Solution for Stack Overflow Question 43147889 "Bowl Draining" effect in Qt

mainwindow.cpp

#include "mainwindow.h"
#include "ui_mainwindow.h"

#include <QAbstractScrollArea>
#include <Qt>
#include <QtMath>
#include <QTimer>
#include <QLineF>
#include <QDebug>


MainWindow::MainWindow(QWidget *parent) :
    QMainWindow(parent),
    ui(new Ui::MainWindow)
{
        ui->setupUi(this);
        scene = new QGraphicsScene(this);
        view = ui->graphicsView;
        view->setScene(scene);
        view->setSceneRect(0,0,512,512);
        viewSize = view->width() /2;
        sceneCenter = QPointF(viewSize,viewSize);
        draw_Radius = 200;
        connect(ui->dial, &QDial::valueChanged, this, &MainWindow::slt_updateAngleFromDial);

        //add green indicator line
        green_line = QLineF(QPointF(draw_Radius, 0), QPointF(draw_Radius + 20, 0));
        QPen greenPen(Qt::green, Qt::SolidLine);
        greenPen.setWidth(3);
        green_needle = scene->addLine(green_line, greenPen);
        green_needle->setTransformOriginPoint(QPointF(0,0));

        //draw static outer circle
        int mSize = draw_Radius *2;
        QRectF rimCircle(0, 0, mSize, mSize);
        // Center about 0,0
        rimCircle.moveCenter(QPointF(0,0));
        mCircle = new QGraphicsEllipseItem(rimCircle);
        QBrush rimTip(Qt::darkCyan, Qt::NoBrush);
        QPen rimPen(Qt::darkCyan, Qt::SolidLine);
        mCircle->setBrush(rimTip);
        mCircle->setPen(rimPen);
        scene->addItem(mCircle);

        //draw static inner circle
        int cSize = 3;
        QRectF circ(0, 0, cSize,cSize);
        circ.moveCenter(QPointF(0,0));
        cCircle = new QGraphicsEllipseItem(circ);
        QBrush rimTip2(Qt::black, Qt::SolidPattern);
        QPen rimPen2(Qt::black, Qt::SolidLine);
        cCircle->setBrush(rimTip2);
        cCircle->setPen(rimPen2);
        scene->addItem(cCircle);

        // Polyline path
        m_pathItem = new QGraphicsPathItem();
        scene->addItem(m_pathItem);

        view->centerOn(0,0);

        startDrainTheBowl_Timer();
    }

    MainWindow::~MainWindow(){delete ui;}

void MainWindow::slt_updateAngleFromDial(int angle){
    draw_angle = angle;
    green_needle->setRotation(draw_angle);
}


void MainWindow::startDrainTheBowl_Timer(){
    QTimer* drainTimer = new QTimer(this);
    connect(drainTimer, SIGNAL(timeout()), this, SLOT(slt_drainTheBowl()));
    drainTimer->start(100);
}

void MainWindow::slt_drainTheBowl()
{
    // Move the points towards center
    QMutableListIterator<QPointF> i(m_points);
    while(i.hasNext())
    {
        i.next();
        QLineF line(QPointF(0,0), i.value());
        // We move a point by decreasing the length from the origin to the point by 1
        qreal length = line.length();
        length -=1;
        line.setLength(length);
        // If the point is now at (or past) the origin, remove from the list
        if(length<=0)
        {
            i.remove();
        }
        else
        {
            // Update the point in the list
            i.setValue(line.p2());
        }
    }


    // Add a new point to the list based on the current angle
    QPointF newPoint;
    newPoint.setY( qSin(qDegreesToRadians((double)draw_angle)) * 200 );
    newPoint.setX( qCos(qDegreesToRadians((double)draw_angle)) * 200 );


    // Set the points into the path item
    QPainterPath path;
    path.moveTo(newPoint);
    for(int i=0; i<m_points.count(); i++)
        path.lineTo(m_points[i]);

    m_points << newPoint;

    m_pathItem->setPath(path);
}