Comments on: Talking to Qt Threads

By: Dave

Dave — Sun, 20 Jun 2010 03:08:46 +0000

Well, it turns out I was wrong with this article. You don't actually have to subclass QThread. You can just *use* one, as noted here: http://blog.exys.org/entries/2010/QThread_affinity.html Here's a revised version: main.cpp:

#include 
#include 
#include 

class MyObject : public QObject
{
    Q_OBJECT

    public:
        MyObject() : QObject(NULL)
        {
            moveToThread(&_thread);
            _thread.start();
        }

    public slots:
        void safelyDoSomething()
        {
            if(QThread::currentThread() != &_thread)
            {
                qDebug() < < __FUNCTION__ << "Wrong thread";
                QMetaObject::invokeMethod(this, "safelyDoSomething",
                        Qt::QueuedConnection);
            }
            else
            {
                qDebug() << __FUNCTION__ << "Right thread";
            }
        }

    private:
        QThread _thread;
};

int main(int argc, char **argv)
{
    QCoreApplication app(argc, argv);

    MyObject object;
    object.safelyDoSomething();

    return app.exec();
}

#include "main.moc"

By: Dave

Dave — Fri, 14 May 2010 03:12:52 +0000

RJ: Here's an example macro that you can place within slots to make them thread safe:

// Just a helper macro:
#define NO_RETURN_VALUE

// This does the real work:
#define THREAD_MAGIC(targetThread, returnValue, args)     \
    if(QThread::currentThread() != targetThread)                     \
    {                                                                \
        QString slotName = __FUNCTION__;                             \
        slotName.remove(QRegExp("^.*::"));                           \
        bool ret = metaObject()->invokeMethod(this,                  \
                qPrintable(slotName), Qt::QueuedConnection,          \
                args.count() >=  1 ? args[0] : QGenericArgument(0),  \
                args.count() >=  2 ? args[1] : QGenericArgument(0),  \
                args.count() >=  3 ? args[2] : QGenericArgument(0),  \
                args.count() >=  4 ? args[3] : QGenericArgument(0),  \
                args.count() >=  5 ? args[4] : QGenericArgument(0),  \
                args.count() >=  6 ? args[5] : QGenericArgument(0),  \
                args.count() >=  7 ? args[6] : QGenericArgument(0),  \
                args.count() >=  8 ? args[7] : QGenericArgument(0),  \
                args.count() >=  9 ? args[8] : QGenericArgument(0),  \
                args.count() >= 10 ? args[9] : QGenericArgument(0)); \
        if(!ret)                                                     \
        {                                                            \
            qFatal(qPrintable(__FUNCTION__ +                         \
              QString(" Could not call QMetaObject::invokeMethod(). " \
              "Check your argument list quantity and types.")));     \
        }                                                            \
        return returnValue;                                          \
     }        

#define MAKE_THREAD_SAFE_0(TargetThread, returnValue)                \
        do {                                                         \
        QList args;                                \
        THREAD_ME_LIKE_A_NEEDLE(TargetThread, returnValue, args);    \
        } while (0);                                                  \


#define THREAD_MAGIC_1(TargetThread, returnValue, ArgType1, ArgName1)         \
        do {                                                         \
        QList args = QList() <<  \
            Q_ARG(ArgType1, ArgName1);                               \
        THREAD_MAGIC(TargetThread, returnValue, args);    \
        } while (0);              
                                                       \

#define THREAD_MAGIC_2(TargetThread, returnValue, ArgType1, ArgName1, ArgType2, ArgName2) \
        do {                                                         \
        QList args = QList() <  \
            Q_ARG(ArgType1, ArgName1) < <                             \
            Q_ARG(ArgType2, ArgName2);                               \
        THREAD_MAGIC(TargetThread, returnValue, args);    \
        } while (0);                                                  \

And now the usage in one of your slots:

void MyThread::mySlot(const QString &name, int count)
{
   THREAD_MAGIC_2(this, NO_RETURN_VALUE, QString, name, int, count);

  // You are now thread safe
}

Notice you can only use this with slots that have between 0 and 10 arguments, which is actually a limitation of Qt's signal/slot system (if I recall correctly). Happy thread hacking!

By: Dave

Dave — Thu, 13 May 2010 22:49:15 +0000

RJ: Great question. I actually wrote a macro a while back. I’ll post a comment here when I get home tonight to explain the missing parts. Maybe I’ll write a new blog post just about the macro method.

By: RJ

RJ — Thu, 13 May 2010 22:41:00 +0000

I want to write a macro for this QMetaObject::invoke trick, since I’m using it in a few places, but I want it to work in slots that also have parameters.

These are non variadic methods tho, don’t know how to get a list of pointers to passed arguments.

Using the following, i can get a list of parameter type strings (needed for QGenericArgument), but i’m missing the param values. Any ideas?

QString qfi( Q_FUNC_INFO );
Q_ASSERT( qfi.startsWith(“void “) );
int methid = metaObject()->indexOfSlot( qfi.remove(0, qfi.indexOf(“::”)+2 ) );
Q_ASSERT( methid != -1 ); // must be a slot, not just a method
QMetaMethod mm = metaObject()->method( methid );

qDebug()<< mm.signature()
<< "index:" << methid
<< "ParamNames: " << mm.parameterNames()
<< "paramtypes: " << mm.parameterTypes();