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devguide-9.htmlThe Document and Graphics Classes devguide-14.htmlThe class Protocols The class HierarchyNode
In Skencil, a document consists of a hierarchy of objects. Each object
has at most one parent and belongs to at most one document. That is, an
object may not belong to any compound object or document, but if it
belongs to one, it cannot be part of two compound objects or two
documents.
An object in this hierarchy knows which parent and document it belongs to.
The class HierarchyNode provides the standard interface for
managing this hierarchy and the references to the parent and the
document for child objects. The default behavior for parents is defined
by the class 
devguide-17.html#N2Compound .
No object derived from this class should override the methods defined
here except as documented.
Instance Variables
HierarchyNode uses the following instance variables:
parentThe parent of the object. Since the parent
also has a reference to its child this introduces a circular
reference. Therefore we must make sure that these references are
deleted when the document is destroyed. The 
#N6Destroy  method takes care of this.
If the object is not currently the child of a compound object
this variable should be 
None.
documentThe document the object belongs to. If the
object has just been created and is not yet inserted into the
document, or when the object is stored in the application's
clipboard, this variable is 
None.
In general, if parent is not None, document should be the
same as 
parent.document.
Methods
Methods defined in HierarchyNode (incomplete):
__init__A
HierarchyNode's __init__ method (and that of all
derived classes) must accept an optional keyword argument called
duplicate with the default value None and it must be
callable with just that argument. This is used by the standard
#N9Duplicate  method.
Destroy()Called by the parent
when the document is destroyed. This method should delete any
circular references. A Compound object should call the
Destroy method of each of its children.
The default implementation just sets parent to None.
Derived classes should extend this method if necessary.
SetParent(parent)Set
self's instance variable 
parent to parent.
SetDocument(document)Set self's instance variable document to document.
Duplicate()Return a duplicate
of self. The default implementation just returns
self.__class__(duplicate = self). Therefore, the class's
constructor must be callable with a single keyword argument
duplicate.
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