Signal from C++ to QML using custom parameter type

cp_mark

Hi all,

I would like to emit a signal from C++ to QML using a custom parameter type.
ie. (in C++)
emit mySignal(CustomParameterType param);

In this case: 'CustomParameterType' is an extended QObject with exposed properties I want to access. (It's really just acting as a glorified struct.)

I have exported my custom class using both:
Q_DECLARE_METATYPE(CustomParameterType)
qRegisterMetaType<CustomParameterType>()

I can successfully catch the signal in QML, but cannot access the details of my 'CustomParameterType'. If I print() the parameter in QML I get: QVariant(CustomParameterType) printed to the log.

I want to be able to access the properties inside 'CustomeParameterType' (ie. parameter.property1). However, I just get undefined errors when I do. It looks like 'CustomParameterType' is getting sent over as a QVariant object?

ambershark

@cp_mark Everything gets sent as a QVariant to qml. You normally have to cast it out. I don't know how to do it in qml as I'm a qml noob but here is what you would do in c++:

// assume variant is in a var call v
CustomParameterType cpt = v.value<CustomParameterType>();

dheerendra

Use qqmlregistertype to register custom data type. This should fix your issue. If not let me know. I will give ready program.

cp_mark

Thanks for the replies everyone.
@ambershark I can't for the life of me find any examples of how to cast a QVariant object in QML :-\
@dheerendra I actually already have that in my code, but I didn't mention it as I thought it was used to instantiate custom objects in QML only. Either way here's what I've added:

qmlRegisterType<CustomParameterType>("MyLib", 1, 0, "CustomParameterType");

I would love to see a working example of my problem. Thanks!

VRonin

@cp_mark said in Signal from C++ to QML using custom parameter type:

emit mySignal(CustomParameterType param);

@cp_mark said in Signal from C++ to QML using custom parameter type:

'CustomParameterType' is an extended QObject

Don't really know what's going on here but you can't pass QObjects by value. Also, QObject subclasses should work seamlessly in QML.

Remove Q_DECLARE_METATYPE and qRegisterMetaType<CustomParameterType>() they are useless and change methods that accept CustomParameterType to use a pointer instead of a value.

Once you did that you'll be able to access its properties from QML

cp_mark

@VRonin that worked. Thanks!
Problem now is destroying the malloc'd CustomParameterType. I am a little familiar with QT memory management, but would like to manually free the object from the QML side of things. This is because I do not have access to a parent object that will end up being destructed (and in turn deconstruct my CustomParameterType).

Eeli K

@cp_mark Do you really use malloc() for C++ objects? See https://stackoverflow.com/questions/184537/in-what-cases-do-i-use-malloc-vs-new.

QML engine uses garbage collection, you don't free memory in QML. I'm not familiar with object ownership, but see http://doc.qt.io/qt-5/qtqml-cppintegration-data.html#data-ownership.

cp_mark

@Eeli-K Nope. I used new . Just come from a C background.