【QT】QGraphicsItem 类 mouseReleaseEvent 事件与 mouseMoveEvent 事件无响应的解决方案

1. 问题描述

QGraphicsItem 类可以响应鼠标按下事件，鼠标释放事件与鼠标移动事件的响应只到达了 GraphicsView。

2. 问题分析

默认QGraphicsItem 类只响应鼠标按下事件，

具体可参见[virtual protected] void QGraphicsItem::mousePressEvent(QGraphicsSceneMouseEvent *event)的说明：

If you do reimplement this function, event will by default be accepted (see QEvent::accept()), and this item is then the mouse grabber. This allows the item to receive future move, release and doubleclick events. If you call QEvent::ignore() on event, this item will lose the mouse grab, and event will propagate to any topmost item beneath. No further mouse events will be delivered to this item unless a new mouse press event is received.

3. 解决方案

1. 在鼠标按下事件的重载函数mousePressEvent(QGraphicsSceneMouseEvent *event)中调用QEvent::accept()，即加入语句：

event->accept();

2. 在QGraphicsItem的构造函数中加入语句(没找到依据)：

setFlag(QGraphicsItem::ItemIsSelectable);