[Development] About ALIAS in Q_PROPERTY

[Simon Hausmann]

On Wednesday 12. March 2014 12.48.20 Olivier Goffart wrote:
> On Monday 10 March 2014 17:43:35 Giuseppe D'Angelo wrote:
> > Can you please explain what this feature is about, why do you think
> > it's useful, how it's supposed to be used, etc.?
> > 
> > On 10 March 2014 17:30, mikhail.svetkin at gmail.com
> > 
> > <mikhail.svetkin at gmail.com> wrote:
> > > Hello, I would like to clarify wherein the complexity
> > > (https://codereview.qt-project.org/#change,80412)?
> > > Ready to listen to any suggestions and implement them.
> 
> This is a follow-up on the earlier discussion:
> "About Q_PROPERTY in custom widget plugin for designer"
> 
> Svetkin's first idea was to extend the READ and WRITE attributes to be more
> complex expressions such as:
> 
>   Q_PROPERTY(QString label READ ui->customWidget->label
>                            WRITE ui->customWidget->setLabel)
> 
> And he even made a patch for it: (https://codereview.qt-project.org/77758)
> There was small issues with the patch. Notably on how complex can the
> expression be, and also on the fact that it would still be hard to have a
> NOTIFY signal.
> 
> I was also wondering if it would not be better to copy QML's idea of
> property aliases:
> 
>  Q_PROPERTY(QString label ALIAS CustomWidget ui->customWidget->label)
> 
> However, now that Svetkin proposed a patch, I feel that it is not so much of
> a good idea. I think the syntax is not so nice especially the need to have
> both the class name and the 'path' to the property. And also because the
> notify signal is still not automatic.
> 
> And overall, i find myself preferring the first idea better. Hence I would
> like more opinions from the list.
> 
> The other question is if we should have that at all. How common is it to
> have property alias like this?

I'm also not sure if it's worth it. Perhaps using Q_PRIVATE_PROPERTY gets us 
close enough?


My worry would be that it has to work together with QML, which requires an API 
to resolve aliases at run-time, in order to install bindings on the _real_ 
property and not the alias (and therefore also remove a previously installed 
binding).

The alias implementation QML goes through quite some lengths to achieve that 
and the automatic connect to the notify signal.


Simon