Newbie - shared object between C++ and QML

SteMMo

Hi all,
I'm trying to share a C++ object with QML code.
I defined a IoBoard class and instantiated.
Then shared it with QML cause I need to recall some functions.
In my debug prints I see that the engine created two objects !

IoBoard ioboard;
ioboard.setType((IoBoard::CONNECTION_SERIAL));

qmlRegisterType<IoBoard>("com.my.locker", 1, 0, "IoBoard");

How can i have a single object?

Thanks!

KH-219Design

Do you have a call anywhere in your application to "QQmlContext::setContextProperty"?

https://doc.qt.io/qt-5/qqmlcontext.html#setContextProperty

If not, then that could be the one piece you are missing.

I have a template project (that is built and tested regularly in GitHub CI to prove its viability) that contains an end-to-end working example:

Here are the calls to setContextProperty: https://github.com/219-design/qt-qml-project-template-with-ci/search?q=setcontextproperty&unscoped_q=setcontextproperty
Here is where the QML code then uses customLoggingCategories (one of the C++ objects exported to QML): https://github.com/219-design/qt-qml-project-template-with-ci/blob/c61bbd4fbfb4aeb4f9c96b5baedeb935e3064ae9/src/lib_app/qml/LogTags.qml
Here is where the QML code uses resourceHelper (another object exported to QML): https://github.com/219-design/qt-qml-project-template-with-ci/blob/79e71e40be0874dd9fcc229b9685a4371697d3cf/src/lib_app/qml/main.qml

You can also download a source distribution of the Qt5 sources and look at their examples, which are often excellent.

You might not be particularly interested in Video Effects, but this official Qt example uses setContextProperty and exposes several C++ objects to QML: https://doc.qt.io/qt-5/qtmultimedia-multimedia-video-qmlvideofx-example.html

So does this one: https://doc.qt.io/qt-5.9/qtquickcontrols2-gallery-example.html

Code:

SGaist

Hi,

How are you using it in your QML code ?

SteMMo

In this way:

IoBoard {
    id: ioBoard
}

{...
  onClicked:  ioBoard.openLock(2)
...}

KH-219Design

Do you have a call anywhere in your application to "QQmlContext::setContextProperty"?

https://doc.qt.io/qt-5/qqmlcontext.html#setContextProperty

If not, then that could be the one piece you are missing.

I have a template project (that is built and tested regularly in GitHub CI to prove its viability) that contains an end-to-end working example:

Here are the calls to setContextProperty: https://github.com/219-design/qt-qml-project-template-with-ci/search?q=setcontextproperty&unscoped_q=setcontextproperty
Here is where the QML code then uses customLoggingCategories (one of the C++ objects exported to QML): https://github.com/219-design/qt-qml-project-template-with-ci/blob/c61bbd4fbfb4aeb4f9c96b5baedeb935e3064ae9/src/lib_app/qml/LogTags.qml
Here is where the QML code uses resourceHelper (another object exported to QML): https://github.com/219-design/qt-qml-project-template-with-ci/blob/79e71e40be0874dd9fcc229b9685a4371697d3cf/src/lib_app/qml/main.qml

You can also download a source distribution of the Qt5 sources and look at their examples, which are often excellent.

You might not be particularly interested in Video Effects, but this official Qt example uses setContextProperty and exposes several C++ objects to QML: https://doc.qt.io/qt-5/qtmultimedia-multimedia-video-qmlvideofx-example.html

So does this one: https://doc.qt.io/qt-5.9/qtquickcontrols2-gallery-example.html

Code:

SGaist

@SteMMo said in Newbie - shared object between C++ and QML:

In this way:

IoBoard {
    id: ioBoard
}

{...
  onClicked:  ioBoard.openLock(2)
...}

And here you are: you instanciate it once in your main function and you create another one here.

Either remove the one from you main or remove this one and use a context property as suggested by @KH-219Design.

Shazter

Is very well described here: https://qml.guide/singletons/ and here https://doc.qt.io/qt-5/qqmlengine.html#qmlRegisterSingletonType. Is good to know which Qt Version you are using, because registration has been changed in Qt 5.15.0. Unfortunately I cannot find the drawing in the Qt Documentation, where was recommended which solution you should use qmlRegisterType / setcontextProperrty etc. in some use cases.