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<!DOCTYPE HTML>
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<title>访问者模式 · GitBook</title>
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Introduction
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访问者模式
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<a href="." >访问者模式</a>
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<h1 id="python设计模式之访问者模式"><a href="http://dongweiming.github.io/python-visitor.html"Permalink to python设计模式之访问者模式"" target="_blank">python设计模式之访问者模式</a></h1><h2 id="访问者模式">访问者模式</h2><p>我觉得Visitor模式是在补修改已有程序结构前提下，通过添加额外的访问者完成对代码功能的拓展
为什么这样用？当你的类层次较多，在某层结构中增加新的方法，要是在基类上面添加或者变更，可能破坏原来的设计， 有兼容问题，所以只在需要的类上面动态添加</p><h2 id="python的例子">python的例子</h2><p>这里是个构建车的例子，每个部件都有一个accept的方法接受我上面说的所谓'访问者'，而这个访问者
以参数的方式传进来，但是其实他是一个含有一些功能的类的实例，它拥有很多个visit开头的方法对应不同的部件。
这样就不需要修改这些部件，而只是修改我们的访问者类的相关部分</p><pre><code># 轮子,引擎, 车身这些定义好了都不需要变动
class Wheel:
def __init__(self, name):
self.name = name
def accept(self, visitor):
# 每个visitor是同样的，但是其中的方法是不一样的，比如这里是visitWheel，
# 然后传入了self，想想？他其实想做什么就能做什么
visitor.visitWheel(self)
class Engine:
def accept(self, visitor):
visitor.visitEngine(self)
class Body:
def accept(self, visitor):
visitor.visitBody(self)
# 我们要组合成车
class Car:
def __init__(self):
self.engine = Engine()
self.body = Body()
self.wheels = [ Wheel("front left"), Wheel("front right"),
Wheel("back left") , Wheel("back right") ]
# 这个也不需要在动，他只是上面部件的组合，只是做了属性的委托
def accept(self,visitor):
visitor.visitCar(self)
self.engine.accept(visitor)
self.body.accept(visitor)
for wheel in self.wheels:
wheel.accept(visitor)
# 这个才是我们的访问者，每次的修改都在这里面
class PrintVisitor:
def visitWheel(self, wheel):
print "Visiting "+wheel.name+" wheel"
def visitEngine(self, engine):
print "Visiting engine"
def visitBody(self, body):
print "Visiting body"
def visitCar(self, car):
print "Visiting car"
if __name__ == '__main__':
car = Car()
visitor = PrintVisitor()
car.accept(visitor)</code></pre>
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