Ownership between Qt C++ and QML
Hi friends, I meet a problem about ownership these days, and i try to find out the answer, but after some search and discussing, i still have questions. So please help me out. Thank you! The question i meet is, i try to use a list in QML, which type is QList<QObject*>, and i doubt that when this object will be destroyed.
I find this in Qt document:
Ownership controls whether or not QML automatically destroys the QObject when the object is garbage collected by the JavaScript engine. The two ownership options are: QDeclarativeEngine::CppOwnership & QDeclarativeEngine::JavaScriptOwnership
So my first question is, what does “object” mean, it just mean QObject or objects inherit QObject ?
In my opinion, object should be QObject or objects inherit QObject. But with the example below:
// header file
- {
- Q_OBJECT
- Q_PROPERTY(int id READ id)
- public:
- ~CMyData() {
- qDebug() << "haha";
- }
- int id() { return 11; }
- };
- {
- Q_OBJECT
- public:
- };
// source file
- {
- obj1 = new CMyData;
- obj2 = new CMyData;
- iList = new QObjectList;
- }
- {
- iList->append(obj1);
- iList->append(obj2);
- return *iList;
- }
The obj1 and obj2 will be destroy under the garbage collection. So, what’s happend ? And i don’t think QList<QObject*> is the object will be destroy after collected by javascript . But how about the QObject(s) in it ?
8 replies
I believe this is the sort of confusion that QDeclarativeListProperty [doc.qt.nokia.com] is intended to solve.
The paragraph after the one you quoted in the QDeclarativeEngine docs
http://doc.qt.nokia.com/4.7-snapshot/qdeclarativeengine.html#ObjectOwnership-enum
says that objects returned from method calls will default to JavaScript ownership. Yes, QObjects includes QObject derived classes.
Hi aalpert, thanks for the reply. I’ll take a look at QDeclarativeListProperty. And let’s go to the question i put in thread, can you tell me how about QList<QObject*> ? I return one instance of the QList<QObject*>,let’s call it “myList”, it’s not a class inherit QObject, but the objects inside it destroyed by the javascript. So, if any QObject derived class’s instance will be destroy by javascript when js run destroy after garbage collection? And there comes another question, if i change the ownership of the parent object, will the children’s ownership be changed? Thank you again :D
- I don’t quite agree according to my experimentation. If I add a new function into above engine code like:
- Q_INVOKABLE CMyData* getData() {
- return new CMyData();
- }
then no matter how many times qml calls this getData() function, the desctruction of CMyData will not happen at all.
But if you change the return type of CMyData* to QObject* , everything works as it should be: “haha” printed out when calls the getData() many times.
- another problem is we could only define QObject* as the parameter of Q_INVOKABLE function, even int*, int&, QObject&, CMyData*, CMyData& can not be recongnized.
Hi aalpert & Alex, i do more test on this. And the usecases are:
I use QList<MyData*> instead of QList<QObject*>, the MyData won’t be destroyed, and also can not be recognized. What’s wrong with me ?
// Some code
- {
- CMyData *data = new CMyData();
- QDeclarativeEngine::setObjectOwnership(data, QDeclarativeEngine::JavaScriptOwnership);
- myList.append(data);
- return myList;
- }
- int main(int argc, char *argv[])
- {
- qmlRegisterType<CMyData>("CMyData", 1, 0, "CMyData");
- qRegisterMetaType<CMyData>("CMyData");
- CEngine *engine = new CEngine;
- QmlApplicationViewer viewer;
- viewer.setOrientation(QmlApplicationViewer::ScreenOrientationAuto);
- viewer.rootContext()->setContextProperty("engine", engine);
- viewer.showExpanded();
- return app.exec();
- }
// In qml file
- ...
- function getData()
- {
- // Trigger garbage collection
- for(var i=0; i<1000; i++)
- {
- var dataList = engine.getData()
- console.debug("Data:", dataList[0])
- }
- }
- ...
Custom type can not be recognized in QList. Anything wrong with my code and my understanding ?
Thank you all :D
Seems related to: https://snusmumriken.nokia.kunder.linpro.no/browse/QTBUG-20377
EDIT: Correct link: https://bugreports.qt.nokia.com/browse/QTBUG-20377 – Volker
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