QVector<CustomClass> as a var property in QML. Qt 5.14 vs 5.12.

Dominik1983

Hello Ladies and Gentlemen.

Unsure if my idea would work, I created a DataPoint class marked with Q_GADGET with three properties defined using Q_PROPERTY. I placed Q_DECLARE_METATYPE(DataPoint) after the class definition. I also added qRegisterMetaType<DataPoint>() to the initialisation method of my main C++ class (let's call it MainClass).

In my MainClass I defined a property:

Q_PROPERTY(QVector<DataPoint> highPressureVector READ getHighPressureVector NOTIFY highPressureVectorChanged)

I fill a corresponding MainClass member (that is used in the getter) m_highPressureVector with three DataPoint objects.

In QML I use the forementioned property like this:

property var points: mainObject ? mainObject.highPressureVector : []

When I log its contents with:

console.log("points = " + points + ", points.length = " + points.length)

I get what I expect:

qml: points = DataPoint(0, 1, red),DataPoint(1,2,red),DataPoint(2,3,red), points.length = 3

The PROBLEM is that I did all this in Qt 5.14.2. I learned that we will use Qt 5.12.8. After compiling the exact same code with Qt 5.12.8 the line that prints points property looks like this:

qml: points = QVariant(QVector<DataPoint>), points.length = undefined

QUESTION: Is there a way to keep using QVector<DataPoint> as my property? Or do I have to make DataPoint inherit from QObject and use QVariantList or something like that?

I also have a second less important question. Why does this happen? I haven't seen anything in the Qt 5.14 version description that would justify such a difference in behaviour.

Dominik1983

It seems that I'm not the only one who has encountered this problem:
https://stackoverflow.com/questions/58284209/how-to-pass-qvectorcustom-q-gadget-to-qml

There is also a bug for that but it's still open even though QVector<GadgetClass> works in Qt 5.14.
https://bugreports.qt.io/browse/QTBUG-73399

It seems that someone changed something in 5.14 that is not well documented but has made a significant change in handling QVectors of gadgets.

Since (for Qt 5.12) QML displays my points property as

QVariant(QVector<DataPoint>)

I assume that it is aware of the existence of DataPoint type. Why does it wrap it in QVariant in Qt 5.12? Is there a way to extract my vector from this variant so that QML treats it as an array?

Dominik1983

OK, I gave up.

Instead of using Q_GADGET I made DataPoint class inherit from QObject and added Q_OBJECT macro to it. Then I turned my property which stores DataPoints into a QList:

Q_PROPERTY(QList<QObject*> highPressureList READ getHighPressureList NOTIFY highPressureListChanged)

I wish I could have kept QVector<GadgetClass> though...

I think that this is the change that is missing in 5.12 and is present in 5.14:
https://bugreports.qt.io/browse/QTBUG-60338

fcarney

Does it make any different to register QVector<DataPoint> as well:

qRegisterMetaType<QVector<DataPoint>>()

Dominik1983

I had some time to return to this problem and switch back DataPoint to Q_GADGET.

@fcarney said in QVector<CustomClass> as a var property in QML. Qt 5.14 vs 5.12.:

Does it make any different to register QVector<DataPoint> as well:
qRegisterMetaType<QVector<DataPoint>>()

Unfortunately it did not help. The vector in QML was visible as:

QVariant(QVector<DataPoint>)

Dominik1983

I found a satisfying solution. When my DataPoint class is a Q_GADGET and is registered as a metatype:

Q_DECLARE_METATYPE(DataPoint);

qRegisterMetaType<DataPoint>();

it can be used in a QVariantList. The list is visible in QML as a JavaScript array. Then I can freely access all properties of each DataPoint in QML.

Adding an object of my DataPoint class to a QVariantList exposed to QML looks like this:

list.append(QVariant::fromValue(DataPoint{x, y, color}));

Reading a single DataPoint object from the QVariantList looks for example like this:

DataPoint lastdp = list.last().value<DataPoint>();