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