How to get Data from GUI into a thread?

[JonB]

@mullman123
Exactly in the opposite direction --- send data from UI thread to secondary thread via signals, just as you would the other way round. Note that you do not pass data in a connect() call, you only specify what the type of the data will be. We do not know what

When i use slots and connects then i noticed that it can make some trouble because obviously the run() function is called before the emit signal reaches the thread

means.

If you need help produce a minimal, compilable example of what you try --- you don't need hundreds of QStrings to illustrate a problem. Whether you pass a whole struct or a single int the principle is the same.

[mullman123]

@JonB
Yeah i tried it with a struct but there is an issue with compiling it. Even though it does not have any complains for passing it to the functions. Is there anything wrong with my connect? Or do i need to do anything specific with my struct?

[J.Hilk]

@mullman123 no, your code is wrong.

struct DataComingFromUI is defined twice!

once in calculationutilization and once in maindow

also you might get away without it, but I would recommend adding a copy operator to your struct as well

[mullman123]

@J-Hilk
Now i got it working. Thanks so far!
The next problem would be how to get my Thread to wait until the data is received from the emit before finishing?
Or is there a better way to pass over my struct to the run process of my thread?
After calling my emit to the thread it does not arrive because i think the thread finishes before it. I am basically calling the recvdata() slot.

My Button action in MainWindow.cpp:

//Button actions
void MainWindow::on_pushButton_CalculateTime_clicked()
{
    setGUIDataCalculateThread();
    emit started_calculationthread(UIData);
    thread_calculateutilization.start();
}

My thread/connect in calculationutilization.cpp

void calculationutilization::run()
{

}

void calculationutilization::recvdata(DataComingFromUI& data)
{
    data.Linetype = "Hello";
}

[SimonSchroeder]

If I understood correctly you have overwritten the run() method of your thread. Then, signal slot connections won't work because no event loop is running inside your thread. It is also not good practice anymore to override the run() method. Instead, just start your thread and the default implementation of run() will start an event loop. You then place a worker object inside that thread with a slot that would be connected to started_calculationthread(UIData). Your slot will get the data and your can use it to do your actual work.

A slight shortcut is to use

QThread::create([](const DataComingFromUI &data){ /* do the work */ }, UIData);

Or just capture UIData with the lambda. In addition, you need to make sure to delete the thread object created by QThread::create by connecting to its finished signal.

[J.Hilk]

@mullman123 said in How to get Data from GUI into a thread?:

void calculationutilization::run()
{

}

seeing this makes me doubt your thread is actually doing thread things.

What exactly is your set up, what method of "thread" are you using

Can you elaborate more, what exactly do you want to archive?
Is it simply some (long taking) operation on your data in the struct ?

If so, then there are easier methods than sub classing QThread, what you seem to have done here. In fact subclassing Thread is very, very early the way to go

[mullman123]

@J-Hilk
What i want to do is use the data from the struct to calculate a simulation of many automated processes.
The Data from the GUI basically sets the parameters and config for the calculation.
Since it can happen that the calculation might take some time i defined it as a thread.

The function is not yet implemented because i struggled with handing over the data from GUI.
But the thread will do kinda lengthy calculations.

[J.Hilk]

@mullman123

alright, so not a fire and forget function, but you want to access the results.

I would suggest the worker thread approach than.

Make your calculating/processing class. make it so that your interface to that class is slots (passing data to the class) and signals (passing data from the class away) only.

When that is working, you can (literally) move it to its own thread in like 10 lines of code

I have this project, that covers all common ways of "threading "in qt:
https://github.com/DeiVadder/QtThreadExample

take a closer look at the "workerObject" project
https://github.com/DeiVadder/QtThreadExample/tree/master/projects/WorkerObject

[mullman123]

@J-Hilk
Thanks for your detailed information.
I will try my luck and give some info what i achieved.

Thanks to everybody so far for the fast help.

[SimonSchroeder]

I have stumbled on the same kind of problems over and over again. You can find a simple wrapper for this kind of multithreading on GitHub: https://github.com/SimonSchroeder/QtThreadHelper

In your specific case you could just write it like this:

workerThread([](const DataComingFromUI & data){ ... }, UIData);

No need to write a full class or something. It is basically an extension of QThread::create which will avoid memory leaks. The only thing to consider is that a new thread is created whenever you want to execute this. With enough work inside the function the overhead of creating a thread is negligible. Because the GUI will not freeze you have to be careful that this cannot be started several times at once such that not too many threads are running at the same time.

BTW, Qt also has QtConcurrent::run(). This uses a thread pool in the background (so, no oversubscribing of the CPU cores) and it makes it quite easy to run a function inside a thread as well.