[Qt-qml] Behaviour when sending QList<QObject*> as QVariant to QML?

[Johan Paul]

Sounds like using a QAbstractListModel would be easier for you than 
using the QList<QObject *> approach.

I've found that using the latter is easy to get started, maybe for 
throwing a PoC or something, but if you really want to use model in QML 
from C++, it's worth implementing it with a QAbstractListModel. In the 
end it's not even that much more code.

Or was there a reason the model had to be a QList<QObject *> thingy?


Cheers,

Johan



> You're already on the right track here.  One thing that might be
> helpful is to do this in your QObject's constructor:
>
> QDeclarativeEngine::setObjectOwnership (this, QDeclarativeEngine::CppOwnership);
>
> We also had to implement reference counting because we shared our list
> objects throughout the program.
>
>
> On Sun, Oct 9, 2011 at 8:05 PM, Pris Matic<prismatic.project at gmail.com>  wrote:
>> I'll try to answer my own question, in the hopes that it might benefit
>> someone else.
>>
>> You definitely need to maintain those QObjects in memory on the C++ side.
>> When I pushed a QList<QObject*>  to the model of a ListView, and then later
>> deleted the QList (using qDeleteAll), I got an instant seg fault. There's
>> also some unclear behaviour as to what happens when you clear the ListView
>> of all elements, and then try to delete the QList. I ran into the same
>> problem (seg fault) even though the model was now pointing to something
>> else. So right now I keep track of everything I send to the declarative
>> engine in this manner and then delete it when I exit the program. It's not
>> ideal, but it's the only solution that works for me.
>>
>>
>> Regards,
>>
>> -Pris
>>
>> On Sat, Oct 8, 2011 at 7:21 PM, Pris Matic<prismatic.project at gmail.com>
>> wrote:
>>>
>>> Hi all,
>>>
>>> I have a question about what exactly happens when you have a
>>> QList<QObject*>  in your C++ code, and then you send it to QML with something
>>> like this:
>>>
>>> QVariant Abc::GetObjectList()
>>> {  return QVariant::fromValue(m_object_list);  }
>>>
>>> Where m_object_list is a QList<QObject*>  where the QObjects have slots
>>> that can be directly invoked from QML (imagine using the list for something
>>> like a ListView).
>>>
>>> Does the QML side of things create a copy of the list, or do I have to
>>> maintain it in C++? Ie if m_object_list was cleared after 'GetObjectList'
>>> was called from QML, then would the list still be valid? If I was using a
>>> ListView, would the ListView need to access that QList if it needed to draw
>>> offscreen delegates as they became visible, etc.
>>>
>>>
>>> Regards,
>>>
>>> -Pris
>>
>>
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>>
>>
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