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Qt: singleton logging class get different instance accross plugins and thread
#pragma once

#include <QtGui>
#include <QMutex>

//! Creates a fake call, to have a cleaner design.
extern void qInfo(const char* fmt);

#define QtTraceMsg  QtMsgType(6)
#define QtInfoMsg   QtMsgType(7)

class MsgHandler: public QObject
{
    Q_OBJECT

signals:
    void newMsg(QtMsgType type, const QString &msg);

public:
    static MsgHandler *instance();
    static void Handler(QtMsgType type, const char *msg);

private:
    MsgHandler() { qRegisterMetaType<QtMsgType>("QtMsgType"); } // important
    static MsgHandler* _instance;
};

#include "MsgHandler.h"

MsgHandler* MsgHandler::_instance = NULL;

MsgHandler * MsgHandler::instance()
{
    static QMutex mutex;

    if (_instance == NULL)
    {
        mutex.lock();
        if (_instance == NULL)
            _instance = new MsgHandler;
        mutex.unlock();
    }

    return _instance;
}

void MsgHandler::Handler(QtMsgType type, const char *msg)
{
    QString s = msg;
    emit instance()->newMsg(type, s);
}

///////////////////////////////////////////////////////////////////////////////
void qInfo(const char *msg)
{
    MsgHandler::instance()->Handler(QtInfoMsg, msg);
}

int main(int argc, char *argv[])
{
    QApplication a(argc, argv);

    // Handle error messages
    qInstallMsgHandler(MsgHandler::Handler);
}

void MyPlugin::run()
{
    qWarning("Test debug"); //works fine
    MsgHandler::instance()->Handler(QtInfoMsg, "info 2"); //creates a new instance!
    qInfo("test info"); //also creates a new instance!
}