use of keypressevent for animating a item in qt

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keypress

QT

animation

in this program I have built a QPropertyanimation .and add to it my item and pos () property. I override KeyPressEvent.and with using of keys consist of "j,f,z" item go forward ,go back and jump. According gravity when item jump should fall.for this purpose i call down function .but item just once jump and don't fall .and I also have another problem .when the first press "j" and "f" (forward,back) item animate desirably but for next times item go forward and go back all of scene. I mean It should animated for example 40 pixel but It animated 800 pixel.

<small></small>class MainWindow : public QMainWindow
{
    Q_OBJECT
    
public:
    explicit MainWindow(QWidget *parent = 0);
    QPoint start;
    QPoint end;
    ~MainWindow();
private:
    QGraphicsView* view;
    QGraphicsScene* scene;
    void keyPressEvent(QKeyEvent* k);
    MyQgraphicsObject* m;
    QPropertyAnimation* pr;
    QElapsedTimer* timer;
    int f;
    int u;
    int b;
    void forward();
    void up();
    void back();
    void down();
};
MainWindow::MainWindow(QWidget *parent) :
    QMainWindow(parent)
{
    view=new QGraphicsView;
    scene=new QGraphicsScene;
    m=new MyQgraphicsObject;
    pr=new QPropertyAnimation(m,"pos");
    view->setScene(scene);
    view->resize(800,800);
    view->setFixedSize(800,800);
    setCentralWidget(view);
    scene->addItem(m);
    start= QPoint(0,0);
    f=30;
    u=-30;
    b=-30;
}
void MainWindow::keyPressEvent(QKeyEvent *k)
{
    switch (k->key())
    {
    case Qt::Key_J:{

        forward();
        break;
    }
    case Qt::Key_Z:
    {
        up();
        down();
        break;
    }
    case Qt::Key_F:
    {
        back();
         break;
    }
    default:
        break;
    }
}

void MainWindow::forward()
{
    end.setX(f);
    pr->setEndValue(end);
    pr->setDuration(1000);
    pr->setEasingCurve(QEasingCurve::Linear);
    pr->start();
    f+=40;
}
void MainWindow::up()
{
    end.setY(u);
    pr->setEndValue(end);
    pr->setDuration(1000);
    pr->setEasingCurve(QEasingCurve::Linear);
    pr->start();
    u-=30;
    pr->pause();
}
void MainWindow::back()
{
    end.setX(b);
    pr->setEndValue(end);
    pr->setDuration(1000);
    pr->setEasingCurve(QEasingCurve::Linear);
    pr->start();
    b-=40;
}
void MainWindow::down()
{
    u+=30;
    end.setY(u);
    pr->setEndValue(end);
    pr->setDuration(1000);
    pr->setEasingCurve(QEasingCurve::Linear);
    pr->start();
}