Sorry, the code was incomplete, it should have been
Q_PROPERTY(qreal x BINDABLE getBindableX READ default FINAL)
//...
QProperty<qreal> m_X;
//...
QBindable<qreal> CPropertyClass::getBindableX() const
{
return &m_X;
}
The purpose is to transmit data from the C++ backend to the QML front end. It’s
a one-way road, QML only has to read and update when properties change.
I have hundreds of such properties in dozens of classes, so I was looking for
the simplest possible way to get it done. Before 6.5, code was more like this
Q_PROPERTY(qreal x READ getX NOTIFY notifyXChanged FINAL)
//..
qreal m_X;
//...
signals:
void notifyXChanged();
//...
qreal getX() const
{
return m_X;
}
//...
void setX(const qreal arg)
{
if (m_X == arg)
{
return;
}
m_X = arg;
emit notifyXChanged();
}
Since 6.5, using BINDABLE keyword and READ DEFAULT is sufficient for QML to
update, and as you can see, the code is much simpler. In the older version, I
need a setter (which is not reachable from QML) because of the change
comparison. The QProperty/QBindable implements that for me: If I set a property
to the same value, no updates are triggered.
Hope that helps making things clear.
Kind regards
Robert
From: Interest <interest-boun...@qt-project.org> On Behalf Of Axel Spoerl via
Interest
Sent: Friday, 24 May 2024 15:02
To: interest@qt-project.org
Subject: Re: [Interest] Is it safe to construct QBindable from QProperty* via
QUntypedBindable?
CAUTION: External email. Do not click on links or open attachments unless you
know the sender and that the content is safe.
Seas Robert,
If memory serves well, there was a recent post about the same question on the
Qt Forum.
Haven't got around to answer, because I wanted to read some code upfront.
The given implementation of getBindable() has no return type, but you say it's
a bindable.
I also assume that there's a reason for actually returning a bindable, rather
than a qreal value.
Probably a calculation is implemented.
The code looks safe to me. The only caveat could be, that the propery is
actually qreal typed.Depending on how it changes hands in QML, it might become
a qreal value at some point, hence loosing the actual binding. But that's not
related to your implementation.
I still find it unusual and haven't seen it anywhere else, than in the
mentioned example.
Don't fully understand the use case of it. But you sound like you do 🙂, so IMHO
there's nothing to worry about.
Cheers,
Axel
________________________________
Von: Interest
<interest-boun...@qt-project.org<mailto:interest-boun...@qt-project.org>> im
Auftrag von Schimkowitsch Robert
<robert.schimkowit...@andritz.com<mailto:robert.schimkowit...@andritz.com>>
Gesendet: Freitag, 24. Mai 2024 14:08
An: interest@qt-project.org<mailto:interest@qt-project.org>
<interest@qt-project.org<mailto:interest@qt-project.org>>
Betreff: [Interest] Is it safe to construct QBindable from QProperty* via
QUntypedBindable?
I use QBindable a lot in C++ classes that expose properties to QML.
My typical property implementation looks as follows:
Q_PROPERTY(qreal x BINDABLE getBindableX READ default FINAL)
//...
QProperty<qreal> m_X;
//...
CPropertyClass::getBindableX() const
{
return &m_X;
}
I got the idea to use "&m_X" to get the QBindable for a property from the
example in QObjectBindableProperty. And initially, I also used
QObjectBindableProperty via Q_OBJECT_BINDABLE_PROPERTY macro.
But once QML natively supported BINDABLE properties, I no longer saw any
advantage, and using QProperty produces less boilerplate.
So the usage of "&m_X" to get QBindable is documented for
QObjectBindableProperty, but only in the example, not in the docs specifically.
It feels a bit dirty to create a derived class via promotion from a base class.
And for QProperty, "&m_X" is not documented even in an example.
So the question is: Is it safe to use? Have I misunderstood something by
choosing QProperty over Q_OBJECT_BINDABLE_PROPERTY?
Robert Schimkowitsch
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