How to manage signals in QThread

masa4

I have a thread in runs a while loop:

void Worker::process()
{
    while(1){
        
        if(myVar == 2)
        {

           ... some codes
          x = true;
        }
        
        else
        {
            x = false;
        }
        emit mySignal(x);
    }
}

I am connecting mySignal to a slot in another class. And i am manipulating widgets according to state of x. But when i run the program, the ui completly unresponsible. myVar's value can change through program. Now is there any automatic way to manage this situation? Or i need to keep x's old value and with an if condition managing if signal will be emitted or not?

jsulm

@masa4 said in How to manage signals in QThread:

while(1){

If you want signals/slots to work do not use endless loops! This is not how Qt works. Your thread should have an event loop, then there is no need for such loops.

masa4

@jsulm Can u be more specific? I want to emit signals when values changed.

Christian Ehrlicher

Emitting signals without an event loop does work, but it's not possible to get a slot called in a thread (from another thread over queued connection) where no event loop is running.
I still would not run an endless loop here - how does myVar changes at all in your code?

masa4

@Christian-Ehrlicher I am getting myVar value from another class(Non qt) which sets this value in a thread. This thread keep checking if computer connected to a device(serial comm). If its, it sets it to 2 and i am setting x(isConnected) to true.

Christian Ehrlicher

Then better use a QTimer to check for a change every x seconds - then you don't even need another thread for it.

JonB

@masa4
Your loop is too "busy", not to mention that it may not be thread-safe (how do you synchronise access to myVar?).

Maybe just a QTimer here for occasional/regular polling might be better?

masa4

@Christian-Ehrlicher But if i use a timer, lets say run it every 1second or maybe 100ms. Every 100ms i will check the myVar variable, and i will change something on ui. For ex. one thing is i will make disabled one of the checkbox if x==false. But when myvar != 2 but x is still true because 100ms interval didnt finish it yet, and i clicked the checkbox because its still enabled, this situation may creates bugs/crashes? Im not sure if i explained well and i really dont sure about it. Does using timer can cause any problems like that?

masa4

@JonB said in How to manage signals in QThread:

Your loop is too "busy", not to mention that it may not be thread-safe (how do you synchronise access to myVar?).

I just read it from another class that communicates with a device. I dont have much idea about synhronization. And what time interval is ok for timer do you know? @jsulm indicated QTimer::start(0) is not a very good idea. Does 100ms reasonable?

J.Hilk

@masa4 this is a very bad idea, besides what th others said, you are currently emitting a signal, basically, every cpu cycle. That is overwhelming your event loop on the main thread.

If you insist on this approach, store the previous true/false state somewhere and only emit the signal if /when the state changes.

Christian Ehrlicher

@masa4 said in How to manage signals in QThread:

For ex. one thing is i will make disabled one of the checkbox if x==false. But when myvar != 2 but x is still true because 100ms interval didnt finish it yet, and i clicked the checkbox because its still enabled,

I don't see a problem here - check the real value just before you execute the action on your external lib again - and even then there might be a race condition because the value may change in the few milliseconds between the check and the acutal implementation.

masa4

@J-Hilk Now I started doing in this way with storing old value of variables, and if it changed emitting signals. Now it runs ok but a little question. after several minutes of running program, my fans starts making a lot of noise. after stopping program, fans are stop too. One of my cpu core runs at 100% when running program. Any idea why this happens?

JonB

@masa4 said in How to manage signals in QThread:

One of my cpu core runs at 100% when running program. Any idea why this happens?

Depends what your code is! Sounds like it's running at 100%, like originally....

J.Hilk

@masa4 like I previously said, your while(1) loop is running full speed all the time -> one core will be at 100 % load for that.

masa4

@J-Hilk @JonB But other way how can i continuously get the values of variables? I am really confused now. So threads not usable in this situation, i need to use timer?

masa4

@masa4 If I dont use while(1), i was not able to read variables continuously, so no point in here to use thread?

jsulm

@masa4 As already said: if you read the values continually all the time then your thread will run at 100%. This has nothing to do with threads as such, it has to do with your code: it simply utilises one CPU core at 100%. Your approach is simply wrong. Do not read the state continuously. Read it several times per second (use a timer for that, no need for additional thread), that is enough to update UI. And then do what @Christian-Ehrlicher already suggested: "I don't see a problem here - check the real value just before you execute the action on your external lib again".

JonB

@masa4
If you insist on reading the variable "continuously" then you are likely to use a CPU core 100%, just like you said you do not want to do. What else would you expect?

If you use a QTimer you may indeed not need a separate thread for this at all.

Christian Ehrlicher

@masa4 said in How to manage signals in QThread:

But other way how can i continuously get the values of variables?

For the third time (or even more): Use a QTimer!