Observer.cpp

/**
 * Observer Design Pattern
 *
 * Intent: Lets you define a subscription mechanism to notify multiple objects
 * about any events that happen to the object they're observing.
 *
 * Note that there's a lot of different terms with similar meaning associated
 * with this pattern. Just remember that the Subject is also called the
 * Publisher and the Observer is often called the Subscriber and vice versa.
 * Also the verbs "observe", "listen" or "track" usually mean the same thing.
 */

#include <iostream>
#include <list>
#include <string>
#include <memory>

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

class ISubject {
 public:
  virtual ~ISubject(){};
  virtual void Attach(IObserver *observer) = 0;
  virtual void Detach(IObserver *observer) = 0;
  virtual void Notify() = 0;
};

/**
 * The Subject owns some important state and notifies observers when the state
 * changes.
 */

class Subject : public ISubject {
 public:
  virtual ~Subject() {
    std::cout << "Goodbye, I was the Subject.\n";
  }

  /**
   * The subscription management methods.
   */
  void Attach(IObserver *observer) override {
    list_observer_.push_back(observer);
  }
  void Detach(IObserver *observer) override {
    list_observer_.remove(observer);
  }
  void Notify() override {
    std::list<IObserver *>::iterator iterator = list_observer_.begin();
    HowManyObserver();
    while (iterator != list_observer_.end()) {
      (*iterator)->Update(message_);
      ++iterator;
    }
  }

  void CreateMessage(std::string message = "Empty") {
    this->message_ = message;
    Notify();
  }
  void HowManyObserver() {
    std::cout << "There are " << list_observer_.size() << " observers in the list.\n";
  }

  /**
   * Usually, the subscription logic is only a fraction of what a Subject can
   * really do. Subjects commonly hold some important business logic, that
   * triggers a notification method whenever something important is about to
   * happen (or after it).
   */
  void SomeBusinessLogic() {
    this->message_ = "change message message";
    Notify();
    std::cout << "I'm about to do some thing important\n";
  }

 private:
  std::list<IObserver *> list_observer_;
  std::string message_;
};

class Observer : public IObserver {
 public:
  Observer(Subject &subject) : subject_(subject) {
    this->subject_.Attach(this);
    std::cout << "Hi, I'm the Observer \"" << ++Observer::static_number_ << "\".\n";
    this->number_ = Observer::static_number_;
  }
  virtual ~Observer() {
    std::cout << "Goodbye, I was the Observer \"" << this->number_ << "\".\n";
  }

  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_;
};

int Observer::static_number_ = 0;

void ClientCode() {
  std::unique_ptr<Subject> subject = std::make_unique<Subject>();
  std::unique_ptr<Observer> observer1 = std::make_unique<Observer>(*subject);
  std::unique_ptr<Observer> observer2 = std::make_unique<Observer>(*subject);
  std::unique_ptr<Observer> observer3 = std::make_unique<Observer>(*subject);

  subject->CreateMessage("Hello World! :D");
  observer3->RemoveMeFromTheList();

  subject->CreateMessage("The weather is hot today! :p");
  std::unique_ptr<Observer> observer4 = std::make_unique<Observer>(*subject);

  observer2->RemoveMeFromTheList();
  std::unique_ptr<Observer> observer5 = std::make_unique<Observer>(*subject);

  subject->CreateMessage("My new car is great! ;)");
  observer5->RemoveMeFromTheList();

  observer4->RemoveMeFromTheList();
  observer1->RemoveMeFromTheList();

  subject->CreateMessage("Test message 1");

  std::unique_ptr<Observer> observer6 = std::make_unique<Observer>(*subject);
  std::unique_ptr<Observer> observer7 = std::make_unique<Observer>(*subject);
  std::unique_ptr<Observer> observer8 = std::make_unique<Observer>(*subject);
  subject->CreateMessage("Test message 2");

  subject->CreateMessage();
}

int main() {
  ClientCode();
  return 0;
}