Time for action – implementing the ability to move the scene

Next it would be good to move the scene around without the need of using the scroll bars. Let us add the functionality for pressing and holding the left mouse button. First, we add two private members to the view: the m_pressed parameter of type bool and the m_lastMousePos element of type QPoint. Then, we reimplement the mousePressEvent() and mouseReleaseEvent() functions as follows:

void MyView::mousePressEvent(QMouseEvent *event) {
  if (Qt::LeftButton == event->button()) {
    m_pressed = true;
    m_lastMousePos = event->pos();
  }
  QGraphicsView::mousePressEvent(event);
}

void MyView::mouseReleaseEvent(QMouseEvent *event) {
  if (Qt::LeftButton == event->button())
    m_pressed = false;
  QGraphicsView::mouseReleaseEvent(event);
}

What just happened?

Within mousePressEvent(), we check whether the left mouse button was pressed. If it was true, we then set m_pressed to true and save the current mouse position in m_lastMousePos...