Sync QQuickPaintedItem and QML painting

-__Mario__-

Hi everyone,

i want to sync QQuickPaintedItem and QML painting. To do this, I've QQuickPaintedItem which is drawing a rectangle at a certain position, signaling this position to QML and drawing also there a rectangle.

QQuickPaintedItem

#include "painter.h"

#include <QPainter>
#include <QTimer>

Painter::Painter(QQuickPaintedItem* parent) :
    pi_(0.0)
{
    QObject::connect(&timer_, &QTimer::timeout, this, &Painter::onTimer);
    timer_.start(30);
}


void Painter::onTimer()
{
    pnt_.setX(300 + sin(pi_) * 150.0);
    pnt_.setY(150);

    pi_ += 0.05;

    update();
}

QPointF Painter::pntPosition() const
{
    return pnt_;
}

void Painter::paint(QPainter* painter)
{
    painter->setBrush(QBrush(QColor(255, 0, 0)));
    QRectF rect(pnt_.x() - 2, pnt_.y() - 2, 4, 4);
    painter->drawRect(rect);

    emit pntPositionChanged();
}

QML

import QtQuick
import TestPainter 1.0

Window {
    width: 640
    height: 480
    visible: true
    title: qsTr("Hello World")

    Painter {
        id: painter
        width: parent.width
        height: parent.height

        Rectangle {
             x: painter.pntPosition.x - 2
             y: painter.pntPosition.y - 2 - 20
             width: 4
             height: 4
             color: "#000000"
        }
    }
}

The result of this code is, that the QML rectangle is lagging:

How can I sync the draw correctly?

Thanks a lot!
Best Mario

jeremy_k

Based on a quick read of the documentation, I don't think that this strategy will work.

https://doc.qt.io/qt-6/qquickpainteditem.html#paint

Note: The QML Scene Graph uses two separate threads, the main thread does things such as processing events or updating animations while a second thread does the actual issuing of graphics resource updates and the recording of draw calls. As a consequence, paint() is not called from the main GUI thread but from the GL enabled renderer thread. At the moment paint() is called, the GUI thread is blocked and this is therefore thread-safe.

Warning: Extreme caution must be used when creating QObjects, emitting signals, starting timers and similar inside this function as these will have affinity to the rendering thread.