Use of Qt Property

hjohn

I have go through
http://doc.qt.io/archives/qt-4.8/properties.html#requirements-for-declaring-properties

https://forum.qt.io/topic/35142/solved-what-s-the-purpose-of-the-qt-property-system

links.But I do not understand purpose of it.One is in QML. But I want to use it without QML.
Here in code

QObject *object = button;

button->setDown(true);
object->setProperty("down", true);

does it mean that object->setproperty() is accessing Q_PROPERTY().

sierdzio

@hjohn said in Use of Qt Property:

Is it mean that object->setproperty() is accessing Q_PROPERTY().

Yes.

hjohn

@sierdzio

We can setDown button directly by using


button->setDown(true);

then what is the purpose of setting property using

object->setProperty("down", true);

sierdzio

what is the purpose of setting property using [setProperty]

Well, in this case it does not make any sense to use the property.

It may make sense if you want to execute a method in a separate thread, then you can order a queued invocation (and stop worrying about mutexes and such).

It may also make some sense if you're working with QObjects you don't know directly (you have no #include for them), then you can query the meta object system for all the properties.

Also, it might be a way of hiding the interface - you can expose the Q_PROPERTY but keep your methods private. I don't quite see why you would do that, but you can ;-) Perhaps in connection with MEMBER properties it would work really well (then there is no need for any access methods at all == less code to write).

Plus other uses mentioned in the other thread.

As you say, though, properties make most sense in QML. In C++, you using methods directly is simpler and faster.

VRonin

Properties are also used to provide a generic way to animate, see QPropertyAnimation

hjohn

@sierdzio
Can you link any example of proper use!

sierdzio

@hjohn said in Use of Qt Property:

@sierdzio
Can you link any example of proper use!

You have already written the code for using properties. If you have any specific case in mind, please tell me which one.

hjohn

@sierdzio said in Use of Qt Property:

It may make sense if you want to execute a method in a separate thread, then you can order a queued invocation (and stop worrying about mutexes and such).

Any Example related this!!

sierdzio

@hjohn said in Use of Qt Property:

@sierdzio said in Use of Qt Property:

It may make sense if you want to execute a method in a separate thread, then you can order a queued invocation (and stop worrying about mutexes and such).

Any Example related this!!

As mentioned in the docs: https://doc.qt.io/qt-5/qmetaobject.html#invokeMethod

QMetaObject::invokeMethod(thread, "quit", Qt::QueuedConnection);

This is similar to just connecting to the quit slot in a queue:

connect(someQObject, &SomeClass::someSignal, thread, &QThread::quit, Qt::QueuedConnection);

hjohn

@sierdzio How to set property of it using Q_PROPERTY()

sierdzio

@hjohn said in Use of Qt Property:

@sierdzio How to set property of it using Q_PROPERTY()

I don't understand the question. How to set what property? What is "it" referring to?

And what has Q_PROPERTY to do with it? That macro only declares the property, it is not used to set anything.