一个context有可以切换多个state，切换到不同的state可以做不同的handle。
该模式将与状态相关的行为抽取到独立的状态类中， 让原对象将工作委派给这些类的实例， 而不是自行进行处理。
from__future__importannotationsfromabcimportABC,abstractmethodclassContext:""" The Context defines the interface of interest to clients. It also maintains a reference to an instance of a State subclass, which represents the current state of the Context. """_state=None""" A reference to the current state of the Context. """def__init__(self,state:State)->None:self.transition_to(state)deftransition_to(self,state:State):""" The Context allows changing the State object at runtime. """print(f"Context: Transition to{type(state).__name__}")self._state=state self._state.context=self""" The Context delegates part of its behavior to the current State object. """defrequest1(self):self._state.handle1()defrequest2(self):self._state.handle2()classState(ABC):""" The base State class declares methods that all Concrete State should implement and also provides a backreference to the Context object, associated with the State. This backreference can be used by States to transition the Context to another State. """@propertydefcontext(self)->Context:returnself._context@context.setterdefcontext(self,context:Context)->None:self._context=context@abstractmethoddefhandle1(self)->None:pass@abstractmethoddefhandle2(self)->None:pass""" Concrete States implement various behaviors, associated with a state of the Context. """classConcreteStateA(State):defhandle1(self)->None:print("ConcreteStateA handles request1.")print("ConcreteStateA wants to change the state of the context.")self.context.transition_to(ConcreteStateB())defhandle2(self)->None:print("ConcreteStateA handles request2.")classConcreteStateB(State):defhandle1(self)->None:print("ConcreteStateB handles request1.")defhandle2(self)->None:print("ConcreteStateB handles request2.")print("ConcreteStateB wants to change the state of the context.")self.context.transition_to(ConcreteStateA())if__name__=="__main__":# The client code.context=Context(ConcreteStateA())context.request1()context.request2()output:
Context: Transition to ConcreteStateA
ConcreteStateA handles request1.
ConcreteStateA wants to change the state of the context.
Context: Transition to ConcreteStateB
ConcreteStateB handles request2.
ConcreteStateB wants to change the state of the context.
Context: Transition to ConcreteStateA
状态模式的优点
避免了过多的条件判断：状态模式通过将每个状态的行为封装到对应的类中，避免了在上下文中使用大量的条件判断语句（如if-else或switch-case）来根据状态执行不同的行为。
符合开闭原则：当需要增加新的状态时，只需要添加新的状态类，而不需要修改上下文类或其他状态类。
使状态转换更加明确：每个状态类只关心自己状态下的行为以及如何转换到其他状态，使得状态转换的逻辑更加清晰。
状态模式的缺点
- 增加了类的数量：每个状态都需要一个对应的类，可能会导致系统中类的数量增加。
- 状态转换逻辑分散：状态转换的逻辑分散在各个具体状态类中，可能会使得状态转换的整体逻辑不够直观。
适用场景
一个对象的行为取决于它的状态，并且它必须在运行时根据状态改变它的行为。
一个操作中含有大量的条件语句，且这些条件依赖于对象的状态。
通过状态模式，我们可以将复杂的条件判断转换为状态类之间的转换，使得代码更加清晰和可维护。
