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”。
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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): passclass 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_stateclass 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) |
总结
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原文链接：https://blog.csdn.net/Jmilk/article/details/122933989
