Qt4: Unresolved timer events

Question

A mini sample file main.cpp:

#include <iostream>
#include <QCoreApplication>

class MyApp : public QCoreApplication
{
private:
    int m_idtimer;

public:
    MyApp(int nargs, char* argc[]) : QCoreApplication(nargs, argc)
    {
        m_idtimer = startTimer(3000); // 3 segs.
    }

protected:
    void timerEvent(QTimerEvent* e)
    {
        char c = '\0';

        std::cout << "timerEvent" << std::endl;
        std::cin >> c;

        if (c == 'q') {
            killTimer(m_idtimer);
            quit();
        }
    }
};

int main(int nargs, char* argc[])
{
    MyApp app(nargs, argc);

    return app.exec();
}

Extra mini Makefile:

LDFLAGS = -I/usr/include/qt4 -I/usr/include/qt4/QtCore
LDLIBS = -lQtCore

Compiling and execution:

$ make main
g++   -I/usr/include/qt4/QtCore  main.cpp  -lQtCore -o main
$ ./main
timerEvent
1
timerEvent
2
timerEvent
3
timerEvent
q
$

Ok and then, my question. I’ve made this sample with the purpose of testing if timer events are accumulative.

When I executing the main program, the next occurs:

• the first timerEvent message is shown after 3 seconds, and timerEvent() waits a character.
• I press 1 inmediatly.
• 3 seconds later, the second timerEvent message appear (as expected).
• I wait some seconds (15 or more) and I press 2
• The third message is shown immediatly (one timer event accumulated).
• I press 3 immediatly.
• And 3 seconds later the fourth message appear (no more timer events accumulated).
• I press q and the program ends.

Question: Why aren there no more timer events accumulated? Does this behaviour depend on the platform?

PD: My Qt version is 4.8, my SO Ubuntu 13.04, and my kernel (linux) 3.8.0-19-generic. The running graphic system is Gnome 3.

Answer 1

Your fifteen second wait will not accumulate timer events because your timerEvent code blocks waiting for the input. Qt can't get back into its event loop until you enter the input. When it does get back to the event loop it checks the elapsed time and notices that more than 3 seconds have elapsed and so it fires off a timer event. The fact that fifteen seconds has elapsed is irrelevant.

This is the expected behaviour and will not (should not) be platform-dependent.