设计模式：工厂方法模式

　2012-05-26 15:40:37　来源：WEB开发网　　　

核心提示：工厂方法模式（Factory Method Pattern）：定义了一个创建对象的接口，但由子类决定要实例化类的哪一个，设计模式：工厂方法模式，工厂方法让类把实例化推迟到子类，原则：要依赖抽象，也就是将生产权限下放，将生产需要的数据和方法全部交给具体工厂实现，不要依赖具体类，案例：Pizza有不同的风味

工厂方法模式（Factory Method Pattern）：
定义了一个创建对象的接口，但由子类决定要实例化类的哪一个。工厂方法让类把实例化推迟到子类。
原则：
要依赖抽象，不要依赖具体类。

案例：
Pizza有不同的风味，但都是由面团（dough）、酱料（sauce）和洒在上面的佐料（topping）加工而成。如果用户订购，需要准备材料、烘烤、切片和包装。

一般的方法是创建不同的Pizza子类实现上述所有数据和方法，然后根据用户订购的类型来初始化不同的类。但这样会在主代码中有大量的条件判断语句，而且类型的每次变更都需要修改代码。

一个Pizza店可能有多个加盟店，分布在不同城市，且风味不同（比如Chicago和NewYork的cheese风味就不同），但是又要由总部提供统一的订购接口给客户，那么我们可以将子类初始化的判断放到各地工厂去实现，也就是将生产权限下放，将生产需要的数据和方法全部交给具体工厂实现。总部只需要根据客户的需要委托相应工厂生产即可。

python

Preparing NY Style Cheese Pizza
    dough: NY Dough
    sauce: NY Sauce
    add toppings:
        NY toopping A
        NY toopping B
Bake for 25 minutes at 350.
Cutting into diagonal slices.
Put into official box.
Mike ordered a NY Style Cheese Pizza.

Preparing Chicago Style Clam Pizza
    dough: Chicago Dough
    sauce: Chicago Sauce
    add toppings:
        NY toopping A
        NY toopping B
        Chicago toopping A
Bake for 25 minutes at 350.
Cutting into square slices.
Put into official box.
John ordered a Chicago Style Clam Pizza.

代码：

#!/usr/bin/python


class Pizza:
    name = ""
    dough = ""
    sauce = ""
    toppings = []

    def prepare(self):
        print "Preparing %s" % self.name
        print "    dough: %s" % self.dough
        print "    sauce: %s" % self.sauce
        print "    add toppings:"
        for n in self.toppings:
            print "        %s" % n

    def bake(self):
        print "Bake for 25 minutes at 350."

    def cut(self):
        print "Cutting into diagonal slices."

    def box(self):
        print "Put into official box."

    def get_name(self):
        return self.name


class PizzaStore:
    def order_pizza(self, pizza_type):
        self.pizza = self.create_pizza(pizza_type)
        self.pizza.prepare()
        self.pizza.bake()
        self.pizza.cut()
        self.pizza.box()
        return self.pizza

    def create_pizza(self, pizza_type):
        pass


class NYStyleCheesePizza(Pizza):
    def __init__(self):
        self.name = "NY Style Cheese Pizza"
        self.dough = "NY Dough"
        self.sauce = "NY Sauce"
        self.toppings.append("NY toopping A")
        self.toppings.append("NY toopping B")


class ChicagoStyleCheesePizza(Pizza):
    def __init__(self):
        self.name = "Chicago Style Cheese Pizza"
        self.dough = "Chicago Dough"
        self.sauce = "Chicago Sauce"
        sefl.toppings.append("Chicago toopping A")

    def cut(self):
        print "Cutting into square slices."


class NYStyleClamPizza(Pizza):
    def __init__(self):
        self.name = "NY Style Clam Pizza"
        self.dough = "NY Dough"
        self.sauce = "NY Sauce"
        self.toppings.append("NY toopping A")
        self.toppings.append("NY toopping B")


class ChicagoStyleClamPizza(Pizza):
    def __init__(self):
        self.name = "Chicago Style Clam Pizza"
        self.dough = "Chicago Dough"
        self.sauce = "Chicago Sauce"
        self.toppings.append("Chicago toopping A")

    def cut(self):
        print "Cutting into square slices."


class NYPizzaStore(PizzaStore):
    def create_pizza(self, pizza_type):
        if pizza_type == "cheese":
            return NYStyleCheesePizza()
        elif pizza_type == "clam":
            return NYStyleClamPizza()
        else:
            return None


class ChicagoPizzaStore(PizzaStore):
    def create_pizza(self, pizza_type):
        if pizza_type == "cheese":
            return ChicagoStyleCheesePizza()
        elif pizza_type == "clam":
            return ChicagoStyleClamPizza()
        else:
            return None

if __name__ == "__main__":
    ny_store = NYPizzaStore()
    chicago_store = ChicagoPizzaStore()

    pizza = ny_store.order_pizza("cheese")
    print "Mike ordered a %s." % pizza.get_name()
    print 

    pizza = chicago_store.order_pizza("clam")
    print "John ordered a %s." % pizza.get_name()
    print

输出：plain