QML property updating and qmlengine getting issue.

[Jimmy Lee]

I am trying to update my qml property by "engine2->rootContext()->setContextProperty("row_data_contxt",m2);" in a sub-thread, which means not in main.cpp, and I used

QQmlContext *currentContext = QQmlEngine::contextForObject(this);
QQmlEngine *engine2 = currentContext->engine();

to get the engine which main.cpp file launched, but it failed as returning a null pointer.

[raven-worx]

@Jimmy-Lee

QQmlEngine::contextForObject(this)

what is the this pointer?!

[Jimmy Lee]

Thanks for your reply, I know I should replace this as the pointer of my thread class, but what should I insert here?

[raven-worx]

@Jimmy-Lee
you actually should insert a pointer to an item which "lives" in a QML engine (like an item).
Thats the purpose of this method after all.

You may also take a look at qmlEngine() method.

[Jimmy Lee]

I have tried declared a QObject* root in my class and set root near the QQmlApplicationEngine engine declared,

MyClass root = engine.rootObjects().first();//in main.cpp

then use this rootas pointer to

 QQmlContext *currentContext = QQmlEngine::contextForObject(root); 
//upline:"read access violation exception"
 QQmlEngine *engine2 = currentContext->engine();     

What 's the right way to pass the enginefrom main.cpp to other class?
(I've read the qmlengine() but what I use is QQmlApplicationEngine)

[dheerendra]

@Jimmy-Lee said in QML property updating and qmlengine getting issue.:

QQmlApplicationEngine

1. You pass the address of QQmlApplicationEngine ? It is inherited from QQmlEngine.
2. Define the method in your class like setMyEngine(QQmlApplicationEngine *eng);
3. Call the method in main.cpp my object.setMyEngine(&engine).

[Jimmy Lee]

Try your method but a Debug Error occurs:

Debug Error! assert method.methodType()==QMetaMethod::Signal

As my code shows following:
main.h

class MyThread : public QObject
{
    Q_OBJECT
public:
    MyThread(QObject* parent = nullptr);
    ~MyThread();
     QQmlApplicationEngine* sub_engine;
     void setEngineAddress(QQmlApplicationEngine* en)
     {
             sub_engine = en;
     }
};

main.cpp

int main()
{
    MyThread *w1;
    QQmlApplicationEngine engine;
    w1->setEngineAddress(&engine);
    w1->start();
}

void MyThread::run()
{
     sub_engine->rootContext()->setContextProperty("row_data_contxt",m2);

}

Is there something wrong? Thanks a lot!

[Jimmy Lee]

@Jimmy-Lee
Has added this->

void MyThread::run()
{
for()
{
     this->sub_engine->rootContext()->setContextProperty("row_data_contxt",m2);

}
}

But the Debug Error still occur once the run()

[Jimmy Lee]

It seems that setContextProperty can`t be execute from another thread in loop mode?

[Jimmy Lee]

In my latest code

void MyThread::run()
{
for()
{
     this->sub_engine->rootContext()->setContextProperty("row_data_contxt",m2);
     Sleep(250);
}
}

Seems each times run at Sleep(25), it will occurs the Virtual C++ Debug error:

What's the due to?

[dheerendra]

@Jimmy-Lee said in QML property updating and qmlengine getting issue.:

MyThread *w1;

You are not creating the QApplication object itself in main.cpp ? Did you allocate the memory for W1 ? What is m2 object ? Did you allocate memory ?

Please do the default project in Qt Creator and try with modifications. Don't remove standard code like Application object, exec() etc.

[Jimmy Lee]

@dheerendra
Sorry I've write what you mentioned in my codes, I didn't write them down here for convenient, my fault.

[Jimmy Lee]

For now, I've used Q_Property(QVariantMap x READ x) method to transmit variable to qml file, and engine.rootcontext()->setcontextproperty("",w1) to send my class instantiation w1 to qml file, which satisfy my requirement.

inspired by: https://forum.qt.io/topic/26464/solved-q_property-qstring-linking-qml-and-c-giving-undefined-error/2

Thanks!