Define and emit signal from main

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Hi!

Is it possible to define a signal and connect it to mainClass?

No.

Do I need to define a class which derives from QObject, instantiate it and move my processing there?

Yes.

:-)

Pradeep Kumar

class YourClass: public QObject{
Q_OBJECT;
...
public signals:
void yourSignal();
};

...

class SecondClass{
Q_OBJECT;
...
public slots:
void slotFunc();
}

YourClass *youObject = new YourClass ;


connect(YourClass, SIGNAL(yourSignal()), SecondClassObject, SLOT(someSlot()));

We need to keep main function minimal.

DRoscoe

@Wieland Thanks.

Pradeep Kumar

@DRoscoe that helped i guess.

Thanks,

VRonin

Can you tell us what are you trying to do? defining and connecting a signal that will not be emitted by anyone is useless. what is your aim?

My guess is that what you are after is either QMetaObject::invokeMethod or QTimer::singleShot(0

DRoscoe

@VRonin I wanted to emit a signal from main.cpp that would be picked up by mainClass when the event loop was started. I never said anything about defining a signal that would be picked up by no one. As soon as you mentioned the single-shot timer, I smacked myself in the head, because I am already doing that exact thing to send a signal to mainClass to tell it to initialize when the event loop starts. I didn't connect the dots to extend that to my current problem.

@Pradeep-Kumar I already know HOW to create properly derived classes for signals and slots. My project currently defines hundreds of them. I was only wondering if I would have to define a class derived from QObject in main.cpp, instantiate it and transfer my main.cpp processing to it. Both your response and that of @Wieland pretty much confirmed what I was hoping to avoid, so yes.. it was helpful. Thanks!

-Dave

kshegunov

You can attach a lambda to the single shot timer (as per @VRonin), so you don't actually have to write separate QObject subclass.

QTimer::singleShot(0, [] () -> void  {
    // We have an active event loop here.
});

Also you can do deferred signal emission with QMetaObject::invokeMethod:

QMetaObject::invokeMetod(emitter, "signalName", Qt::QueuedConnection); //< You can even pass arguments

But I'm with @VRonin, what are you trying to do exactly?

DRoscoe

@kshegunov I think I've been pretty clear about what I am trying to do. I want to emit a signal from my main.cpp entry point to be picked up by my derived mainClass. If the particulars matter, the mainClass has a thread-safe logger that isn't created until the class is initialized. Having the event queued on its event loop ensures it gets written in a thread-safe manner after the logger is instantiated and the log file is created. It is primarily to capture issues with command line arguments in an environment where a console window will not be available (such as in an aircraft cockpit).

kshegunov

@DRoscoe said in Define and emit signal from main:

I think I've been pretty clear about what I am trying to do.

Apparently not. But in any case a signal will always require an emitter object, it can't float in the empty space, so I suppose your best bet is to just queue the call through the event loop with a zero-timeout timer event (i.e. QTimer::singleShot). If you just want to call a slot after the event loop starts then you can also use the QMetaObject::invokeMethod approach:

QMetaObject::invokeMethod(receiver, "slotName", Qt::QueuedConnection); //< You can even pass arguments

DRoscoe

@kshegunov thank you!

kshegunov

You're welcome.

PS. My personal preference is with invokeMethod() but the function unfortunately doesn't support the pointer-to-member syntax, so its usage is getting ever so rare.