How to get a return value from QMetaObject::invokeMethod

Question

I am calling from my thread the following:

QMetaObject::invokeMethod(pProcessor,
                          "doTask",
                          Qt::QueuedConnection,
                          Q_RETURN_ARG(quint32, taskId),
                          Q_ARG(quint64,   objId),
                          Q_ARG(quint8,    intId),
                          Q_ARG(QString,   name),
                          Q_ARG(QString,   comment)
                          );

but it just fails, no matter what I do. If I take out Q_RETURN_ARG(quint32, taskId), the method is invoked, but I need the taskId, which I cannot get. Any help is much appreciated.

score 29 · Accepted Answer · answered Sep 10 '13 at 18:31

I'm assuming you want to call a method of an object from non owner thread & wants to get the return value. In order to do that use "Qt::BlockingQueuedConnection" as a connection type.

quint32 taskId; // Declare taskId.
qRegisterMetaType<quint32>("quint32");
QMetaObject::invokeMethod(pProcessor,
                      "doTask",
                      Qt::BlockingQueuedConnection,
                      Q_RETURN_ARG(quint32, taskId),
                      Q_ARG(quint64,   objId),
                      Q_ARG(quint8,    intId),
                      Q_ARG(QString,   name),
                      Q_ARG(QString,   comment)
                      );

Incase your method returns non standard return type you have to register your type before calling QMetaObject::invokeMethod(...). Refer http://qt-project.org/doc/qt-5.0/qtcore/qmetatype.html#qRegisterMetaType.

score 1 · Answer 2 · answered Sep 10 '13 at 18:17

The documentation for invokeMethod states that

The return value of the member function call is placed in ret. If the invocation is asynchronous, the return value cannot be evaluated.

I guess this happens because the code below this function call is continued to be executed, while your doTask method may not yet be called. Try using Qt::DirectConnection therefore.

score 0 · Answer 3 · answered Sep 10 '13 at 18:18

from docs: If the invocation is asynchronous, the return value cannot be evaluated.

Then you should switch to Qt::DirectConnection, or change your design. QFuture, of course, could help.

I blogged here about a different solution, using C++ lambda facility to issue calls from background threads to GUI thread (see the text after 'My Solution' label).

How to get a return value from QMetaObject::invokeMethod

3 Answers3