觀察者 Observer 模式用於解決一個常見的問題 當其他物件改變狀態時 如果一組物件需要進行相應的更新 那麼應該如何處理呢？

使用兩種物件的型別以實現觀察者模式

//: C10:Observer.h
// From "Thinking in C++, Volume 2", by Bruce Eckel & Chuck Allison.
// (c) 1995-2004 MindView, Inc. All Rights Reserved.
// See source code use permissions stated in the file 'License.txt',
// distributed with the code package available at www.MindView.net.
// The Observer interface.
#ifndef OBSERVER_H
#define OBSERVER_H

class Observable;
class Argument {};

class Observer {
public:
  // Called by the observed object, whenever
  // the observed object is changed:
  virtual void update(Observable* o, Argument* arg) = 0;
  virtual ~Observer() {}
};
#endif // OBSERVER_H ///:~
 

這種方法中Observer與Observable互動作用 必須首先宣告Observable 另外 類Argument是空的 在更新過程中它只擔任一個基類角色

被觀察物件的型別是Observable

//: C10:Observable.h
// From "Thinking in C++, Volume 2", by Bruce Eckel & Chuck Allison.
// (c) 1995-2004 MindView, Inc. All Rights Reserved.
// See source code use permissions stated in the file 'License.txt',
// distributed with the code package available at www.MindView.net.
// The Observable class.
#ifndef OBSERVABLE_H
#define OBSERVABLE_H
#include <set>
#include "Observer.h"

class Observable {
  bool changed;
  std::set<Observer*> observers;
protected:
  virtual void setChanged() { changed = true; }
  virtual void clearChanged() { changed = false; }
public:
  virtual void addObserver(Observer& o) {
    observers.insert(&o);
  }
  virtual void deleteObserver(Observer& o) {
    observers.erase(&o);
  }
  virtual void deleteObservers() {
    observers.clear();
  }
  virtual int countObservers() {
    return observers.size();
  }
  virtual bool hasChanged() { return changed; }
  // If this object has changed, notify all
  // of its observers:
  virtual void notifyObservers(Argument* arg = 0) {
    if(!hasChanged()) return;
    clearChanged(); // Not "changed" anymore
    std::set<Observer*>::iterator it;
    for(it = observers.begin();it != observers.end(); it++)
      (*it)->update(this, arg);
  }
  virtual ~Observable() {}
};
#endif // OBSERVABLE_H ///:~
 

只要有方法用Observable註冊 一個 Observer並有方法為Observable 更新其Observer 不必在意成員函式的設定

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