Correct syntax for setting C++ property in QML

mzimmers

Hi all -

I've implemented a display with a MouseArea that, when clicked, change the value of a property defined in a C++ class.

class Opmode : public QObject
{
    Q_OBJECT
    Q_PROPERTY(ModeEnum mode READ mode WRITE setMode NOTIFY modeChanged);
public:
    explicit Opmode(QObject *parent = nullptr);
    ModeEnum mode() { return m_opmode; }
    void setMode(const ModeEnum &m);
    ...

In my QML:

    MouseArea {
        anchors.fill: parent
        onClicked: {
            control.checked = true
            opMode.mode = mode <== this works
//            opMode.setMode(mode) <== this doesn't
    ...

I guess I've never done this before. Is there some reason that I can't use the setMode function in the class? And, is my assignment OK? It seems to work, but I'd like to know for sure.

Thanks...

bjdm

The QML engine uses the QProperty macro to register the cpp method setMode as the symbol to use when setting the property -- it doesn't expose the method itself. opMode.mode = mode is the correct syntax. More here.

J.Hilk

@mzimmers you aren't meant to call the getter and setter of a Q_PROPERTY from the QML side. Your assignment is ok, it's how it's supposed to be.

For completeness, if you make your setMode a (public)slot or mark it explicitly with the Q_INVOKAVBLE makro, then you can also call the setter from QML

mzimmers

@J-Hilk said in Correct syntax for setting C++ property in QML:

For completeness, if you make your setMode a (public)slot or mark it explicitly with the Q_INVOKAVBLE makro, then you can also call the setter from QML

Well, setMode is in my Q_INVOKABLE macro. It's not declared as a slot, but I thought there was nothing special about slot functions?

EDIT:

I just tried marking setMode() as a slot, and now that line works. Thanks for the help, guys, though I'm not sure I fully understand why it needs to be declared a slot.

JoeCFD

@mzimmers True. Here the setter may be used by other C++ classes. In QML, it is not needed in this case. Also do not define it if it is not used.
I often define variables like the following when the setter is not used.
Q_PROPERTY( bool android MEMBER m_android CONSTANT )
Q_PROPERTY( bool charging MEMBER m_charging NOTIFY chargingChanged )

mzimmers

From a best practices standpoint, is one of these preferred over the other?

opMode.mode = mode
opMode.setMode(mode)

JoeCFD

@mzimmers If you still remember encapsulation of C++, opMode.setMode(mode) is preferred in C++ classes. But in QML, encapsulation has been sort of dropped. Any call from C++ classes use setter. In QML, you can use opMode.mode = mode. However, if this change or opMode.mode is used in some other QML files and you need to send out modeChanged signal, you will need opMode.setMode(mode).

JoeCFD

@mzimmers My previous answer may not be right completely. You can not call opMode.setMode(mode) because mode is not registered as type? Is it an enum? If yes, define it
https://forum.qt.io/topic/109031/passing-enum-into-c-function-like-parameter-from-qml-how/8

opMode.mode = mode /* no notification /
opMode.setMode(mode) / with notification since emit modeChanged is defined in this function */

/* qml supported types */
https://doc.qt.io/qt-6/qtqml-cppintegration-data.html

GrecKo

@mzimmers said in Correct syntax for setting C++ property in QML:

From a best practices standpoint, is one of these preferred over the other?
opMode.mode = mode
opMode.setMode(mode)

opMode.mode = mode is. Prefer to use properties when possible.

Contrary to what JoeCFD is saying, this is more encapsulated than calling setMode. You are not acting on the C++ variable anyway, you are going through the QML engine and it can make use of this for stuff like behaviors for example.

When doing opMode.mode = mode the modeChanged signal WILL be called (provided that your WRITE method is correctly coded).

mzimmers

For the sake of completeness in this topic, I think I've covered all my other bases. I've defined the enums appropriately:

// opmode.h
namespace OpmodeNS {
    Q_NAMESPACE
    typedef enum ModeEnum {
        Standard,
        Saving,
        Boost,
        Service
    } opmodeEnums;
    Q_ENUM_NS(ModeEnum)
}

and registered it accordingly:

// main.cpp
// register the Opmode enums so they can be used in QML.
qmlRegisterUncreatableMetaObject(
    OpmodeNS::staticMetaObject, // meta object created by Q_NAMESPACE macro
    "Opmodes",                // import statement (can be any string)
    1, 0,                           // major and minor version of the import
    "OpModes",                    // name used in QML (does not have to match C++ name)
    "Error: only enums"             // error in case someone tries to create an object
 );

And my WRITE method:

// opmode.cpp
void Opmode::setMode(const ModeEnum &m)
{
    if (m != m_opmode) {
        m_opmode = m;
        emit modeChanged(m_opmode);
    }
}

Thanks to everyone for the input on this; as usual, it's been educational.

JoeCFD

@GrecKo
In C++ the way of mode access in QML like opMode.mode = mode is like public access and is not preferred in C++. That is what I meant.
Contrary to what JoeCFD is saying, this is more encapsulated than calling setMode. You are not acting on the C++ variable anyway, you are going through the QML engine and it can make use of this for stuff like behaviors for example.

Good to know.
When doing opMode.mode = mode the modeChanged signal WILL be called (provided that your WRITE method is correctly coded).