[Development] QProperty and library coding guide

[Sze Howe Koh]

On Sat, 18 Jul 2020 at 03:58, Ulf Hermann <ulf.hermann at qt.io> wrote:
>
> > I must be missing something.  I tend to think of eventloops with notify
> > signals are an improvement over polling.  You statement seems to say
> > that asking for a value is better, and notifications are to be avoided?
>
> I'm not saying that you should go back to polling. In the case that you
> have genuine events, for example input or network activity, signals may
> just be the thing you need. This whole discussion is specifically about
> properties and their _change_ signals.
>
> Mind that the propagation of dirty flags across a binding graph is a
> kind of "change signal" in its own right. With QProperty, we just don't
> calculate the values right away when we notice that a related property
> has changed. Why is that?
>
> Properties can change fairly often, and synchronously triggering a chain
> of calculations whenever some property changes can be inefficient and
> can lead to unexpected intermediate values. This is why we introduce
> lazy evaluation for property bindings.
>
> With this system, on some level, you indeed have to poll a property to
> make something happen outside the system. However, certain technologies
> are inherently based on polling. For example the scene graph rendering a
> frame every 16ms will poll for changed items every time. For this, a
> lazy evaluation strategy makes more sense than the usual change signals.
>
> If you want to take advantage of lazy evaluation in such places, you
> should stop using the change signals there, though. Whenever a property
> has a change signal, it and everything it depends on needs to be
> evaluated eagerly.
>
> best,
> Ulf

Will language bindings be able to take advantage of the new QProperty
system? For example, will it be possible to create a lazy-evaluated
property binding from non-C++ code?

(For reference, Qt for Python currently creates new properties this
way: https://wiki.qt.io/Qt_for_Python_UsingQtProperties )


Regards,
Sze-Howe