Set context property of a custom object is an object true

MoaMoaK

I'm trying to have access to a method of a custom object in QML through context.

// C++
// item is type MyClass
QQuickWidget *infoView = new QQuickWidget();
QQmlContext *rootCtx = infoView->rootContext();
rootCtx->setContextProperty( "pl_item", item ); // item is the object I want to access
infoView->setSource( QUrl::fromLocalFile(":/playlist/InfoMovie.qml") );

viewStack->addWidget( infoView ); // viewStack is a QStackedLayout displayed
viewStack->setCurrentWidget( infoView );

// HPP of MyClass
class MyClass : public QObject
{
    Q_OBJECT
public:
    MyClass( );
    Q_INVOKABLE QString getT() { return QString("plop"); };
};

//InfoMovie.qml
Item {
    Text {
        text: "<b>"+pl_item.getT()+"</b>"
    }
    // Other stuff displayed

When executing this code I get the following error :

file::/playlist/InfoMovie.qml:39: TypeError: Property 'getT' of object true is not a function

What i understand is that for the QML, pl_item is of type "true" (??) and not "MyClass" (so it can't invoke the methods) but I don't get why the type is "true". When setting a breakpoint on the c++ line with setContextProperty, I can see that item is really a well instantiated MyClass object. And the qml file is displayed properly for the rest of the file. So where can the error be ? I really have no idea where this "object true" comes from.

MoaMoaK

I did further tests and here is what i found. Either I didn't understand something or there is a bug :

//C++
// Same as above
QQuickWidget *infoView = new QQuickWidget();
QQmlContext *rootCtx = infoView->rootContext();

// New
log(typeid(item).name());
rootCtx->setContextProperty( "pl_item", item );
QVariant test = rootCtx->contextProperty("pl_item");
log(typeid(item).name());
log(test.typeName());

// Same as above
infoView->setSource( QUrl::fromLocalFile(":/playlist/InfoMovie.qml") );
viewStack->addWidget( infoView ); // viewStack is a QStackedLayout displayed
viewStack->setCurrentWidget( infoView );

The debugger gives me item of type MyClass (same memory address as the object i want) and test of type QVariant (bool) (value true). And the log output :

PK7MyClass
PK7MyClass
bool

At least that's coherent with the fact that i'm getting a true value in the QML part.... But where does that boolean comes from ?! This code + the qml above is exactly what i've written (not simplified code). I don't understand ...

Eeli K

@MoaMoaK try

rootCtx->setContextProperty( "pl_item", &item );

Eeli K

@MoaMoaK Ok, I did some demangling and testing and found out that if you create your item like this:

MyClass const* item =  new MyClass();

it gives the error, but without const it works as expected. I hope in the future you will give all the relevant information - in this case you left out the code which created the item object.

Eeli K

@MoaMoaK You could say this is a permanent bug by design in C++; it does implicit type conversion from any pointer (const or not) to bool. setContextProperty accepts const* because it accepts a QVariant, which can be created with bool.

In any case it wouldn't make much sense to be able to pass QObject const* to setContextProperty because the data constness would be lost.

MoaMoaK

OH Thx a lot that was it. Without the const, it works.
So my code went from

void AnotherClass::showInfo(MyClass const *item)
{
    // Same stuff as above (2nd post)
}

to

void AnotherClass::showInfo(MyClass *item)
{
    // Same stuff as above (2nd post)
}