設計模式之十五--訪問者模式

訪問者模式：表示一個作用於某對象結構中各個元素的操作。它是你可以在不改變個元素的類的前提下定義作用於這些元素的新操作。

訪問者模式比較適用於數據結構比較固定的系統中，實現數據結構和算法的分離。但是一般來看數據結構都很不固定，所以用的比較少。

from abc import ABCMeta, abstractmethod


class Visitor(metaclass=ABCMeta):

    def __init__(self, name):
        self.name =  name

    @abstractmethod
    def visit_element_a(self, element):
        pass

    @abstractmethod
    def visit_element_b(self, element):
        pass


class ConcreteVisitorA(Visitor):

    def __init__(self, name):
        super().__init__(name)

    def visit_element_a(self, element):
        print("{0} visits {1}".format(self.name, element.name))

    def visit_element_b(self, element):
        print("{0} visits {1}".format(self.name, element.name))


class ConcreteVisitorB(Visitor):

    def __init__(self, name):
        super().__init__(name)

    def visit_element_a(self, element):
        print("{0} visits {1}".format(self.name, element.name))

    def visit_element_b(self, element):
        print("{0} visits {1}".format(self.name, element.name))


class ObjectStructure:

    def __init__(self):
        self.elements = set()

    def attach(self, element):
        self.elements.add(element)

    def detach(self, element):
        self.elements.remove(element)

    def accept(self, visitor):
        for element in self.elements:
            element.accept(visitor)


class Element(metaclass=ABCMeta):

    def __init__(self, name):
        self.name = name

    @abstractmethod
    def accept(self, visitor):
        pass


class ConcreteElementA(Element):

    def __init__(self, name):
        super().__init__(name)

    def accept(self, visitor):
        visitor.visit_element_a(self)


class ConcreteElementB(Element):

    def __init__(self, name):
        super().__init__(name)

    def accept(self, visitor):
        visitor.visit_element_b(self)


if __name__ == "__main__":

    visitor_a = ConcreteVisitorA("VisitorA")
    visitor_b = ConcreteVisitorB("VisitorB")
    element_a = ConcreteElementA("ElementA")
    element_b = ConcreteElementB("ElementB")
    object_structure = ObjectStructure()

    object_structure.attach(element_a)
    object_structure.attach(element_b)

    object_structure.accept(visitor_a)
    object_structure.accept(visitor_b)