How to properly using infinite loop in Qt GUI?

Question

I am new to and learning the Qt GUI framework.

I have an ultrasonic sensor wired up to the Raspberry Pi, to measure water level. If I were coding in C, I would have used a while(1) loop to constantly read the sensor input. But when I put while(1) inside MainWindow.cpp, the window cannot be displayed. However, using qDebug() I can still print out the sensor value, which means my while(1) still running but the main window won't appear. I found out in this answer that because of while(1),

MainWindow ctor never returns, so w.show() is never called and a.exec() (main message loop) is never executed.

To solve this, I use QTimer instead of a loop: connect the timeout() SIGNAL to a SLOT which is a function to read the sensor value one-time:

waterLevelTimer = new QTimer(this);
connect(waterLevelTimer, SIGNAL(timeout()), this, SLOT(getWaterLevel()));
waterLevelTimer->start(100); // "loop" once every 100 millisecond

With this method, I can read the sensor value with the fastest interval is 1 millisecond and the GUI still displayed fine.

But should I use QTimer to mimic a while(1) loop? Is there a better way to have an infinite loop to read GPIOs while still being able to use GUI for other work?

You should never have infinite loops in event-driven program (which basically all GUI programs are). And a constructor should really only initialize an object, not perform work. — Some programmer dude
– Some programmer dude, Commented Jul 24, 2021 at 9:26
And the solution you your problem is a timer, not an infinite loop. As a possible secondary solution you might be able to use a worker-thread but that could add other complications with synchronization and communication between the GUI and the thread. — Some programmer dude
– Some programmer dude, Commented Jul 24, 2021 at 9:28
Thanks for the comments! Where should I put my work-performing code? I followed several Qt beginner examples and they all code inside MainWindow.cpp. — Lê Thành Vinh
– Lê Thành Vinh, Commented Jul 24, 2021 at 9:35
If you need to do something at periodic intervals a timer (QTimer) is perfect, that's what it's for. It's bettern than an infinite loop where you call sleep or something similar. — Some programmer dude
– Some programmer dude, Commented Jul 24, 2021 at 9:39

falkb · Accepted Answer · 2021-07-26 14:33:18Z

2

The main thread where the GUI of Qt runs never should be blocked by long-lasting operations like an infinite while-loop, because otherwise you would block the event system and nothing will work anymore.

Instead you usually create a worker thread in parallel (see QThread) where you do your loop in the run function of the thread. Maybe also use such timer as you suggest, which works if the executed code is faster than the timer duration.

QThread *thread = QThread::create([]{
    while(1)
        checkSomething();
});
thread->start();

edited Jul 26, 2021 at 14:33

answered Jul 26, 2021 at 14:21

falkb

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2 Comments

Lê Thành Vinh Over a year ago

QThread seems perfect for me. But your last sentence, what will happen if the work takes longer than the timer's interval?

falkb Over a year ago

If the timer triggers faster than the executed code takes, it will lead to congestion

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