Calling a variable from C++ in QML

[mamoud]

I have a header file, and i defined the class, the signals as follows:
Mysystem.h

class Mysystem : public SystemCan
{
Q_OBJECT
Q_PROPERTY(QString pNumber MEMBER m_pNumber NOTIFY pNumberChanged)

public:
explicit Mysystem(quint8 x, quint64 y, QObject *parent = 0);

signals:
    void pNumberChanged();

private:
    QString m_pNumber;
};

and functions are set in .h file and .cpp file as well.

When I tried to get the value of pNumber I did this also in my main c++ file:

main.cpp

Mysystem mysystem;  // but here i got an error because i should give 2 arguments, but i dont know how to do this


viewer.rootContext()->setContextProperty("thepnumber", &mysystem);

In my QML file i just want to read this variable like this:

console.log (thepnumber.pNumber)

I wonder how to solve this argument problem, or if there is another way to solve this, i would appreciate that

[VRonin]

@mamoud said in Calling a variable from C++ in QML:

Q_PROPERTY(QString pNumber MEMBER m_pNumber NOTIFY pNumberChanged)

This will not do what you think it will do. Q_PROPERTY doesn't do any magic on the NOTIFY argument, it will not get automatically triggered if you set the property, you should split it into READ and WRITE and implement emitting the signal in the WRITE method

@mamoud said in Calling a variable from C++ in QML:

but here i got an error because i should give 2 arguments, but i dont know how to do this

Just pass them to the constructor: Mysystem mysystem(0,0);

[KroMignon]

@mamoud To create the instance:

Mysystem mysystem(0,1); // x= 0, y =1

viewer.rootContext()->setContextProperty("thepnumber", &mysystem);

[J.Hilk]

@mamoud

https://www.learncpp.com/cpp-tutorial/85-constructors/

[mamoud]

@VRonin

thanks
do you mean like this:

Q_PROPERTY(QString pNumber READ pNumber_read WRITE pNumber_write NOTIFY pNumber_changed)

and adding this as well:

public: QVariant pNumber_read() const;
public slots: void pNumber_write(QString value);  // i am not sure about this
signals: void pNumber_changed();
private: QString m_pNumber;

[VRonin]

@mamoud

Q_PROPERTY(QString pNumber READ pNumber WRITE setPNumber NOTIFY pNumberChanged)

public:
const QString& pNumber() const {return m_pNumber;}
void setPNumber(const QString& val){
if(val==m_pNumber) return;
m_pNumber=val;
pNumberChanged(m_pNumber);
}
signals:
void pNumberChanged(const QString& pN);
private:
QString m_pNumber;

[J.Hilk]

Pro (QtCreator) tip:

write

Q_PROPERTY(QString pNumber READ pNumber WRITE setPNumber NOTIFY pNumberChanged)

right click on Q_PROPERTY
select Refactor
click on Generate Missing Q_Property Members

saves a lot of time

[mamoud]

Great thank you,

but i still have this problem, that i cannot read the value in QML when i write this:

console.log (thepnumber.pNumber)

it shows nothing

[J.Hilk]

@mamoud said in Calling a variable from C++ in QML:

Great thank you,

but i still have this problem, that i cannot read the value in QML when i write this:

console.log (thepnumber.pNumber)

it shows nothing

more context please, where exactly is this called?

---

//General solution
Connections{
target: thepnumber

onpNumberChanged: console.log(thepnumber.pNumber)
}

[mamoud]

I have a QML called PageInfo.qml

Rectangle {
    id: infopage

    function pageEnter() {
        console.log("The part is: " + thepnumber.pNumber)
    }

[J.Hilk]

@mamoud said in Calling a variable from C++ in QML:

pageEnter

and when you call pageEnter?

[mamoud]

Once i enter this page it will be called

[J.Hilk]

@mamoud
if you write

function pageEnter() {
console.log("Enter Function to get Number")
console.log("The part is: " + thepnumber.pNumber)
}

do you see any console logs?

[VRonin]

Did you set a value to m_pNumber? if you never assign anything to it than it's totally natural that the log shows an empty string.

[mamoud]

Yeah i see
Enter Function to get Number
and also i see
The part is:
but no value is shown for thepnumber.pNumber

[mamoud]

@VRonin

No but this m_pNumber should get values from a CAN signal (canalyzer) which is set in .cpp file