Is it possible to invoke C++ function before an item draw by qml?

[Alien]

Hello,

I'm looking for a way to invoke my c++ function before qml item draw on the screen. for example imagine that you have a rectangle in qml which x and y properties calculated in C++ function with one parameter like below:

C++ Header:

#ifndef CTEST_H
#define CTEST_H

#include <QObject>

class CTest : public QObject
{
    Q_OBJECT
    Q_PROPERTY(int C_X READ getX WRITE setX NOTIFY xChanged)
    Q_PROPERTY(int C_Y READ getY WRITE setY NOTIFY yChanged)
private:
    int x;
    int y;
public:
    explicit CTest(QObject *parent = nullptr);

    int getX() const;
    void setX(int value);

    int getY() const;
    void setY(int value);

signals:
       void xChanged();
       void yChanged();
public slots:
    void someCalculation(int data);
};

#endif // CTEST_H

C++ Source:

#include "ctest.h"
#include <QtMath>

int CTest::getX() const
{
    return x;
}

void CTest::setX(int value)
{
    if(x == value)
        return;
    x=value;
    emit xChanged();
}

int CTest::getY() const
{
    return y;
}

void CTest::setY(int value)
{
    if(y == value)
        return;
    y=value;
    emit yChanged();
}

void CTest::someCalculation(int data)
{
    int temp = static_cast<int>(pow(data,3));
    setX(temp/10);
    setY(temp/5);
}

CTest::CTest(QObject *parent) : QObject(parent)
{

}

Qml doc:

import QtQuick 2.12
import QtQuick.Window 2.12
import io.test 1.0
Window
{
    visible: true
    width: 640
    height: 480
    title: qsTr("Hello World")

    CTest
    {
        id:ctestID
    }

    Rectangle
    {
        id:rectID
        color: "purple"
        //Where should I invoke CTest.someCalculation(7) 
        x:ctestID.C_X
        y:ctestID.C_Y
        width: 100
        height: 100
    }
}

It seems that C++ function can invoke on qml event hadler like onClicked or something similar but here I don't have any event handler (Also Component and onCompleted is not adequate to this example because I need calculation before drwing the rectangle).

Best Regards,
Alien

[rrlopez]

You can do it on the constructor of your class, if you want to have a value fixed (like the 7 on your example). This way, the calculation is done when the ctestID class is created and hence, the item will have proper dimensions.
Edit: If you don't want to do it on the constructor, you can always implement an activate public function to be called whenever you want.

[Alien]

Dear @rrlopez Ok I see thanks