[solved] Qt Signal/Slots in one class but emitted from different threads



  • Hi,

    i saw a lot of post concerning this topic, but all topics did not match my problem.

    The project is devided in two parts, a library where all processing is done and the GUI. The library is completly in C++/MFC and should not be changed that much because it is also used in none Qt projects which have to be running without Qt dependencies. The library defines an interface to make calls to an GUI. So the Qt GUI also implements this interface in order to receive the calls from the library. The library runs several threads.

    Some of the processing of the GUI calls should not block the library from running. So the GUI class defines slots and signals, the signals are emitted in the implemented interface functions. The signals and slots are connected using:

    @"connect(this, SIGNAL(xyz), this SLOT(xyz));"@

    I think the problem is that the intface functions that emit the signals are running in thread a but the GUI runs in thread b. The right way would be create QObjects inside the library and define the signals on that side, but this is not possible in my case. Any suggestions?



  • You could go a bit another way:

    use

    @
    QMetaObject::invokeMethod(this, "foo", Qt::QueuedConnection);
    @

    There you can also post parameters

    @
    QMetaObject::invokeMethod(this, "foo", Qt::BlockingQueuedConnection, Q_ARG(<param type>, param));
    @


  • Moderators

    The connection is using pointers to the same instance, isn't it?
    I assume that you are using the same object through pointers in different threads. Then this might be possible. However, the signal-slot mimic will more or less generate a direct function call which will be executed in the thread you are emitting the signal. I do not see a way to tell the thread that it has to execute the function in another thread.
    I guess you need to separate the different tasks in two objects. Otherwise it will not function.



  • You can also use the connection type in your connect statement, if you need.

    It depends on what you want to do in response to those signals though. You can not trigger anything that modifies the GUI from another tread than the GUI thread. That can only be done using queued connections, never with direct method calls or direct connections (amount to pretty much the same thing).



  • [quote author="Gerolf" date="1304063870"]You could go a bit another way:

    use

    @
    QMetaObject::invokeMethod(this, "foo", Qt::QueuedConnection);
    @

    There you can also post parameters

    @
    QMetaObject::invokeMethod(this, "foo", Qt::BlockingQueuedConnection, Q_ARG(<param type>, param));
    @
    [/quote]

    That is working, thank you.



  • Using the typed connect also works with emit signal then:

    @
    "connect(this, SIGNAL(xyz), this SLOT(xyz), Qt::QueuedConnection);"
    @



  • [quote author="Gerolf" date="1304064590"]Using the typed connect also works with emit signal then:

    @
    "connect(this, SIGNAL(xyz), this SLOT(xyz), Qt::QueuedConnection);"
    @[/quote]

    I tried that before and had no success



  • If you then emit inside your class, it did not work? strange... should do logically the same as QMetaObject::invokeMethod



  • [quote author="Gerolf" date="1304067659"]If you then emit inside your class, it did not work? strange... should do logically the same as QMetaObject::invokeMethod[/quote]

    That was the first thing i tried, from what i read i thought it should work. But the code never reached the slot function



  • [quote author="bazz-dee" date="1304068700"]
    [quote author="Gerolf" date="1304067659"]If you then emit inside your class, it did not work? strange... should do logically the same as QMetaObject::invokeMethod[/quote]

    That was the first thing i tried, from what i read i thought it should work. But the code never reached the slot function[/quote]

    perhaps, the code was wrong? Did you add the parameter types inmj signals and also the brackets? param,eter trypes in the slot ?

    @
    connect(this, SIGNAL(xyz(type1, type2)), this SLOT(xyz()));
    @



  • i had the parameters in both



  • only the types, or also the parameter names?



  • only the types. i still have it running with signals/slots where the signals are emitted from the GUI thread


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