class Subject:
    def __init__(self):
        self.observers = []
    
    def attach(self, observer):
        self.observers.append(observer)
    
    def notify(self):
        for observer in self.observers:
            observer.update(self)

class Observer:
    def update(self, subject):
        pass

class ConcreteSubject(Subject):
    def __init__(self):
        super().__init__()
        self._state = 0

    @property
    def state(self):
        return self._state

    @state.setter
    def state(self, value):
        self._state = value
        self.notify()

class ConcreteObserver(Observer):
    def update(self, subject):
        print(f"Observer: Subject's state changed to {subject.state}")

subject = ConcreteSubject()
observer1 = ConcreteObserver()
observer2 = ConcreteObserver()

subject.attach(observer1)
subject.attach(observer2)

subject.state = 10  # Output: Observer: Subject's state changed to 10