Qt multithreading and let the second thread update the main (UI) thread

[jefftee]

//inside the slot
 
//get the query term, and pass it to the action, it will update view through signal/slot, so longer need to pass view as param
 
std::shared_ptr<QueryAction> action = std::make_shared<QueryAction>(queryTerm);
 
//this in turn will run a while loop polling the database, and finish gracefully once polling is complete
action->execute();
 
QThread *t = new QThread();
action->moveToThread(t);

//inside the slot

//get the query term, and pass it to the action, it will update view through signal/slot, so longer need to pass view as param

std::shared_ptr<QueryAction> action = std::make_shared<QueryAction>(queryTerm);

//this in turn will run a while loop polling the database, and finish gracefully once polling is complete
action->execute();

QThread *t = new QThread();
action->moveToThread(t);

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Not sure what action->execute() is intended to do in line 8. If your intent is to execute something in the new thread, that's not the correct way to accomplish that. For starters, you haven't created the new QThread yet, nor has it been started (QThread::start). Typically you would emit a signal that is connected to a slot in your Worker object, which can be easily done with a QTimer::singleShot.

The pointer on line 10 is a local pointer, so you will lose the ability to address your QThread after your slot returns. Make the pointer a class member variable, which will allow you to use it later on.

You should also make all of your signal/slot connections for the Worker object and QThread after line 11.

Lastly, after you have created the QThread, moved your Worker object to the QThread, and made your signal/slot connections, you need to start the QThread so that it begins running its event loop.

After you have done all of that, then QTimer::singleShot with a 0ms wait to queue the signal to execute in the new thread, etc.

Once you have all of that working, you should add a boolean to your Worker object that you can use to tell it to stop processing. i.e. You have started doing a long running task in the new thread and the user wants to cancel the task or close the program, etc. Your long running task would then check the boolean to determine if stop has been requested, etc. To make it all thread safe, you should protect the boolean member variable with a QMutex and QMutexLocker whenever you read or change the boolean value.

Hope that helps.