Python设计模式中的状态模式你了解吗

状态模式

状态模式，当对象的内部状态发生了改变的时候，允许对象执行不同的流程。

优点：

• 封装了状态转换规则。
• 枚举了可能的状态，在枚举状态之前需要确定状态的种类。
• 将所有与某个状态有关的行为放到一个类中，并且可以方便地增加新的状态，只需要改变对象状态即可改变对象的行为。
• 允许状态转换逻辑与状态对象合成一体，而不是某一个巨大的条件语句块。
• 可以让多个环境对象共享一个状态对象，从而减少系统中对象的个数。

缺点：

• 状态模式的使用必然会增加系统类和对象的个数。
• 状态模式的结构与实现都较为复杂，如果使用不当将导致程序结构和代码的混乱。
• 状态模式对 “开闭原则” 并不太好，对于可以切换状态的状态模式，增加新的状态类需要修改那些负责状态转换的源代码，否则无法切换到新增状态，而且修改某个状态类的行为也需修改对应类的源代码。

应用场景

• 行为随状态改变而改变的场景。
• 条件、分支语句的代替者。

代码示例

这是一个状态图，具有 “有 25 分钱”、“没有 25 分钱”、“售出糖果”、“糖果售罄” 这 4 个状态。同时也对应 4 个动作：“投入 25 分钱”，“退回 25 分钱”，“转动曲柄” 和 “发放糖果”。

class state:
    # 定义state基类
    def insert_quarter(self):
        pass
    def eject_quarter(self):
        pass
    def turn_crank(self):
        pass
    def dispense(self):
        pass

class soldoutstate(state):
    # 继承state 类
    def __init__(self, gumball_machine):
        self.gumball_machine = gumball_machine
    def __str__(self):
        return "sold_out"
    def insert_quarter(self):
        print("you can't insert a quarter, the machine is sold out")
    def eject_quarter(self):
        print("you can't eject, you haven't inserted a quarter yet")
    def turn_crank(self):
        print("you turned, but ther are no gumballs")
    def dispense(self):
        print("no gumball dispensed")

class soldstate(state):
    # 继承state 类
    def __init__(self, gumball_machine):
        self.gumball_machine = gumball_machine
    def __str__(self):
        return "sold"
    def insert_quarter(self):
        print("please wait, we're already giving you a gumball")
    def eject_quarter(self):
        print("sorry, you already turned the crank")
    def turn_crank(self):
        print("turning twice doesn't get you another gumball")
    def dispense(self):
        self.gumball_machine.release_ball()
        if gumball_machine.count > 0:
            self.gumball_machine.state = self.gumball_machine.no_quarter_state
        else:
            print("oops, out of gumballs!")
            self.gumball_machine.state = self.gumball_machine.soldout_state

class noquarterstate(state):
    # 继承state 类
    def __init__(self, gumball_machine):
        self.gumball_machine = gumball_machine
    def __str__(self):
        return "no_quarter"
    def insert_quarter(self):
        # 投币 并且改变状态
        print("you inserted a quarter")
        self.gumball_machine.state = self.gumball_machine.has_quarter_state
    def eject_quarter(self):
        print("you haven't insert a quarter")
    def turn_crank(self):
        print("you turned, but there's no quarter")
    def dispense(self):
        print("you need to pay first")

class hasquarterstate(state):
    # 继承state 类
    def __init__(self, gumball_machine):
        self.gumball_machine = gumball_machine
    def __str__(self):
        return "has_quarter"
    def insert_quarter(self):
        print("you can't insert another quarter")
    def eject_quarter(self):
        print("quarter returned")
        self.gumball_machine.state = self.gumball_machine.no_quarter_state
    def turn_crank(self):
        print("you turned...")
        self.gumball_machine.state = self.gumball_machine.sold_state
    def dispense(self):
        print("no gumball dispensed")

class gumballmachine:
    def __init__(self, count=0):
        self.count = count
        # 找出所有状态，并创建实例变量来持有当前状态，然后定义状态的值
        self.soldout_state = soldoutstate(self)
        self.no_quarter_state = noquarterstate(self)
        self.has_quarter_state = hasquarterstate(self)
        self.sold_state = soldstate(self)
        if count > 0:
            self.state = self.no_quarter_state
        else:
            self.state = self.soldout_state
    def __str__(self):
        return ">>> gumball machine current state: %s" % self.state
    def insert_quarter(self):
        # 投入25分钱
        self.state.insert_quarter()
    def eject_quarter(self):
        # 退回25分
        self.state.eject_quarter()
        # print("state", self.state, type(self.state))
    def turn_crank(self):
        # 转动曲柄
        # print("state", self.state, type(self.state))
        self.state.turn_crank()
    def release_ball(self):
        # 发放糖果
        print("a gumball comes rolling out the slot...")
        if self.count > 0:
            self.count -= 1
        
if __name__ == "__main__":
    # 以下是代码测试
    gumball_machine = gumballmachine(5) # 装入5 个糖果
    print(gumball_machine)
    gumball_machine.insert_quarter() # 投入25分钱
    gumball_machine.turn_crank() # 转动曲柄
    print(gumball_machine)
    gumball_machine.insert_quarter() #投入25分钱
    gumball_machine.eject_quarter()  # 退钱
    gumball_machine.turn_crank()     # 转动曲柄
    print(gumball_machine)
    gumball_machine.insert_quarter() # 投入25分钱
    gumball_machine.turn_crank() # 转动曲柄 
    gumball_machine.insert_quarter() # 投入25分钱 
    gumball_machine.turn_crank()  # 转动曲柄
    gumball_machine.eject_quarter() # 退钱
    print(gumball_machine)

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