To set limit to a draggable line segment

[viniltc]

Hi all,

I'm trying to implement a line segment which can be draggable by mouse between two points (shown below where green and red lines must be draggable between black dash lines).

I can now drag the lines by mouse (which I followed a sample program given by @mrjj , here).

As a next step I want to set a limit to this draggable area, which I'm having issues to implement with. I really appreciate if someone can help me to do this. I now explain the situation in a bit detail:

public:
    MainWindow(QWidget *parent = nullptr);
    ~MainWindow();
     void paintEvent(QPaintEvent *e);

public slots:

      void changeP1value(int);


private:
    Ui::MainWindow *ui;
     bool dragging = false; 
     QPoint p1 = QPoint(400,100);
     QPoint p2 = QPoint(450,100);
     QPoint p3 = QPoint(450,100);
     QPoint p4 = QPoint(500,100);

     QPoint *CurPoint1=nullptr;
     QPoint *CurPoint2=nullptr;

protected:
     virtual void mousePressEvent(QMouseEvent *event) override;
     virtual void mouseReleaseEvent(QMouseEvent *event) override;
     virtual void mouseMoveEvent(QMouseEvent *event) override;
};
#endif // MAINWINDOW_H

and in .cpp

// small helper to give us the distance
int distance(QPoint x1, QPoint x2)
{
    return abs(x2.y() - x1.y());
}

// the Paint event to draw lines
void MainWindow::paintEvent(QPaintEvent *e)
{
    QPainter painter1(this);

    QPen pointpen(Qt::black);
    pointpen.setWidth(5);
    QPen linepen1(Qt::red);
    linepen1.setWidth(2);
    QPen linepen2(Qt::green);
    linepen2.setWidth(2);

    painter1.setPen(linepen1);
    painter1.drawLine(p1,p2);
    painter1.setPen(linepen2);
    painter1.drawLine(p3,p4);

}

// when user clicks
void MainWindow::mousePressEvent(QMouseEvent *event)
{
    QPoint mp = event->pos(); // where is mouse
// test if we hit the line. give user 20 pixels slack as its hard to hit one pixel
    if (distance ( mp, p1) < 20 && ( mp.x() > p1.x() && mp.x() < p2.x() ) ) {
        dragging = true; // flag we are dragging
        
        
        CurPoint1 = &p1;
        CurPoint2 = &p2;
    }
    else if (distance ( mp, p3) < 20 && ( mp.x() > p3.x() && mp.x() < p4.x() ) ) {
        dragging = true;
        CurPoint1 = &p3;
        CurPoint2 = &p4;
    }
}

void MainWindow::mouseReleaseEvent(QMouseEvent *event)
{
    dragging = false; // if user release mouse we are not draggign anymore
}

// then when mouse move
void MainWindow::mouseMoveEvent(QMouseEvent *event)
{
 // If we are dragging, call your normal slider changed function to update your points.

    if (dragging)
    {
      if(CurPoint1->y()>50 && CurPoint1->y()<150) 
        {
        changeP1value(event->y());
        }
      else
      update();
    }

}

void MainWindow::changeP1value(int value)
{

    CurPoint1->setY(value);
    CurPoint2->setY(value);
    update();

}

In the mouseMoveEvent, I've condition to check if I'm dragging a line and if the hight is between 50 and 150:

 if (dragging)
    {
      if(CurPoint1->y()>50 && CurPoint1->y()<150) //----> it get stucked and cannot be dragged down again 
        {
        changeP1value(event->y());
        }
      else
      update();
    }

In this case I can drag , the line stops at the set limits but I cant drag down again. Can you spot an issue?

Thanks in advance

[viniltc]

That problem got solved finally!
The correct way is to check the new y value, not the current value:

void MainWindow::mouseMoveEvent(QMouseEvent *event)
{
  if (dragging)
  {
    if(event->y() > 50 && event->y() < 150)  //--> check new y value,
    {
      changeP1value(event->y());
    }
    else
      update();
  }
}