Save QPainter after called update - qt

Question

I want to draw straight lines on qml , but every time I draw a line, the previous disappears, I wonder if it has any thing to do with the update method, any ways to solve this problem.
main.qml
MouseArea{
id:fidpoint
anchors.fill: parent
onPressed: {
switch(addstate.currentText){
case 'Track':
map.setTstart(mouseX,mouseY);
draw_line.setColor("black");
draw_line.setStart(Qt.point(mouseX,mouseY));
draw_line.setEnd(Qt.point(mouseX,mouseY));
draw_line.update();
break;
}
}
onReleased: {
switch(addstate.currentText){
case 'Track':
map.addTrack(mouseX,mouseY);
draw_line.setColor("black");
draw_line.setEnd(Qt.point(mouseX,mouseY));
draw_line.update();
break;
}
}
onPositionChanged: {
switch(addstate.currentText){
case 'Track':
draw_line.setColor("black");
draw_line.setEnd(Qt.point(mouseX,mouseY));
draw_line.update();
break;
}
}
}
draw_line is the Object's id that I registered to qml from main.cpp
paint.cpp
void Paint::paint(QPainter *painter)
{
QPen pen(m_color, 2);
painter->setPen(pen);
painter->setRenderHint(QPainter::Antialiasing,true);
painter->drawLine(startNode,endNode);
}
paint is the class that inherits from QQuickPaintedItem

Answer 1

A possible option would be to create an image and save the changes there, but this would solve your problem in the short term. A better option is that draw_line draw only one line, then add it as a child of the item you want the canvas to be.
lineitem.h
#ifndef LINEITEM_H
#define LINEITEM_H
#include <QQuickPaintedItem>
class LineItem: public QQuickPaintedItem
{
Q_OBJECT
Q_PROPERTY(QPoint startPos READ startPos WRITE setStartPos NOTIFY startPosChanged)
Q_PROPERTY(QPoint endPos READ endPos WRITE setEndPos NOTIFY endPosChanged)
Q_PROPERTY(QColor lineColor READ lineColor WRITE setLineColor NOTIFY lineColorChanged)
public:
using QQuickPaintedItem::QQuickPaintedItem;
void paint(QPainter *painter);
QPoint startPos() const;
void setStartPos(const QPoint &startPos);
QPoint endPos() const;
void setEndPos(const QPoint &endPos);
QColor lineColor() const;
void setLineColor(const QColor &lineColor);
signals:
void startPosChanged();
void endPosChanged();
void lineColorChanged();
private:
QPoint m_startPos;
QPoint m_endPos;
QColor m_lineColor;
};
#endif // LINEITEM_H
lineitem.cpp
#include "lineitem.h"
#include <QPainter>
void LineItem::paint(QPainter *painter)
{
painter->setRenderHint(QPainter::Antialiasing, true);
QPen pen(m_lineColor, 2);
painter->setPen(pen);
painter->drawLine(m_startPos, m_endPos);
}
QPoint LineItem::startPos() const
{
return m_startPos;
}
void LineItem::setStartPos(const QPoint &startPos)
{
if(m_startPos == startPos)
return;
m_startPos = startPos;
emit startPosChanged();
update();
}
QPoint LineItem::endPos() const
{
return m_endPos;
}
void LineItem::setEndPos(const QPoint &endPos)
{
if(m_endPos == endPos)
return;
m_endPos = endPos;
emit endPosChanged();
update();
}
QColor LineItem::lineColor() const
{
return m_lineColor;
}
void LineItem::setLineColor(const QColor &lineColor)
{
if(m_lineColor == lineColor)
return;
m_lineColor = lineColor;
emit lineColorChanged();
update();
}
main.qml
import QtQuick 2.9
import QtQuick.Window 2.2
import com.eyllanesc.org 1.0
Window {
id: win
visible: true
width: 640
height: 480
title: qsTr("Hello World")
property LineItem currentItem: null
Rectangle{
id: canvas
anchors.fill: parent
MouseArea{
anchors.fill: parent
onPressed: {
currentItem = create_lineitem(canvas)
currentItem.lineColor = "green"
currentItem.anchors.fill = canvas
currentItem.startPos = Qt.point(mouseX,mouseY)
currentItem.endPos = Qt.point(mouseX,mouseY)
}
onReleased: currentItem.endPos = Qt.point(mouseX,mouseY)
onPositionChanged: currentItem.endPos = Qt.point(mouseX,mouseY)
}
}
function create_lineitem(parentItem, color) {
return Qt.createQmlObject('import com.eyllanesc.org 1.0; LineItem{}',
parentItem);
}
}
main.cpp
#include "lineitem.h"
#include <QGuiApplication>
#include <QQmlApplicationEngine>
int main(int argc, char *argv[])
{
QCoreApplication::setAttribute(Qt::AA_EnableHighDpiScaling);
QGuiApplication app(argc, argv);
qmlRegisterType<LineItem>("com.eyllanesc.org", 1, 0, "LineItem");
QQmlApplicationEngine engine;
engine.load(QUrl(QStringLiteral("qrc:/main.qml")));
if (engine.rootObjects().isEmpty())
return -1;
return app.exec();
}
The complete example can be found in the following link.