Error when using QMetaObject::invokeMethod( /*param*/, /*param*/, Qt::QueuedConnection, Q_ARG(QString, /*param*/))

[JJLim]

hi, may I know how to use QString as the parameter in the QMetaObject::invokerMethod()?

[Christian Ehrlicher]

What did you try so far? Why do you need this at all?

[JJLim]

I wanna update the qml value dynamically actually. I had tried using QTimer and now I find out this method and wonder how to involve QString by using this method

[eyllanesc]

@JJLim

1. If you want to modify any element of QML from C++ then better create a QObject, export it to QML and make a binding.

2. Connections with QML elements must be of type Qt::DirectConnection so if you want to invoke a method and pass it a string you must use QMetaObject::invokeMethod(obj, "foo", Qt::DirectConnection, Q_ARG(QString, "bar"));

[jeremy_k]

@eyllanesc said in Error when using QMetaObject::invokeMethod( /*param*/, /*param*/, Qt::QueuedConnection, Q_ARG(QString, /*param*/)):

2. Connections with QML elements must be of type Qt::DirectConnection so if you want to invoke a method and pass it a string you must use QMetaObject::invokeMethod(obj, "foo", Qt::DirectConnection, Q_ARG(QString, "bar"));

Can you reference anything to support the direct connection requirement? The commit history going back to 4.7 development uses a direct connection for QML property bindings, but that could be for efficiency.

A test program shows that auto, queued, and blocking queued connections all work with 5.15.5, given the relevant threading restrictions are followed.

[JJLim]

Thanks for your advice. I found that I can prevent this error by directly converting it to QString in Q_ARG. The demo is like below

QMetaObject::invokeMethod( /param/, /param/, Qt::QueuedConnection, Q_ARG(QString, QString::fromStdString("test")));