How to determine if signal is already connected?

[SPlatten]

@J-Hilk , perhaps I haven't made myself clear, I have two widgets that share spinners and a slider, in the constructor of the same two widgets I want to check if the spinners and slider have already had a signal connected, if the answer is no then I want to go ahead with the connection.

[Pl45m4]

@SPlatten said in How to determine if signal is already connected?:

I have two widgets that share spinners and a slider, in the constructor of the same two widgets I want to check if the spinners and slider have already had a signal connected, if the answer is no then I want to go ahead with the connection.

But why? You wrote the code. If you haven't connected it already, it's probably not connected :) And if you did, it should stay connected unless you disconnect it manually somewhere, somehow.

Don't quite unterstand the issue you have here :)

[SPlatten]

@Pl45m4 , the issue is I have two instances of the same widget, there are three common widgets for setting attributes on instances of the two, in the constructor of the widget I want to automatically check the shared widgets and then only connect if they haven't already been connected.

The question I am asking is can I detected using the Qt library calls if a signal has already been connected to a widget.

[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?