How to make a connection to Component destruction() signal from cpp side

Eligijus

Hello,

Connecting Component destruction() signal to cpp slot from qml side is quite simple - Component.onDestruction: mySlot().

But I'm intrested how to catch this signal or make a connection from cpp side.

raven-worx

@Eligijus
since a QQuickItem inherits QObject simply connect to QObject::destroyed() signal

Eligijus

@raven-worx Component destruction() signal is emited when object property values are still valid/not destroyed. As I still want to access some property data from this object, both QObject::destroyed() and ~QObject() get called too late.

raven-worx

@Eligijus
The problem is that the attached property of Component (private QQmlComponentAttached class) hooks directly into the QML engine internals.
Deleting an item from C++ always has the "drawback" that the derived destructor is called first (due to the virtual destructor of QObject).
So i would expect that the Component.onDestruction only works for QML items with QML ownership.

About what items are we actually talking? Whats the use case exactly?

Eligijus

@raven-worx My use case is that I want to save property array value to file upon destruction.
Let's say I have Class Test

class Test : public QObject
{
    Q_OBJECT
public:
    explicit Test(QObject *parent = nullptr);
    ~Test();

    Q_INVOKABLE void store();
};

store method for simplicity currently prints value to debug console

void Test::store()
{
    const QMetaObject *o = metaObject();
    int offset = o->propertyOffset();
    int count = o->propertyCount();

    for (int i = offset; i < count; ++i) {
        QMetaProperty metaProperty = o->property(i);
        QVariant defaultValue = property(metaProperty.name());

        if (defaultValue.userType() == qMetaTypeId<QJSValue>())
            defaultValue = defaultValue.value<QJSValue>().toVariant();

        qDebug() << metaProperty.name() << defaultValue;
    }
}

I create Test in qml:

Test {
        id: t
        property var foo: [100, 200, 300]
    }

And if store() is called in constructor here's the output:

foo QVariant(Invalid)

But if I connect store() to Component destruction() in qml I get my expected output:

foo QVariant(QVariantList, (QVariant(int, 100), QVariant(int, 200), QVariant(int, 300)))

My problem is that I don't want to add additional code to qml side and make a connection from cpp side because there are many Test objects created in qml.

raven-worx

@Eligijus
Try this approach:

if( QObject *attached = qmlAttachedPropertiesObject<QQmlComponent>(this, true) )
     connect( attached, SIGNAL(destruction()), this, SLOT(store()) ); // explicitly using the old syntax here, since QQmlComponentAttached is a private type

Note the qmlAttachedPropertiesObject() call with the second parameter set to true to create the attached object if it doesn't exist yet.

Eligijus

@raven-worx That works. Thanks for answer and comment why use an old connect sytax there very informative.
I ended up putting this code snippet inside componentComplete() method inherited from QQmlParserStatus because putting it directly in constructor didn't work.

raven-worx

@Eligijus said in How to make a connection to Component destruction() signal from cpp side:

Thanks for answer and comment why use an old connect sytax there very informative.

Just to point out the alternative to the old connect syntax:
the connection could also be done by searching for the corresponding QMetaMethod in the QMetaObject of the attached object and connect it.
But it's way shorter with the old syntax.