觀察者模式：

1、事件釋出者

事件釋出者下面是抽象出一個IPublisher基類，可以支援有多種不同型別的釋出者；另外一種方式是可以把管理觀察者的抽出作為一個類，事件釋出者包含這個類即可（見類圖）。

class IPublisher {
 public:
  virtual ~IPublisher(){};
  virtual void AddObserver(IObserver *observer) = 0;
  virtual void DeleObserver(IObserver *observer) = 0;
  virtual void Notify() = 0;
};

class Publisher
 
 : public IPublisher {
 public:
  virtual ~Publisher() {}
  void AddObserver(IObserver *observer) override {}
  void DeleObserver(IObserver *observer) override {}
  void Notify() override {}private:
  std::list<IObserver *> list_observer_;
};

這裡具體事件釋出者Publisher裡面重寫了幾個方法，並且維護一個list用來儲存所有的觀察者，其中Notify方法

2、觀察者

class IObserver {
 public:
  virtual ~IObserver(){};
  virtual void Update(const std::string &message_from_subject) = 0;
};

class Observer : public IObserver {
 public:
  Observer(Subject &subject) : subject_(subject) {}virtual ~Observer() {}void Update(const std::string &message_from_subject) override
 
 {
    message_from_subject_ = message_from_subject;
    PrintInfo();
  }
  void RemoveMeFromTheList() {
    subject_.Detach(this);
    std::cout << "Observer \"" << number_ << "\" removed from the list.\n";
  }
  void PrintInfo() {
    std::cout << "Observer \"" << this->number_ << "\": a new message is available --> " << this->message_from_subject_ << "\n";
  }

 private:
  std::string message_from_subject_;
  Subject &subject_;
  static int static_number_;
  int number_;
};