设计模式五，Observer

观察者（Observer)模式用于解决一个相当常见的问题：当某些其他对象改变状态时，如果一组对象需要进行相应的更新，那么应该如何处理呢？假定有一些数据（即文档）和两个视图，一个图形视图和一个文本视图，在更改“文档”数据时，必须通知这些视图更新它们自身，这就是观察者模式所要完成的任务。

以下代码完成Observer模式，采用内部内的方式，不过虽然实现了此模式，但内部类的代码不断重复，不是好的方法。

找到更适合的方法再考虑进行更新。

#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 ///:~

#ifndef OBSERVABLE_H

#define OBSERVABLE_H

#include 

#include "Observer.h"

class Observable {

  bool changed;

  std::set 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::iterator it;

    for(it = observers.begin();it != observers.end(); it++)

      (*it)->update(this, arg);

  }

  virtual ~Observable() {}

};

#endif // OBSERVABLE_H ///:~

//: C10:ObservedFlower.cpp

// Demonstration of "observer" pattern.

#include 

#include 

#include <string>

#include 

#include "Observable.h"

using namespace std;

class Flower {

    bool isOpen;

public:

    Flower() : isOpen(false),

        openNotifier(this), closeNotifier(this) {}

    void open() { // Opens its petals

        isOpen = true;

        openNotifier.notifyObservers();

        closeNotifier.open();

    }

    void close() { // Closes its petals

        isOpen = false;

        closeNotifier.notifyObservers();

        openNotifier.close();

    }

    // Using the "inner class" idiom:

    class OpenNotifier;

    friend class Flower::OpenNotifier;

    class OpenNotifier : public Observable {

        Flower* parent;

        bool alreadyOpen;

    public:

        OpenNotifier(Flower* f) : parent(f),

            alreadyOpen(false) {}

        void notifyObservers(Argument* arg = 0) {

            if(parent->isOpen && !alreadyOpen) {

                setChanged();

                Observable::notifyObservers();

                alreadyOpen = true;

            }

        }

        void close() { alreadyOpen = false; }

    } openNotifier;

    class CloseNotifier;

    friend class Flower::CloseNotifier;

    class CloseNotifier : public Observable {

        Flower* parent;

        bool alreadyClosed;

    public:

        CloseNotifier(Flower* f) : parent(f),

            alreadyClosed(false) {}

        void notifyObservers(Argument* arg = 0) {

            if(!parent->isOpen && !alreadyClosed) {

                setChanged();

                Observable::notifyObservers();

                alreadyClosed = true;

            }

        }

        void open() { alreadyClosed = false; }

    } closeNotifier;

};

class Bee {

    string name;

    // An "inner class" for observing openings:

    class OpenObserver;

    friend class Bee::OpenObserver;

    class OpenObserver : public Observer {

        Bee* parent;

    public:

        OpenObserver(Bee* b) : parent(b) {}

        void update(Observable*, Argument *) {

            cout << "Bee " << parent->name

                << "'s breakfast time!" << endl;

        }

    } openObsrv;

    // Another "inner class" for closings:

    class CloseObserver;

    friend class Bee::CloseObserver;

    class CloseObserver : public Observer {

        Bee* parent;

    public:

        CloseObserver(Bee* b) : parent(b) {}

        void update(Observable*, Argument *) {

            cout << "Bee " << parent->name

                << "'s bed time!" << endl;

        }

    } closeObsrv;

public:

    Bee(string nm) : name(nm),

        openObsrv(this), closeObsrv(this) {}

    Observer& openObserver() { return openObsrv; }

    Observer& closeObserver() { return closeObsrv;}

};

class Hummingbird {

    string name;

    class OpenObserver;

    friend class Hummingbird::OpenObserver;

    class OpenObserver : public Observer {

        Hummingbird* parent;

    public:

        OpenObserver(Hummingbird* h) : parent(h) {}

        void update(Observable*, Argument *) {

            cout << "Hummingbird " << parent->name

                << "'s breakfast time!" << endl;

        }

    } openObsrv;

    class CloseObserver;

    friend class Hummingbird::CloseObserver;

    class CloseObserver : public Observer {

        Hummingbird* parent;

    public:

        CloseObserver(Hummingbird* h) : parent(h) {}

        void update(Observable*, Argument *) {

            cout << "Hummingbird " << parent->name

                << "'s bed time!" << endl;

        }

    } closeObsrv;

public:

    Hummingbird(string nm) : name(nm),

        openObsrv(this), closeObsrv(this) {}

    Observer& openObserver() { return openObsrv; }

    Observer& closeObserver() { return closeObsrv;}

};

int main() {

    Flower f;

    Bee ba("A"), bb("B");

    Hummingbird ha("A"), hb("B");

    f.openNotifier.addObserver(ha.openObserver());

    f.openNotifier.addObserver(hb.openObserver());

    f.openNotifier.addObserver(ba.openObserver());

    f.openNotifier.addObserver(bb.openObserver());

    f.closeNotifier.addObserver(ha.closeObserver());

    f.closeNotifier.addObserver(hb.closeObserver());

    f.closeNotifier.addObserver(ba.closeObserver());

    f.closeNotifier.addObserver(bb.closeObserver());

    // Hummingbird B decides to sleep in:

    f.openNotifier.deleteObserver(hb.openObserver());

    // Something changes that interests observers:

    f.open();

    f.open(); // It's already open, no change.

    // Bee A doesn't want to go to bed:

    f.closeNotifier.deleteObserver(

        ba.closeObserver());

    f.close();

    f.close(); // It's already closed; no change

    f.openNotifier.deleteObservers();

    f.open();

    f.close();

} ///:~

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