Return value from QObject in another thread via signals/slots

[St.Stanislav]

@jsulm Yes, I thought about this approach, but what if I want to process something twice? In example above I want to process numbers 2 and 4. In this case ProcessingObject will emit the same signal twice but with the different arguments. So I need to add some queue of values that I'm processing. Then I need some identifiers for prevent mixing up results. It's not very complicated, but I hope that more simple built-in approach is existed.

[JonB]

@St-Stanislav
Hang on! IMHO at least, slots should not return a value. The assumption you are making is that there will only be one slot attached to the signal, and that is unwarranted/not guaranteed at all. Strange things happen when multiple slots are attached and you rely on the return result.

I agree it can be a "shame" that you cannot use signals/slots to get a return value, but that's not what they are for. Not to mention, it completely breaks if the connection is queued, for whatever reason.

I wouldn't advise it, but a better way (to allow for multiple slots) would be to pass a reference parameter for the "result".

[St.Stanislav]

@JonB Thank you for reply! Yes, I understand signas/slots nature and its purpose, so that's why I try to find correct workaround :) So if I understand correctly, you assume something like this:

signals:
void processingRequested(float& result, float value);

slots:
void multiply(float &result, float value) {
    result = value * 2;
}

Am I right? So how sender will know, when result will be changed by receiver?

[SGaist]

Can you describe what exactly you are trying to achieve ?

Understanding what you want to do will help us with the design.

[St.Stanislav]

@SGaist So I have program that displays data from external device. I have two threads (with appropriate objects): one is for data processing and second is for interface and displayed data holding. If in processing object one parameter is changed, I need to replot data from the displaying object based on that parameter. This parameter is something like calibration for X-axis and object has needed methods. So in my suggest I need to send X-coordinate from data in the displaying object to the processing object, compute new result there and apply it in displaying object with replacing the previous one. I hope I explained it clearly enough, anyway I've tried :)

Hm, so maybe I can emit signal that parameter was changed, send it to displaying object and apply it there. I need implement calibration method in displaying object as well, so it will be implemented twice. Don't know, is it good way, but it should be as workaround.

[SGaist]

The propagation of the change through signal and slots is the correct way to do it.

Why would you need to implement things twice ? You should have one object responsible for these mathematical operations rather than duplicate them in several places.

[St.Stanislav]

@SGaist Yes, and math operations are provided by Processing object: I read raw data from serial port, send raw array to Processing object and Processing Object resend them further to display:
SerialPort => Processing (math) => Display
And everything is done with signals/slots: Serial port emits data, Processing object catch this data, process it and emits processed data, Display catch it and display. But sometimes I need to change already displayed data with the same math processing:
Display => Processing (math) => Display
So that's why I'm not sure, how to do it correctly :)

[SGaist]

Do you have an example of such a case ?

[St.Stanislav]

@SGaist Sorry for my late reply. What kind of examples do you mean? Which code I use for processing and emitting or just an abstract description of what emit and catch in slots?

[SGaist]

An example of what could trigger that double evaluation that is giving you trouble.