[Interest] QAbstractEventDispatcher - registerSocketNotifier(), unregisterSocketNotifier()

[Thiago Macieira]

On Monday, 30 December 2019 15:31:14 -03 Narolewski Jakub wrote:
> I did not notice if unregisterTimer() and registerTimer() are also called
> so often but their non-destructive switching could also come in handy.
> Do you think that changing current QSocketNotifier::setEnabled( bool )
> behaviour to actually enable / disable notifiers is a good idea?

I don't see why not. In fact, I'm positive it is a good idea. In all 
implementations, there's a cost associated with registering a notifier. So the 
worst case scenario is that in an implementation it's impossible to disable, 
forcing us to fall back to deregistering with the low-level implementation. 
That's exactly what we do today, so it's no worse. We do need some bookkeeing 
so we can re-register on setSocketDescriptorEnabled(), but I guess that it's 
the bookkeeping that's already there.

The drawback is of the increased memory consumption by QSocketNotifiers that 
stay mostly disabled. That is the case for the write notifiers for all 
sockets, which are only enabled when they have something to write. I think 
that's an acceptable trade-off.

> I don't know much about Qt at this level or how much of current code
> depends on them actually registering and deregistering.
> 
> Also, does QSocketNotifier 'know' when it is supposed to release underlying
> socket?

Today, on setEnabled(false) or destruction. If we implement the new feature, 
only on destruction.

> Could registering and deregistering be moved entirely to its constructor
> and destructor?

That's what I hope.

> I don't know how QAbstractEventDispatcher interops with QTimer, I will try
> to research it a bit more.

It's basically the same.

The one thing to look after is that QAbstractSocket has a shortcut to skip 
some of the signal-slot mechanisms of QSocketNotifier and be notified 
immediately by way of a virtual. So there's a second place to deal with 
registering/unregistering, aside from QSocketNotifier.

-- 
Thiago Macieira - thiago.macieira (AT) intel.com
  Software Architect - Intel System Software Products