What are some best practices for debugging Qt signals and slots?

Question

Debugging signals and slots can be hard, because the debugger does not jump to the slots of a signal when it is emitted. What are some best practices for debugging Qt signals an

Answer 1

In addition to what have been said, here are additional tricks.

If you use QTest for your unit tests, then you can pass -vs argument to the executable and all signals will be shown in the console.

I looked at how QTest works, and it registers callbacks that is triggered when signal and slots are executed using QSignalDumper class. However, this API is not exported and might break any time. Here is how I was able to hook on all signals and slots on Linux with Qt 5.10 using GCC.

// QSignalSpyCallbackSet is defined in qt5/qtbase/src/corelib/kernel/qobject_p.h

struct QSignalSpyCallbackSet
{
    typedef void (*BeginCallback)(QObject *caller, int signal_or_method_index, void **argv);
    typedef void (*EndCallback)(QObject *caller, int signal_or_method_index);
    BeginCallback signal_begin_callback,
                    slot_begin_callback;
    EndCallback signal_end_callback,
                slot_end_callback;
};
typedef void (*register_spy_callbacks)(const QSignalSpyCallbackSet &callback_set);

static void showObject(QObject *caller, int signal_index, const QString &msg)
{
   const QMetaObject *metaObject = caller->metaObject();
   QMetaMethod member = metaObject->method(signal_index);
   qDebug() << msg << metaObject->className() << qPrintable(member.name());
}

static void onSignalBegin(QObject *caller, int signal_index, void **argv)
{
   showObject(caller, signal_index, "onSignalBegin");
}

static void onSlotBegin(QObject *caller, int signal_index, void **argv)
{
   showObject(caller, signal_index, "onSlotBegin");
}

int main(int argc, char *argv[])
{
   static QSignalSpyCallbackSet set = { onSignalBegin, onSlotBegin, 0, 0 };
   QLibrary qtcore("libQt5Core");
   register_spy_callbacks reg = (register_spy_callbacks)qtcore.resolve("_Z32qt_register_signal_spy_callbacksRK21QSignalSpyCallbackSet");

   if (reg) {
      reg(set);
   }
   ...
}

I believe that Qt should expose that API, because we could use it for so many things beyond debugging, such as monitoring the time spent in a slot, get statistics, etc.