关于“itemChange()”的问题QGraphicsItem 的

发布于 2024-11-28 17:05:08 字数 571 浏览 0 评论 0原文

在 itemChange 的函数中，首先，我获取将要添加的子项，然后使用dynamic_cast将其转换为“MyItem”，但转换总是失败。

 QVariant MyItem::itemChange ( GraphicsItemChange change, const QVariant & value )
{

if (change==ItemChildAddedChange)
{
  QGraphicsItem* item=value.value<QGraphicsItem*>();
if (item)
{
     MyItem* myItem=dynamic_cast<MyItem*>(item);//myItem always be NULL,
//although I know the item is 'MyItem' type.
     if (myItem)
      {
       qDebug()<<"successful!";
       }
       }
}
return QGraphicsItem::itemChange(change,value);
}

非常感谢！

原文

In the function of itemChange, first ,I get the child item that will be added, then I use dynamic_cast cast it to 'MyItem', but the cast always fail.

 QVariant MyItem::itemChange ( GraphicsItemChange change, const QVariant & value )
{

if (change==ItemChildAddedChange)
{
  QGraphicsItem* item=value.value<QGraphicsItem*>();
if (item)
{
     MyItem* myItem=dynamic_cast<MyItem*>(item);//myItem always be NULL,
//although I know the item is 'MyItem' type.
     if (myItem)
      {
       qDebug()<<"successful!";
       }
       }
}
return QGraphicsItem::itemChange(change,value);
}

Thanks very much!

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萝莉病 2024-12-05 17:05:08

请注意 itemChange 上的注释：

请注意，当执行此操作时，新的子级可能无法完全构造
通知已发送；在子进程上调用纯虚函数可以
导致崩溃。

如果对象未完全构造，dynamic_cast 也可能失败。（我不太明白这方面的规范，但在某些情况下会，有些情况下不会。）如果您在构建项目之后设置父项，它将起作用：

#include <QtGui>

class MyGraphicsItem : public QGraphicsRectItem {
public:
  MyGraphicsItem(QGraphicsItem *parent, QGraphicsScene *scene)
    : QGraphicsRectItem(0.0, 0.0, 200.0, 200.0, parent, scene) {
    setBrush(QBrush(Qt::red));
  }
protected:
  QVariant itemChange(GraphicsItemChange change, const QVariant &value) {
    if (change == QGraphicsItem::ItemChildAddedChange) {
      QGraphicsItem* item = value.value<QGraphicsItem*>();
      if (item) {
        MyGraphicsItem* my_item=dynamic_cast<MyGraphicsItem*>(item);
        if (my_item) {
          qDebug() << "successful!";
        }
      }
    }
    return QGraphicsRectItem::itemChange(change, value);
  }
};

int main(int argc, char **argv) {
  QApplication app(argc, argv);

  QGraphicsScene scene;
  MyGraphicsItem *item = new MyGraphicsItem(NULL, &scene);

  // This will work.
  MyGraphicsItem *item2 = new MyGraphicsItem(NULL, &scene);
  item2->setParentItem(item);

//  // This will not work.
//  MyGraphicsItem *item2 = new MyGraphicsItem(item, &scene);

  QGraphicsView view;
  view.setScene(&scene);
  view.show();

  return app.exec();
}

Note the comment on itemChange:

Note that the new child might not be fully constructed when this
notification is sent; calling pure virtual functions on the child can
lead to a crash.

dynamic_cast can also fail if the object is not fully constructed. (I don't quite understand the spec on this, but there are some cases where it will, some where it will not.) If you set the parent after constructing the item, it will work:

#include <QtGui>

class MyGraphicsItem : public QGraphicsRectItem {
public:
  MyGraphicsItem(QGraphicsItem *parent, QGraphicsScene *scene)
    : QGraphicsRectItem(0.0, 0.0, 200.0, 200.0, parent, scene) {
    setBrush(QBrush(Qt::red));
  }
protected:
  QVariant itemChange(GraphicsItemChange change, const QVariant &value) {
    if (change == QGraphicsItem::ItemChildAddedChange) {
      QGraphicsItem* item = value.value<QGraphicsItem*>();
      if (item) {
        MyGraphicsItem* my_item=dynamic_cast<MyGraphicsItem*>(item);
        if (my_item) {
          qDebug() << "successful!";
        }
      }
    }
    return QGraphicsRectItem::itemChange(change, value);
  }
};

int main(int argc, char **argv) {
  QApplication app(argc, argv);

  QGraphicsScene scene;
  MyGraphicsItem *item = new MyGraphicsItem(NULL, &scene);

  // This will work.
  MyGraphicsItem *item2 = new MyGraphicsItem(NULL, &scene);
  item2->setParentItem(item);

//  // This will not work.
//  MyGraphicsItem *item2 = new MyGraphicsItem(item, &scene);

  QGraphicsView view;
  view.setScene(&scene);
  view.show();

  return app.exec();
}

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