[Qt-interest] QEventLoop and QEvents

[Eric Clark]

> -----Original Message-----
> From: qt-interest-bounces at trolltech.com [mailto:qt-interest-
> bounces at trolltech.com] On Behalf Of Frank Hemer
> Sent: Friday, February 19, 2010 4:22 PM
> To: qt-interest at trolltech.com
> Subject: Re: [Qt-interest] QEventLoop and QEvents
> 
> On Friday 19 February 2010 22:27:12 Eric Clark wrote:
> > > -----Original Message-----
> > > From: qt-interest-bounces at trolltech.com [mailto:qt-interest-
> > > bounces at trolltech.com] On Behalf Of Frank Hemer
> > > Sent: Friday, February 19, 2010 3:21 PM
> > > To: qt-interest at trolltech.com
> > > Subject: Re: [Qt-interest] QEventLoop and QEvents
> > >
> > > On Friday 19 February 2010 20:21:07 Eric Clark wrote:
> > > > When I say that it is a little bit of time, it is milliseconds.
> This
> > >
> > > is how
> > >
> > > > it HAS to work. Without going into great detail, this particular
> > >
> > > widget is
> > >
> > > > a GLWidget. There is an active 3D simulation going on in the
> widget
> > >
> > > and
> > >
> > > > when you click on the widget, a 3D ray-trace is performed to find
> the
> > >
> > > 3D
> > >
> > > > object that was clicked on. During the active simulation, there
> are
> > >
> > > events
> > >
> > > > that happen that cause manipulation of the 3D objects, such as
> > >
> > > cutting
> > >
> > > > holes in the objects, or possibly removing the object from the
> viewer
> > >
> > > all
> > >
> > > > together. The simulation runs on a separate thread than the GUI
> and
> > >
> > > as
> > >
> > > > such, it is VERY unsafe to allow the ray-trace to happen while
> the
> > > > simulation is active. Instead, the 3D viewer reacts to mouse
> events
> > >
> > > when it
> > >
> > > > repaints. During repainting of the GLWidget, the simulation is
> > >
> > > temporarily
> > >
> > > > paused and any events that occurred between painting will also be
> > > > processed. Believe me, I know what I am doing as far as deferring
> the
> > > > events. The time between repaints is so fast that no user will
> ever
> > >
> > > notice
> > >
> > > > the delay in reaction of a mouse click. Not only that, what
> happens
> > >
> > > when
> > >
> > > > you click on an object is that it gets highlighted in the 3D
> viewer,
> > >
> > > this
> > >
> > > > highlighting would even be ok if it did take longer than normal,
> but
> > >
> > > the
> > >
> > > > reaction time now is on the order of milliseconds and no human
> eye
> > >
> > > can
> > >
> > > > possibly see the delay.
> > > >
> > > > Now that everyone knows what is really going on. I would love for
> > >
> > > someone
> > >
> > > > to answer my question. Is there any way to tell Qt to not delete
> the
> > >
> > > event?
> > >
> > > > Or do I need to make a copy of the event?
> > >
> > > You can catch events (mouse and key) in an QApplication-installed
> > > eventHandler, store a copy and  post them later.
> > > That works quite well, however be aware of some surprising effects
> if
> > > for ex.
> > > a Window pops up right before the events get posted.
> > > This will most likely NOT work for other system-internal events
> though.
> >
> > I am actually using an event handler to catch the events. That is how
> I
> > defer them. I was trying to take the event and post it to a different
> event
> > loop that I could call processEvents on when it was time to do so.
> However,
> > the events were being deleted before it was time to call
> processEvents on
> > them. So, I tried to make a copy of the event and post it to the
> other loop
> > from within my event handler, but that seemed to cause some very
> strange
> > behavior. Maybe I did something wrong, I am not sure...
> 
> An eventloop is _PROCESSING_ the events, its not a storage class.
> 
> Use a QQueue to store the event copies and their receiver.
> And post them later using QApplication::postEvent.

I thought that an event loop used a queue to store the events until processEvents
was called on it... Is this incorrect? If I never call exec on the event loop, will
it not store the events that I post to it until I either call exec or processEvents?

Eric

> 
> Frank
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