-- flyaflya [2005-08-04 09:28:01]
Contents
1. Visitor(访问者）
1.1. 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.
1.2. 代码
1 class Visitor:
2 def __init__(self):
3 self._methodDic={}
4
5 def default(self, other):
6 print "What's this:", other
7
8 def addMethod(self, method):
9 self._methodDic[method.getKey()]=method
10
11 def __call__(self, other):
12 method=self._methodDic.get(\
other.__class__.__name__,self.default)
13 return method(other)
1.3. 例子
在MyVisit的函数call中定义对target的具体访问操作。
1 class MyVisit:
2 """
3 Instead of deriving from Visitor the work is
4 done by instances with this interface.
5 """
6 def __init__(self, otherClass):
7 self._msg='Visit: %s'%otherClass.__name__
8 self._key=otherClass.__name__
9
10 def __call__(self, target):
11 print self._msg, target
12
13 def getKey(self):
14 return self._key
15
16 # 被访问者
17 class E1:pass
18 class E2:pass
19 class E3:pass
20
21 # 用法
22
23 collection=[E1(), E1(), E2(), E3()]
24
25 visitor=Visitor()
26 visitor.addMethod(MyVisit(E1))
27 visitor.addMethod(MyVisit(E2))
28
29 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
1 visitor = Visitor()
2 visitor.addMethod(MyVisit(E1))
3 a = E1()
4 visitor(a)
输出：
Visit: E1 <__main__.E1 instance at 0x00A91EE0>