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-- flyaflya [<<DateTime(2005-08-04T09:28:01Z)>>]
<<TableOfContents>>
== Visitor(访问者） ==
=== 意图 ===
作用于某个对象群中各个对象的操作. 它可以使你在不改变这些对象本身的情况下,定义作用于这些对象的新操作.

=== 代码 ===
{{{
#!python
class Visitor:
    def __init__(self):
        self._methodDic={}

    def default(self, other):
        print "What's this:", other

    def addMethod(self, method):
        self._methodDic[method.getKey()]=method

    def __call__(self, other):
        method=self._methodDic.get(\
                other.__class__.__name__,self.default)
        return method(other)
}}}
=== 例子 ===
在MyVisit的函数__call__中定义对target的具体访问操作。
{{{
#!python
class MyVisit:
    """
    Instead of deriving from Visitor the work is
    done by instances with this interface.
    """
    def __init__(self, otherClass):
        self._msg='Visit: %s'%otherClass.__name__
        self._key=otherClass.__name__

    def __call__(self, target):
        print self._msg, target

    def getKey(self):
        return self._key

# 被访问者
class E1:pass
class E2:pass
class E3:pass

# 用法

collection=[E1(), E1(), E2(), E3()]

visitor=Visitor()
visitor.addMethod(MyVisit(E1))
visitor.addMethod(MyVisit(E2))

map(visitor, collection)
}}}
###########################    
输出：

{{{Visit: E1 <__main__.E1 instance at 7ff6d0>

Visit: E1 <__main__.E1 instance at 7ff730>

Visit: E2 <__main__.E2 instance at 7ff780>

What's this: <__main__.E3 instance at 7ff7b0>
}}}

# 简化用法
{{{
#!python
visitor = Visitor()
visitor.addMethod(MyVisit(E1))
a = E1()
visitor(a)
}}}
#########################
输出：

{{{Visit: E1 <__main__.E1 instance at 0x00A91EE0>
}}}

=== 特殊说明 ===