Query signal for value in lambda function



  • I am trying to use the following syntax to propagate a signal up my widget hierarchy

      connect(imgSeqCtrl, &ImgSeqCtrlWidget::step,  [this]() {  emit mySignal(1);  });
    
    

    where step is a signal with the following signature

    void step(int);

    Now, I want to emit a different signal depending on the sign of the step's signal value. Is there a possibility to query the steps actual value inside the lambda or is there now way around an additional dispatch function in which I do the case query?


  • Qt Champions 2016

    What is the signature of ImgSeqCtrlWidget::step?



  • Yes, you need to change your lambda to:

    [this](int value) { 
    if (value < 0) 
        emit signal1(); 
    else 
        emit signal2();
    }
    

    Basically the lambda needs to take the same (or less) arguments as your initial signal.



  • thank you!

    @kshegunov
    void ImgSeqCtrlWidget::step(int);



  • @Sewing Just to complete the picture, you don't need a Lambda to propagate a Sigal, you can connect Signals to Signals as long as the arguments are compatible, which theay are in your case:

    connect(imgSeqCtrl, &ImgSeqCtrlWidget::step,  this, &myClassName::mySignal);
    


  • Yes, but what if I want to send my signal, equipped with two different values?



  • @Sewing well that depends, if you want to add a new argument to your Signal than you'll need a function to do that, that function can be an actual SLOT of your class or a lambda.

    however droping arguments, no problem

    //SignalA(int index, QString text);
    //SignalB(int index);
    
    //This os ok
    connect(objectA, &&QObject::SignalA, objectB, &QObject::SignalB);
    
    //This is not
    connect(objectB, &&QObject::SignalB, objectA, &QObject::SignalA);
    
    //Howver iirc this should work:
    //SignalA(int index, QString text = "DefaultText");
    //SignalB(int index);
    connect(objectB, SIGNAL(SignalB(int)), objectA, SIGNAL(SignalA(int,QString)));
    


  • thank you!


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