qt - 如何扩大 QGraphicsItem 的悬停区域

Question

我有一个带有相当小的点标记的 QGraphicsScene。我想扩大这些标记的区域以使拖动更容易。标记是一个十字，距离原点 +/- 2 像素。我已经重新实现了

QGraphicsItem::contains(const QPointF & point ) const
{
  return QRectF(-10,-10,20,20);
}

和

void hoverEnterEvent(QGraphicsSceneHoverEvent* event)
{
  setPen(QPen(Qt::red));
  update();
}

但标记只有在被光标直接击中时才会变成红色(甚至有点挑剔)。如何扩大“悬停区域”？

Answer 1

如简短评论中所述:通常这些东西是通过边界矩形或形状函数处理的，尝试重载它们。查看形状下 QGraphicsItem 的 qt 帮助(http://doc.qt.io/qt-4.8/qgraphicsitem.html#shape):

Returns the shape of this item as a QPainterPath in local coordinates. The shape is used for many things, including collision detection, hit tests, and for the QGraphicsScene::items() functions.

The default implementation calls boundingRect() to return a simple rectangular shape, but subclasses can reimplement this function to return a more accurate shape for non-rectangular items. For example, a round item may choose to return an elliptic shape for better collision detection. For example:

QPainterPath RoundItem::shape() const { QPainterPath path; path.addEllipse(boundingRect()); return path; } The outline of a shape can vary depending on the width and style of the pen used when drawing. If you want to include this outline in the item's shape, you can create a shape from the stroke using QPainterPathStroker.

This function is called by the default implementations of contains() and collidesWithPath().

所以基本上发生的是所有想要访问与项目关联的“区域”的函数，调用形状然后执行例如对生成的画家路径进行包含或碰撞检测。因此，如果您有小元素，您应该扩大形状区域。例如，假设一条线是您的目标，那么您的形状实现可能如下所示:

QPainterPath Segment::shape() const{
  QLineF temp(qLineF(scaled(Plotable::cScaleFactor)));
  QPolygonF poly;
  temp.translate(0,pen.widthF()/2.0);
  poly.push_back(temp.p1());
  poly.push_back(temp.p2());
  temp.translate(0,-pen.widthF());
  poly.push_back(temp.p2());
  poly.push_back(temp.p1());
  QPainterPath path;
  path.addPolygon(poly);
  return path;
}

Pen 是段的成员，我用它的宽度来扩大形状区域。但是您也可以采用与对象的实际尺寸有良好关系的任何其他东西。