Models: QList<QObject*> vs QList<Derived*>

[qragnar]

Hi,

Fairly simple question: It seems that when I use a GridView with a model of QList<Derived*>, where Derived inherits from QObject, the delegate of my GridView doesn't show anything. If, OTOH I stick with a model QList<QObject*>, it works as expected. (Each QObject pointer points to a Derived).

Is this normal and expected? Can I use a QList<Derived*> as a model instead, iow. have I just missed something?

[SGaist]

Hi,

I'm no QML specialist yet, but did you register your custom type ? Have a look "here":http://qt-project.org/doc/qt-5/qtqml-cppintegration-data.html#qobject-derived-types

Hope it helps

[qragnar]

If by that, you mean:
@qmlRegisterType<Derived>("MyApp", 1,0, "Derived");@

then yes, I did.

[SGaist]

Also the QList<Derived *> type ?

[qragnar]

I'm not sure I see why that would be required, but I tried it anyway:

@qmlRegisterType<QList<Derived*> >("MyApp", 1,0, "DerivedList");@

This causes a compile-error:

/usr/include/qt5/QtQml/qqml.h:204: error: 'staticMetaObject' is not a member of 'QList<Derived*>'

I find this even weirder, since /usr/include/qt5 does not exist. I'm using the SailfishSDK and it's under /opt, not /usr. Ah well, I guess the compiler just reports the path incorrectly.

[Ildar]

Also, you may using "QQmlListProperty":http://qt-project.org/doc/qt-5/qqmllistproperty.html

[Ildar]

[quote author="qragnar" date="1410240330"]I'm not sure I see why that would be required, but I tried it anyway:

@qmlRegisterType<QList<Derived*> >("MyApp", 1,0, "DerivedList");@

This causes a compile-error:

/usr/include/qt5/QtQml/qqml.h:204: error: 'staticMetaObject' is not a member of 'QList<Derived*>'
[/quote]

I think this is because QList is not QObject subclass

[SGaist]

Indeed, I've mixed two different things together. Sorry

[qragnar]

[quote author="Ildar" date="1410240434"]Also, you may using "QQmlListProperty":http://qt-project.org/doc/qt-5/qqmllistproperty.html[/quote]

I'm not really sure I understand how this applies to a model. Are you suggesting I simply wrap the QList as a property of a model?

[Ildar]

Yes, you may create simple wrap for QList<Derived*>
I think QQmlListProperty more appropriate way to set collection from C++ to QML. For example:
@Q_PROPERTY(QQmlListProperty<Derived> derivedList READ derivedListProperty NOTIFY derivedListChanged)

private:
QQmlListProperty<Derived> derivedListProperty()
{
return QQmlListProperty<Derived>(this,
NULL,
&MyClass::append,
&MyClass::count,
&MyClass::at,
&MyClass::clear);
}

static void append(QQmlListProperty<Derived> *property, Derived *value)
{
MyClass myClass = qobject_cast<MyClass>(property->object);

if(myClass != NULL)
{
        myClass->add(value);
}

}

static Derived *at(QQmlListProperty<Derivedt> *property, int index)
{
MyClacc myClass = qobject_cast<MyClass>(property->object);

if(myClass != NULL)
{
    return myClass->at(index);
}
else
{
    return NULL;
}

}

static void clear(QQmlListProperty<Derived> *property)
{
MyClass myClass = qobject_cast<MyClass>(property->object);

if(myClass != NULL)
{
    myClass->clear();
}

}

static int count(QQmlListProperty<Derived> *property)
{
MyClass myClass = qobject_cast<MyClass>(property->object);

if(myClass != NULL)
{
    return myClass->size();
}
else
{
    return 0;
}

}
@

And now in QML you can using myClass.derivedList as simple QML list

[qragnar]

Hmm... Still not sure I'm getting this. I tried your code. Thank you very much for that, btw! However, the GridView doesn't get populated with anything:

@GridView {
id: buttonGrid
anchors.fill: parent
model: MyClass.derivedList
// ...
}@

buttonGrid is empty. I've inserted some qDebug() calls and it seems that derivedListProperty() does not even get called...

[Ildar]

Did you register your custom type?
@qmlRegisterType<MyClass>("MyApp", 1,0, "MyClass");@

And did you export MyClass instance to QML?
@
QQmlApplicationEngine *qmlEngine = new QQmlApplicationEngine(this);
qmlEngine->rootContext()->setContextProperty("myClass", new MyClass());
@
Or did you create MyClass instance in QML?
@
MyClass {
id: myClass
}
GridView {
id: buttonGrid
anchors.fill: parent
model: myClass.derivedList
// ...
}@

[qragnar]

Yes, I have registered the type using qmlRegisterType. And main.cpp contains the following:

@QGuiApplication *app = SailfishApp::application(argc, argv);
QQuickView *view = SailfishApp::createView();
QQmlContext *ctxt = view->rootContext();

MyClass mc;
// this is irrelevant here, but included for clarity
// it is just to make it possible for mc to use setContectProperty
// in this case, it isn't used, though
mc.setQmlContext(ctxt);
mc.loadData("default");
view->setResizeMode(QQuickView::SizeRootObjectToView);
ctxt->setContextProperty("MyClass", QVariant::fromValue(&mc));

@

As you can see, this is really a SailFish app

[Ildar]

Try ctxt->setContextProperty("myClass", QVariant::fromValue(&mc));
and using as model: myClass.derivedList (start with lower letter "m").
P.S.
MyClass - this is type
myClass - this is instance of object

[Ildar]

And more better set parents to objects. For example:
@MyClass *myClass = new MyClass(this);@
This is because if object parent is null than QML take ownership of object.

[qragnar]

Well, to be precise the type could start with a lower case and the instance name with an upper case, but I suppose using the same name for both could be more problematic.

In any case, it is now working and I can set the model to derivedList. Thanks very much for that suggestion. However, I still have my original problem, which is the reason wanting to use something other than QList<QObject*>. The original problem is that I can't called a method defined in Derived from the Delegate, since QML doesn't know that model is Derived, only QObject.

Update:
Actuallly, I realised after posting this that I can dereference like this:

@onClicked: {
myClass.derivedList[index].doSomeStuff()
myClass.derivedList[index].callAnotherMethod()
}
@

So my original issue is actually solved now. Thanks!

[Ildar]

bq. Well, to be precise the type could start with a lower case and the instance name with an upper case, but I suppose using the same name for both could be more problematic.

Of cource) Simply, I using "Qt coding style":http://qt-project.org/wiki/Qt_Coding_Style