设计模式 - Observer实现

Observer模式的典型实现：

Subject.h NOTE：1.Subject类依赖Obserer接口类。2.Attach又叫Register/Subscribe/AddObserver，Detach又叫Release/Unsubscribe/RemoveObserver。TODO：Attach方法应该判断一个Observer有没有被重复添加。Detach应确保Obserer存在。考虑Subject对象销毁如何通知Observer。

#ifndef DP_SUBJECT_H
#define DP_SUBJECT_H

#include "Observer.h"
#include <list>

class Subject {
public:
  virtual ~Subject() {
    delete observers;
  }
  virtual void Attach(Observer* observer) {
    observers->push_front(observer);
  }
  virtual void Detach(Observer* observer) {
    observers->remove(observer);
  }
  virtual void Notify() {
    std::list<Observer*>::iterator it;
    for (it = observers->begin(); it != observers->end(); ++it) {
      (*it)->Update(this);
    }
  }
protected:
  Subject() {
    observers = new std::list<Observer*>;
  }
private:
  std::list<Observer*>* observers;
};

#endif

Observer.h

#ifndef DP_OBSERVER_H
#define DP_OBSERVER_H

class Subject;

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

#endif

ConcreteSubject.h NOTE：SetState方法里直接调用Notify可以减轻用户负担。

#ifndef DP_CONCRETESUBJECT_H
#define DP_CONCRETESUBJECT_H

#include "Subject.h"
#include <string>

class ConcreteSubject : public Subject {
public:
  ConcreteSubject()
    : subjectState("") {
  }
  void SetState(const std::string& state) {
    subjectState = state;
  }
  std::string GetState() {
    return subjectState;
  }
private:
  std::string subjectState;
};

#endif

ConcreteObserver.h NOTE：1.细节通过调用GetState向ConcreteSubject查询，属于拉模型。2.要是Update参数包括全部或部分细节，则是推模型。3.定义一个Collection成员存储多个Subject，不在构造函数里初始化绑定Subject，而是通过动态的方式注册，可以实现观察多个Subject。 TODO：Update函数应该对dynamic_cast的异常情况进行处理。

#ifndef DP_CONCRETEOBSERVER_H
#define DP_CONCRETEOBSERVER_H

#include "Observer.h"
#include <string>

class ConcreteSubject;

class ConcreteObserver : public Observer {
public:
  ConcreteObserver(Subject* subject)
    : observerState("") {
    this->subject = subject;
    subject->Attach(this);
  }
  virtual ~ConcreteObserver() {
    subject->Detach(this);
    delete subject;
  }
  virtual void Update(Subject* subject) {
    if (this->subject == subject) {
      observerState = dynamic_cast<ConcreteSubject*>(subject)->GetState();
    }
  }
private:
  Subject* subject;
  std::string observerState;
};

#endif

Main.cpp

#include "ConcreteSubject.h"
#include "ConcreteObserver.h"

int main() {
  ConcreteSubject* subject = new ConcreteSubject();
  Observer* observer1 = new ConcreteObserver(subject);
  Observer* observer2 = new ConcreteObserver(subject);

  subject->SetState("A");
  subject->Notify();
  subject->SetState("B");
  subject->Notify();

  return 0;
}