Re: using timers in the GUI (was: Re: Testing MXE-Octave)

[Jacob Dawid]

> Yeah, I don't understand the point of the timer either.  Wouldn't it
> be sufficient to just perform the update when the Octave interpreter
> is at points where it is safe to perform updates?  For example, in 
> the
> readline callback when Octave is waiting for input and perhaps some
> other select locations.  Jacob, could you comment about why you chose
> to use a timer for this purpose?

That's pretty complicated.

As far as I recall, the problem was that Qt's model/view mechanism is 
not - different from what I initially supposed - threadsafe. The 
updating of the model has to be done from within the Qt thread. As I 
experienced crashes because of that, I sequenced the access to a 
specific "intermediate" memory by semaphores (basically just a 
simplified version of a symbol list). The octave symbol table is queried 
and the intermediate memory updated whenever the interpreter is ready 
and the intermediate memory is *not read* by the Qt thread (otherwise it 
would wait for the Qt thread to finish its job). At times, where the 
octave interpreter is not accessing the intermediate memory (ie. 
writing), the Qt thread periodically updates the model based on the 
intermediate memory. Since there is no guarantee that the procedure 
invoked by the timer will be finished within an interval, the timer is a 
single shot timer. It will only fire once, then the timer is restarted 
by the procedure when the model has been fully updated. That guarantees 
idle times between updates.