设计模式 - Observer

意图：定义对象间的一种一对多的依赖关系,当一个对象的状态发生改变时, 所有依赖于它的对象都得到通知并被自动更新。

class Subject;

class Observer
{
public:
    Observer()
    {}
    virtual ~Observer()
    {}
    virtual void Update(Subject* subject) = 0;
};

class Subject
{
public:
    Subject():subject_state(0), observers()
    {}
    virtual ~Subject()
    {}
    int GetState()
    {
        return this->subject_state;
    }
    void Attach(Observer* observer)
    {
        this->observers.push_back(observer);
    }
    void Notify()
    {
        for(list<Observer*>::iterator it=observers.begin(); it!=observers.end(); ++it)
        {
            (*it)->Update(this);
        }
    }

protected:
    int subject_state;
    list<Observer*> observers;
};

class ConcreteSubject: public Subject
{
public:
    ConcreteSubject()
    {}
    ~ConcreteSubject()
    {}
    void Changed()
    {
        ++subject_state;
        Notify();
    }
};

class ConcreteObserverA: public Observer
{
public:
    ConcreteObserverA(): observer_state(0)
    {}
    ~ConcreteObserverA()
    {}
    void Update(Subject* subject)
    {
        cout<<"ConcreteObserverA::Update()"<<endl;
        this->observer_state = subject->GetState();
    }

private:
    int observer_state;
};

class ConcreteObserverB: public Observer
{
public:
    ConcreteObserverB(): observer_state(0)
    {}
    ~ConcreteObserverB()
    {}
    void Update(Subject* subject)
    {
        cout<<"ConcreteObserverB::Update()"<<endl;
        this->observer_state = subject->GetState();
    }

private:
    int observer_state;
};