[Qt-interest] Basic QThread question

[Phil]

On Tue, 27 Apr 2010 05:40:33 pm Sean Harmer wrote:
> Hi,
> 
> It is probably best to use a QTimer with a 1s timeout value in your worker
> thread. Then connect the timeout signal to a slot that queries your device
>  and then emits a signal value( int x ) say that you have connected to in
>  the main thread.
> 
> You just need to be careful about which thread context your slot gets
>  called in because the thread object will have affinity with the main
>  thread not itself. The easiest way to do this is to create an object in
>  the run() function of your QThread subclass that does the actual work
>  rather than having the QThread class do the work.
> 
> Something like this:
> 
> class DevicePoller : public QObject
> {
> Q_OBJECT
> public:
>     DevicePoller( QObject* parent = 0 )
> 
>     : QObject( parent ), m_timer( new QTimer( this ) )
> 
>     {
>         connect( m_timer, SIGNAL( timeout() ), SLOT( poll() ) );
>         m_timer->start( 1000 );
>     }
> 
> public slots:
>     void poll()
>     {
>         // Talk to your device here
>         ...
>         int x = ...
> 
>         // Let the world know the value retrieved
>         emit value( x );
>     }
> 
> signals:
>     void value( int x );
> 
> private:
>     QTimer* m_timer;
> };
> 
> 
> class PollingThread : public QThread
> {
> Q_OBJECT
> public:
>     PollingThread( QObject* parent = 0 )
> 
>     : QThread( parent )
> 
>     {}
> 
>     DevicePoller* poller() const { return m_poller; }
> 
> protected:
>     virtual void run()
>     {
>         // Create an object to do the polling
>         m_poller = new DevicePoller;
> 
>         // Start the event loop
>         exec();
> 
>         // Clean up
>         delete m_poller;
>         m_poller = 0;
>     }
> 
> private:
>     DevicePoller* m_poller;
> };
> 
> 
> Then in your main thread somewhere have something like this:
> 
> DevicePoller* m_workerThread = new DevicePoller( this );

The above line has been corrected to PollingThread* etc.
 
> // Will use a Qt::QueuedConnection as objects live in different threads

This is the area where I'm having trouble. Is the following line correct?

> connect( m_workerThread->poller(), SIGNAL( value( int ) ),
>          this, SLOT( processValue( int ) ) );

Isn't the polling done in DevicePoller-> poll()? If I do create a poller 
function then I'm greeted with an error message stating:

"invalid use of void expression"

If I substitute "DevicePoller" for " m_workerThread->poller()" then the 
programme runs but not as a thread. The GUI is still blocked each second.

I'm still confused and, as always, any help will be greatly appreciated.

-- 
Regards,
Phil