QThread and QEventLoop - Idle Processing

[jlemaitre]

If you want to do idle processing in the thread besides the main task the thread is doing, use a timer instead of sleep().

In the initial post of the op, I understood the idle processing is the main task. (Polling the device is the only task of the thread, and it must be done at idle time.) In that case, is its timer-based solution (re-coded below) appropriate?

class DevicePoller : public QThread
{
    Q_OBJECT
    QTimer m_timer;
slots:
    void poll();
protected:
    virtual void run()
    {
        connect( m_timer, SIGNAL( timeout() ), this, SLOT( poll() ) );
        m_timer.start( 0 );
        exec();
    }
};

class DevicePoller : public QThread
{
    Q_OBJECT
    QTimer m_timer;
slots:
    void poll();
protected:
    virtual void run()
    {
        connect( m_timer, SIGNAL( timeout() ), this, SLOT( poll() ) );
        m_timer.start( 0 );
        exec();
    }
};

To copy to clipboard, switch view to plain text mode 

Or, is it best to avoid timers at all, and launch a thread with QThread::IdlePriority:

class DevicePoller : public QThread
{
protected:
    virtual void run()
    {
        while( !finished ) poll();
    }
};
DevicePoller mythread;
mythread.start( QThread::IdlePriority );
// ...

class DevicePoller : public QThread
{
protected:
    virtual void run()
    {
        while( !finished ) poll();
    }
};
DevicePoller mythread;
mythread.start( QThread::IdlePriority );
// ...

To copy to clipboard, switch view to plain text mode 

Or, am I wrong and there is another strategy?

Thanks for the rapid answer anyway!

[wysota]

In the initial post of the op, I understood the idle processing is the main task. (Polling the device is the only task of the thread, and it must be done at idle time.) In that case, is its timer-based solution (re-coded below) appropriate?

It's a periodic task, not idle processing.

class DevicePoller : public QThread
{
    Q_OBJECT
    QTimer m_timer;
slots:
    void poll();
protected:
    virtual void run()
    {
        connect( m_timer, SIGNAL( timeout() ), this, SLOT( poll() ) );
        m_timer.start( 0 );
        exec();
    }
};

class DevicePoller : public QThread
{
    Q_OBJECT
    QTimer m_timer;
slots:
    void poll();
protected:
    virtual void run()
    {
        connect( m_timer, SIGNAL( timeout() ), this, SLOT( poll() ) );
        m_timer.start( 0 );
        exec();
    }
};

To copy to clipboard, switch view to plain text mode 

There is no point in using threads here, you can use the main thread with this approach.

Or, is it best to avoid timers at all, and launch a thread with QThread::IdlePriority:

class DevicePoller : public QThread
{
protected:
    virtual void run()
    {
        while( !finished ) poll();
    }
};
DevicePoller mythread;
mythread.start( QThread::IdlePriority );
// ...

class DevicePoller : public QThread
{
protected:
    virtual void run()
    {
        while( !finished ) poll();
    }
};
DevicePoller mythread;
mythread.start( QThread::IdlePriority );
// ...

To copy to clipboard, switch view to plain text mode 

Or, am I wrong and there is another strategy?

Thread priorities are just hints for the OS, it may ignore them so that's not a good approach -- you'll end up with a busy loop.