Return value from QObject in another thread via signals/slots

St.Stanislav

Hello!

I have faced with the following situation: I have ProcessingObject in the one thread and its interface ProcessingObjectInterface in another. ProcessingObject has some inner structure and process data. I want to send data from ProcessingObjectInterface to ProcessingObject, process it there and receive result in ProcessingObjectInterface.
Sending data is simple, just using signas/slots mechanism, but what is the simplest and correct way to receive result in sender?

SGaist

Hi,

Do you mean get the result in the returned value of the function call ?

jsulm

@St-Stanislav said in Return value from QObject in another thread via signals/slots:

but what is the simplest and correct way to receive result in sender?

In the same way you send data?
ProcessingObject would emit a signal when it has finished its work and send the result as parameter in a signal.

St.Stanislav

@SGaist Yes, exactly. So in example I suppose that ProcessingObject just multiplies argument by 2. So I want to send signal

void processingRequested(float value = 2);

To slot

float multiply(float value) {
    return value * 2;
}

and receive last function's returning value.

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.