Loop with inter thread communication - best design?

robro

Hello,

I would like to know what is the best way to realize an inter thread communication in the following case.

There is a thread A where a loop is running and there is a thread B where either QSerialPort or QCanBus is used.
The thread A does several things and also needs to write on the Com-Port oder CanBus. Then the loop must wait until a command is received on the same port or bus.

My idea in pseudocode is the following:

Thread A:
int plannedRuns;
int loopValue; 
signal: writeToBus(); 

connect(ThreadA, writeToBus, ThreadB, writeFrame, QueuedConnection)

void startMeasurement(int runs)
{
   plannedRuns = runs;
   loopValue = 0;
   workingFunction();
}

void workingFunction()
{
    doStuff();
    loopValue++;
    if(loopValue < plannedRuns) emit writeToBus();
}

Thread B:
writeFrame(); //Writes frames
signal: frameAvailable() //Signal when new frames are there
connect(ThreadB, frameAvailable, ThreadA, workingFunction, QueuedConnection)

Is this a good way?

Thank you very much!

SGaist

Hi,

So you would like to follow the producer/model ?

robro

@SGaist said in Loop with inter thread communication - best design?:

So you would like to follow the producer/model ?

I think this would be the easiest.
Or do you have another suggestion?
As I am not a professional programmer I am always open for suggestions :-)

SGaist

It will depend on what exactly is inside that package. Can you describe one use-cases.

David Jacobson

I'm just speculating about your use case but, if your "Thread A" is mostly Qt UI processing, and "Thread B, ..." is a separate i/o package, I like to use a pattern I refer to as "delayed widgets" in the UI Thread A; where basically, Thread B is isolated with a "worker" class that implements QThreadPool, and the UI Thread A starts a periodic timer that "wakes up" on a reasonable interval and updates a set of status widgets from member variables real-time-updated by the worker class Thread B. An example of my UI Thread A setup code would be:

self.timerDelayedWidgets = QTimer(self)
self.timerDelayedWidgets.timeout.connect(self.display_DelayedWidgets)
self.timerDelayedWidgets.start(self._DELAYED_WIDGETS_INTERVAL_MILLI)

robro

Thanks for the answers.
OK, my use case is a little bit different.

In Thread A a sensor is measured, its data is processed and stored.
After every measurement the sensor has to be moved.
This is done communicating with motors over a SerialBus or CanBus.
After the new position is reached I get a "target reached" info on the Bus and then I can start a new measurement.

Actually after every measurement while Thread A is still processing the data, Thread B can already start moving.

SGaist

So in fact, you could queue your measure results for processing while moving your motor and starting the next measurement.

robro

@SGaist
Yes! This sounds really good.
How would an implementation roughly look like?
Basically like the pseudocode I provided, but you would put everything what can be processed later into a queue for a third thread?

I still would like to keep threads for bus communication and basic processing separate as there is alot of traffic on the bus and I don't like the bus class to do other things than sending and receving bus messages.

SGaist

You can use a QQueue to store the data that needs processing.

Emit a signal from the class where you process the messages when it's a measure result.

You can then use a QWaitCondition to keep your measure processing thread from spinning for nothing if the queue is empty.