How to temporarily switch to the GUI thread - c++

Question

I have a program that takes a long action, I run this function on a different thread. Periodically I need to update the information for the user, so I send a signal to the GUI thread. But sometimes I need the user to make a choice, I need to display the QDialog on the GUI thread and pause the slow thread while the user selects an option, and when the user completes the selection, return the value to the slow thread and continue it
it should look something like this:
But I don’t know how to stop and continue the thread and whether it should be done this way.
Header:
class Example:public QObject
{
//...
Q_OBJECT
void mainLoop();
Example();
signals:
void updateGUI(const QString &message);
void sendQuestion(const QString &message);
void continueMainLoop(const QString &answer);
private slots:
void updatuGUIslot(const QString &message);
void showQuestionDialog(const QString &message);
};
Source:
Example::Example()
{
connect(this,&Example::updateGUI,this,&Example::updatuGUIslot);
connect(this,&Example::sendQuestion,this,&Example::showQuestionDialog);
std::thread t(&Example::mainLoop,this);
t.detach();
// in the project it is not in the constructor
}
void Example::mainLoop()
{
while(some condition1)
{
// slow action
if(some condition2)
emit updateGUI("message");
if(some condition3)
{
QString result;
ThreadPtr th = this_thread(); // pseudocode
connect(this,&Example::continueMainLoop,this,[&](const QString &answer)
{
result = answer;
th.continue(); // pseudocode
});
emit sendQuestion("question");
th.wait(); // pseudocode
}
// slow action
}
}
void Example::showQuestionDialog(const QString &message)
{
// show dialog with question
emit continueMainLoop("answer");
}
void Example::updatuGUIslot(const QString &message)
{
// update GUI
}

Answer 1

you need to invoke the method with BlockingQueuedConnection before condition3 for checking which option that selected by the user.
bool updateGui ;
QMetaObject::invokeMethod(this, "showDialog",Qt::BlockingQueuedConnection,
Q_RETURN_ARG(bool, updateGui));
if(updateGui)
{
//update GUI
}