类的继承结构如下：
class VTKRENDERINGCORE_EXPORT vtkAbstractPicker : public vtkObject
class VTKRENDERINGCORE_EXPORT vtkAbstractPropPicker : public vtkAbstractPicker
class VTKRENDERINGCORE_EXPORT vtkPicker : public vtkAbstractPropPicker
class VTKINTERACTIONWIDGETS_EXPORT vtkResliceCursorPicker : public vtkPicker//交互部分自定义的一个类
