[PyQt] Processing signals from a called method while the method executes

[Jan Haag]

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On 7/2/10 3:38 PM, alanm wrote:
> On Friday 02 July 2010 1:17:06 am Andreas Pakulat wrote:
>>
>> Don't block the event-loop with your processor. The signals are
>> delivered as you expect, but the widget is not redrawn with that
>> message. The reason is that your processing blocks the Qt event loop and
>> hence no painting is done.
>>
>> Possible options to not block the event loop that long would be to use a
>> QTimer to schedule each part of your processing after the next
>> event-loop run. The other option would be moving the processing into a
>> separate thread, you should only go this route after making familiar
>> with multi-threading though.
>>
>> Andreas
> 
> Thanks for the reply.  I don't think I understand, though.  I read in the 
> QTimer documentation where I can do a zero-millisecond timer to schedule 
> things after the next event run, but changing
> 
> sleep(10)
> 
> To:
> 
> QTimer.singleShot(0, lambda: sleep(10))
> 
> Seemed like it should do that.  It doesn't.  Apparently the 
> processor.process() method is still blocking.   Where am I going wrong here?

What this actually does is to start a single-shot QTimer (timing out
immediately) to run a callback during the next run of the event-loop;
Thus far, you understood things correctly. However, _INSIDE_ this
callback, you still set the GUI-thread and, as a consequence, the
event-loop to sleep for 10 seconds. This is obviously not what you
wanted... A better idea would be to call

QTimer.singleShot(10000, your_actual_processing_callback)

which delivers a timeout event triggering your callback after 10 seconds
(10000 ms).
The difference is that a QTimer runs asynchronously in a thread by
itself, while sleep runs synchronously in the thread it is called in,
thus blocking the event loop which does the same.

The processing callback should be reasonably short-running, otherwise
you'll run into the same problem again; in this case, your best bet is
to work your way into multithreading using the QThread class and
triggering your processing-thread's run method as the timer callback.
However, threads are often a good way to mess things up completely
(deadlocks etc...), so don't go for them unless you really have to.

Jan
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