How to determine if signal is already connected?

[Pl45m4]

@SPlatten

Besides calling connect again and check its output, there is

int QObject::receivers(const char *signal)

• https://doc.qt.io/qt-5/qobject.html#receivers

Dont know if it's recommended to use...

Warning: This function violates the object-oriented principle of modularity. However, it might be useful when you need to perform expensive initialization only if something is connected to a signal.

I guess not, but maybe it helps :)

[SPlatten]

@Pl45m4 , doesn't calling connect just create another connection? Thats where I am now.

[J.Hilk]

@SPlatten
@Pl45m4 is on the right track here,

you can/should subclass your spinner and create a function,

bool isValueChanged(){
   static const QMetaMethod valueChangedSignal = QMetaMethod::fromSignal(&MyObject::valueChanged);
return isSignalConnected(valueChangedSignal);
}

https://doc.qt.io/qt-5/qobject.html#isSignalConnected

[SPlatten]

@J-Hilk , thank you, penny has dropped, I see what I have to do now.

[Pl45m4]

@J-Hilk

lol, found receivers() and sender() but not isSignalConnected().

Sometimes it's hard to find something, if you don't know what you are looking for :D
(function names)

[aleksejs]

I'm working with Qt not so long, but I see how cumbersome is signal-slot mechanism in comparison to Borland Event-Handler mechanism already.
In Borland you write

button->OnClick = buttonClickHandler;

or

if( !button->OnClick )
    button->OnClick = buttonClickHandler;

Until now I haven't seen better Event mechanism than Borland's.

[JonB]

@aleksejs
How did that work/what did you specify when buttonClickHandler is a method in a class?

[Christian Ehrlicher]

... Or if two distinct functions should be called where none is aware of the other?

[Szamo]

@aleksejs
Borland made it simple and thus easy to use. "Simple" means that 1 signal could have 0 or 1 slot (button->OnClick is the signal, buttonClickHandler is the slot). If one wanted to connect more slots to 1 signal then had to implement handling more slots (listeners) in the 1 connected slot. For a simple GUI 1 slot was enough of course. "Simple" also means that threads were not considered at all, it could not call a slot in another thread like Qt can. It simply called the slot and if more threads were used then one had to implement handling race conditions (using mutex or such).
@JonB
Borland extended C++ introducing __closure keyword by which a class instance pointer (this) and a member method pointer could be bound together. Thus a function pointer could be used for a non member or a member method. Thus it does not matter if buttonClickHandler is a method in a class.

[JonB]

@Szamo
Well I cannot comment if Borland C++ required an extension to C++ to achieve something. That is not the way to go at least these days. And I don't think it was available on non-Windows platforms.

The restrictions you describe about 1 signal -> 1 slot would limit me compared to Qt. I would have to implement the multi-slot-handling queue. Similarly if I wanted to use threads. So of course Borland's code looks "simpler", because it is simpler and may require you to do more coding.

For my own part I don't find Qt's signals/slots "cumbersome". If someone likes the syntax of button->OnClick = buttonClickHandler rather than the connect(....) statements look at Python/PySide/PyQt which implements a style of button.onClick.connect(handler) instead. So the detail of connecting is "syntactic sugar", one could implement some kind of Borland/Python-style wrapper if wanted.