自定义QGraphicsItem的继承顺序

注意:一定要将public QObject放在public QGraphicsItem的前面,并且在该类的定义中添加Q_OBJECT宏.

编译时出现警告 Warning: Class Node implements the interface QGraphicsItem but does not list it in Q_INTERFACES. qobject_cast to QGraphicsItem will not work!

在类的声明（Q_OBJECT下面）中添加:Q_INTERFACES(QGraphicsItem)可解决该问题.