QTimer in a GUI application

HamiltonQ

Hello, everyone!

I'm starting to produce applications with Qt In one of my applications I've come across an unexpected output:

QObject :: startTimer: QTimer can only be used with threads started with QThread.

I'm starting my timer (timer-> start ()) in the class that contains the UI elements (QMainWindow). I do this out of the constructor, in the function below:

void client :: showPorts () {

SettingsDialog showDialogPorts * = new SettingsDialog (this);
      serial = new QSerialPort ();
      timer = new QTimer ();
      connect (time, SIGNAL (timeout ()), this, SLOT (doMyTask ()));
      timer-> start (20);

      .
      .
      .
      .
      .

}

What's wrong with using QTimer in the GUI class (QMainWindow)? Thanks!!

SGaist

Hi and welcome to devnet,

Are you sure it's that timer that triggers the message ? Not any thread using it ?

HamiltonQ

Hey, man! You are fast, thanks!! I'm not sure, but I want to control my event from a timer that I have control, so I used the QTimer * timer = new QTimer ();

HamiltonQ

It seems to me that there is some problem when I use in class that contains access to UI, am I right?

JKSH

[quote author="HamiltonQ" date="1421974393"]It seems to me that there is some problem when I use in class that contains access to UI, am I right?[/quote]No, that's not a problem.

How many threads does your program have?

HamiltonQ

I don't initiated any thread purposely. I do not know if the application automatically starts some thread to call some method or to start some object.

HamiltonQ

I just want to start a count to trigger a function every 20 ms in the same class that contains the call to the UI. It's possible?

alex_malyu

It should not be a problem unless doMyTask () can initiate another call to showPorts () in which cay you may keep creating timers until system is exhausted

Alex.

HamiltonQ

[quote author="alex_malyu" date="1421985237"]It should not be a problem unless doMyTask () can initiate another call to showPorts () in which cay you may keep creating timers until system is exhausted

Alex.[/quote]

Thanks, Alex!

But, why doMyTask() have to call showPorts()? I just wanna call doMyTask every 20 ms.

HamiltonQ

I changed of place the connect ( connect (time, SIGNAL (timeout ()), this, SLOT (doMyTask ())); ), I put him on constructor, but nothing changed, I keep getting the output:

QObject :: startTimer: QTimer can only be used with threads started with QThread.

kuzulis

do you have an QApplication instance?

HamiltonQ

Hi!

QApplication instance is in the main.cpp file, but I'm starting the Timer on client.cpp file, which contains the call to the UI

kuzulis

Then you can google by the "QTimer can only be used with threads started with QThread." pattern. ;)

HamiltonQ

For three days I am trying to solve this. I have seen numerous topics, but none of them do what I want to do, which is to use the QTimer in the class that contains the UI. The example that the documentation give is for NON-GUI application. But I wanna exactly this

SGaist

Then let's first get back to the basics.

Do you still see this message if you create a default Qt Widget application and add

@
mainwindow.h

class MainWindow: public QWidget
{
// default code

public slots:
void testTimer();
}

mainwindow.cpp

MainWindow:MainWindow(QWidget *parent)
: QWidget(parent)
{
QTimer *timer = new QTimer(this);
connect(timer, SIGNAL(timeout()), SLOT(testTimer()));

timer->start(20);
}

void MainWindow::testTimer()
{
qDebug() << "Timed out !";
}
@

to the default code ?

HamiltonQ

Hi, Samuel!

This works, I tested. But my base class is QMainWindow. I tried to change the base class for QWiget but it happened several errors.

Thanks, man!

SGaist

Just keep your QMainWindow based class, the only thing that was important was the part related to the QTimer

HamiltonQ

I have to change the base class to use QTimer?

JKSH

Can you show us your code for doWork()?

Note that in newer versions of Qt, the actual message is "QObject::startTimer: Timers can only be used with threads started with QThread". ("Timers", not "QTimer"!)

All QObjects have built-in timers. I don't think the message came from your QTimer, but rather from your QSerialPort.

SGaist

No you don't, QMainWindow is a QWidget as well