How can I make my QDial react to a different mouse event than the one predefined?

Question

I have a subclass of a QDial and I want to change the values by moving my mouse up and down. At the moment, the basic implementation allows you to do a circular movement with the mouse to change the values. But I'd like to change them by moving my mouse either up (let's say that once I have pressed my dial, I'll move my mouse to the top of my screen) or down (to the bottom of the screen).

Here's a picture (beware, MS Paint skills ahead) of what I want to achieve. On the left, it's the basic behavior, and on the right, the behavior that I'd like to have.

I don't see how I could get that with mousePressEvent or mouseMoveEvent . Does anybody have an idea?

Answer 1

Unless I've misunderstood what you want this should be fairly straight forward with something like...

class dial: public QDial {
public:
  dial (QWidget *parent = nullptr)
    : QDial(parent)
    , m_dragging(false)
    , m_scale_factor(1.0f)
    {}
protected:
  virtual void mousePressEvent (QMouseEvent *event) override
    {
      m_mouse_press_point = event->pos();
      m_dragging = true;
      m_base_value = value();
    }
  virtual void mouseReleaseEvent (QMouseEvent *event) override
    {
      m_dragging = false;
    }
  virtual void mouseMoveEvent (QMouseEvent *event) override
    {
      if (m_dragging) {
        int new_value = m_base_value + m_scale_factor * (m_mouse_press_point.y() - event->y());
        setValue(new_value);
      }
    }
private:
  bool   m_dragging;
  QPoint m_mouse_press_point;
  int    m_base_value;
  float  m_scale_factor;
};

You'll probably want to adjust m_scale_factor based on screen resolution or similar.

If you don't want to subclass QDial the same logic could be implemented with a stateful event filter.