.
    QmlDocument *qml = QmlDocument::create("asset:///main.qml").parent(this);


    ThemeHelper &x = ThemeHelper::getInstance();
    qml->setContextProperty("_themeHelper", &x);

    [...]