Qt Multiple Async While Loops and qApp->processEvents(); - c++

Question

I am trying to do a project is creating some graphics on window with Qt GUI C++ 5.6.2.
I have two methods named 'createVerticalSpeedIndicator' and 'createAirSpeedIndicator'. These methods need to create some graphics with a while(1) loop and use qApp->processEvents(); on window and they are doing it perfectly when one of them is working the other one is deactive. But I need to run both of them simultanously and always.
What can I do to run them simultanously and always.
Thank you So much

Answer 1

The solution is to invert the control flow. The while() { ... processEvents() ... } is an anti-pattern in asynchronous code, because it assumes that you have locus of control whereas you really don't. You're lucky that you didn't run out of stack since processEvents could potentially re-enter the createXxx methods.
Here's a complete example of a transformation:
// Input
void Class::createVerticalSpeedIndicator() {
for (int i = 0; i < 100; i ++) {
doStep(i);
QCoreApplication::processEvents();
}
}
// Step 1 - factor out state
void Class::createVerticalSpeedIndicator() {
int i = 0;
while (i < 100) {
doStep(i);
QCoreApplication::processEvents();
i++;
}
};
// Step 2 - convert to continuation form
void Class::createVerticalSpeedIndicator() {
int i = 0;
auto continuation = [=]() mutable {
if (!(i < 100))
return false;
doStep(i);
QCoreApplication::processEvents();
i++;
return true;
};
while (continuation());
};
// Step 3 - execute the continuation asynchronously
auto Class::createVerticalSpeedIndicator() {
int i = 0;
return async(this, [=]() mutable {
if (!(i < 100))
return false;
doStep(i);
i++; // note the removal of processEvents here
return true;
});
};
template <typename F> void async(QObject * parent, F && continuation) {
auto timer = new QTimer(parent);
timer->start(0);
connect(timer, &QTimer::timeout, [timer, c = std::move(continuation)]{
if (!c())
timer->deleteLater();
});
}
At that point you can apply the same transformation to createAirSpeedIndicator and start them in the constructor of your class:
Class::Class(QWidget * parent) : QWidget(parent) {
...
createVerticalSpeedIndicator();
createAirSpeedIndicator();
}
Both tasks will run asynchronously and pseudo-concurrently within the main thread, i.e. each task will alternatively execute a single step.
Suppose we wanted to chain the tasks, i.e. have a task start only after the previous one has finished. The modification in the user code can be simple:
Class::Class(QWidget * parent) : QWidget(parent) {
...
createVerticalSpeedIndicator()
>> createAirSpeedIndicator()
>> someOtherTask();
}
The async function must now return a class that allows such connections to be made:
struct TaskTimer {
QTimer * timer;
TaskTimer & operator>>(const TaskTimer & next) {
next.timer->stop();
connect(timer, &QObject::destroyed, next.timer, [timer = next.timer]{
timer->start(0);
});
timer = next.timer;
return *this;
}
};
template <typename F> TaskTimer async(QObject * parent, F && continuation) {
TaskTimer task{new QTimer(parent)};
task.timer->start(0);
connect(task.timer, &QTimer::timeout,
[timer = task.timer, c = std::move(continuation)]{
if (!c())
timer->deleteLater();
});
return task;
}