共计 7391 个字符，预计需要花费 19 分钟才能阅读完成。
问题：有一个糖果公司需要设计一个糖果售卖机，控制流程如下图，需要怎么实现？
这是一个状态图，每个圆圈都是一种状态。很明显，有有 25 分钱、没有 25 分钱、售出糖果、糖果售罄四个状态，同时也对应四个动作：投入 25 分钱，退回 25 分钱，转动曲柄和发放糖果。
那如何从状态图得到真正的代码呢？
简单代码实现如下：
#! -*- coding: utf-8 -*-
class GumballMachine:
# 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
STATE_SOLD_OUT = 0
STATE_NO_QUARTER = 1
STATE_HAS_QUARTER = 2
STATE_SOLD = 3
state = STATE_SOLD_OUT
def __init__(self, count=0):
self.count = count
if count > 0:
self.state = self.STATE_NO_QUARTER
def __str__(self):
return “Gumball machine current state: %s” % self.state
def insert_quarter(self):
# 投入 25 分钱
if self.state == self.STATE_HAS_QUARTER: # 如果已经投过
print(“You can’t insert another quarter”)
elif self.state == self.STATE_NO_QUARTER: # 如果没有投过
self.state = self.STATE_HAS_QUARTER
print(“You inserted a quarter”)
elif self.state == self.STATE_SOLD_OUT: # 如果已经售罄
print(“You can’t insert a quarter, the machine is sold out”)
elif self.state == self.STATE_SOLD: # 如果刚刚买了糖果
print(“Please wait, we’re already giving you a gumball”)
def eject_quarter(self):
# 退回 25 分
if self.state == self.STATE_HAS_QUARTER:
print(“Quarter returned”)
self.state = self.STATE_NO_QUARTER
elif self.state == self.STATE_NO_QUARTER:
print(“You haven’t inserted a quarter”)
elif self.state == self.STATE_SOLD:
print(“Sorry, you alread turned the crank”)
elif self.state == self.SOLD_OUT:
print(“You can’t eject, you haven’t inserted”)
def turn_crank(self):
# 转动曲柄
if self.state == self.STATE_SOLD:
print(“Turning twice doesn’t get you another gumball”)
elif self.state == self.STATE_NO_QUARTER:
print(“You turned but there’s no quarter”)
elif self.state == self.STATE_SOLD_OUT:
print(“You turned, but there are no gumballs”)
elif self.state == self.STATE_HAS_QUARTER:
print(“You turned…”)
self.state = self.STATE_SOLD
self.dispense()
def dispense(self):
# 发放糖果
if self.state == self.STATE_SOLD:
print(“A gumball comes rolling out the slot”)
self.count -= 1
if self.count == 0:
self.state = self.STATE_SOLD_OUT
else:
self.state = self.STATE_NO_QUARTER
elif self.state == self.STATE_NO_QUARTER:
print(“You need to pay first”)
elif self.state == self.STATE_SOLD_OUT:
print(“No gumball dispensed”)
elif self.state == self.STATE_HAS_QUARTER:
print(“No gumball dispensed”)
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)
这段代码有几个问题：
没有遵守开放 - 关闭原则
更像是面向过程的设计
状态转化被埋藏在条件语句中
未来加入新的需求，需要改动的较多，不易维护，可能会出 bug
如何改进呢？
考虑封装变化，把每个状态的行为都放在各自的类中，每个状态只要实现自己的动作，用加入新类的方式来实现新状态的加入。
定义 State 父类，在这个类中，糖果机的每个动作都有一个应对的方法
为机器中的每个状态实现状态类，这些类将负责在对应的状态下进行机器的行为
摆脱旧的条件代码，将动作委托到状态类
新的实现代码如下：
#! -*- coding: utf-8 -*-
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)
重构后的代码相对于之前的代码做了哪些事情呢？
将每个状态的行为局部话到自己的类中
删除 if 语句
将状态类对修改关闭，对糖果季类对扩展开放
下图是刚初始状态图示：
上面重构部分代码使用的就是状态模式：
定义
状态模式: 状态模式允许对象在内部状态改变时改变它的行为，对象看起来好像修改了它的类。
状态模式的类图如下：
状态模式是将多个行为封装在状态对象中，context 的行为随时可委托到其中一个状态中。当前状态在不同的状态对象中改变，以反映出 context 内部的状态，context 的行为也会随之改变。
扩展
如果，现在要在这四个状态的基础上再加一个状态（购买糖果后，有 10% 的概率再得一个），该如何实现呢？
# 添加 WinnerState 类，只有 dispense 方法不同，可以从 SoldState 类继承
class WinnerState(SoldState):
def __str__(self):
return “winner”
def dispense(self):
print(“You’re a WINNER! You get two gumballs for your quarter”)
self.gumball_machine.release_ball()
if gumball_machine.count == 0:
self.gumball_machine.state = self.gumball_machine.soldout_state
else:
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
# 修改 turn_crank 方法
class HasQuarterState(State):
…
def turn_crank(self):
print(“You turned…”)
winner = random.randint(0, 9)
if winner == 4 and self.gumball_machine.count > 1: # 如果库存大于 1 并且随机数等于 4（可以是 0 到 9 任意值）
self.gumball_machine.state = self.gumball_machine.winner_state
else:
self.gumball_machine.state = self.gumball_machine.sold_state
# 在 GumballMachine 中初始化
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.winner_state = WinnerState(self)
…
总结
状态模式允许一个对象给予内部状态而拥有不同的行为
状态模式用类代表状态
Context 会将行为委托给当前状态对象
通过将每状态封装进一个类，把改变局部化
状态装欢可以由 State 类或 Context 类控制
使用状态模式会增加类的数目
状态类可以被多个 Context 实例共享
本文例子来自《Head First 设计模式》。
最后，感谢女朋友支持和包容，比❤️
也可以在公号输入以下关键字获取历史文章：公号 & 小程序 | 设计模式 | 并发 & 协程
