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);
}