How to retrieve the QPainter object in QML Canvas object - c++

Question

I have a QML Canvas, on which I'm drawing in C++ by overriding the paint(QPainter *painter) method and using a bunch of statements that use that painter object.
Stuff like...
void myGraphDisplay::paint (QPainter* painter) {
QPainterPath path;
path.MoveTo(0, 0);
path.LineTo(100, 100);
painter->strokePath(path, painter->pen());
etc.
Now some time later, another function wants to draw on the canvas, but the painter object is no longer available. I have tried saving it as a private member of the myGraphDisplay class but my application crashes if I try to access it again in the later function.
void myGraphDisplay::updateGraph () {
QPainterPath path;
path.MoveTo(100, 0);
path.LineTo(0, 100);
painter->strokePath(path, painter->pen()); // where do I get "painter" here?
I also tried
QPainter painter(this);
as shown on the QT reference pages but that gives me an error...
no matching function for call to QPainter::QPainter(myGraphDisplay*)
How do I get the current QPainter object? If it's any help, updateGraph() is a Q_INVOKABLE called from the same QML.

Answer 1

The rules state that the process of painting a QQuickItem occurs in the paint method, not in another method. The generic solution is:
Save the painting information in some attribute of the class.
Invoke the paint method
Implement the logic in the paint method.
*.h
private:
QPainterPath m_path;
*.cpp
myGraphDisplay::myGraphDisplay(QQuickItem *parent): QQuickItem(parent){
// default path
m_path.MoveTo(0, 0);
m_path.LineTo(100, 100);
}
void myGraphDisplay::updateGraph(){
QPainterPath path;
path.MoveTo(100, 0);
path.LineTo(0, 100);
m_path = path;
update();
}
void myGraphDisplay::paint (QPainter* painter) {
painter->strokePath(m_path, painter->pen());
}