observer.c

Design Pattern C Source Code

/*
*/
class BaseClassSubject {
public:
    void Operation (int newValue);
};

class MySubject : public BaseClassSubject {
public:
    void Operation (int newValue);
private:
    int _myInstVar;
};

/*
*/
void MySubject::Operation (int newValue) {
    BaseClassSubject::Operation(newValue);
        // trigger notification

    _myInstVar += newValue;
        // update subclass state (too late!)
}
/*
*/
typedef int TextRange;
class Text {
public:
    void Cut(int newValue);
    void ReplaceRange(int newValue);
    void Notify();
};

/*
*/
void Text::Cut (TextRange r) {
     ReplaceRange(r);       // redefined in subclasses
     Notify();
}
/*
*/
#include "List.H"
/*
*/
class Subject;

class Observer {
public:
    virtual ~ Observer();
    virtual void Update(Subject* theChangedSubject) = 0;
protected:
    Observer();
};
/*
*/
class Subject {
public:
    virtual ~Subject();
/*
*/
    virtual void Attach(Observer*);
    virtual void Detach(Observer*);
    virtual void Notify();
protected:
    Subject();
private:
    List<Observer*> *_observers;
};
/*
*/
void Subject::Attach (Observer* o) {
    _observers->Append(o);
}
/*
*/
void Subject::Detach (Observer* o) {
    _observers->Remove(o);
}
/*
*/
void Subject::Notify () {
    ListIterator<Observer*> i(_observers);

    for (i.First(); !i.IsDone(); i.Next()) {
        i.CurrentItem()->Update(this);
    }
}
/*
*/
class ClockTimer : public Subject {
public:
    ClockTimer();
/*
*/
    virtual int GetHour();
    virtual int GetMinute();
    virtual int GetSecond();
/*
*/
    void Tick();
};
/*
*/
void ClockTimer::Tick () {
    // update internal time-keeping state
    // ...
    Notify();
}
/*
*/
class Widget {
public:
    virtual void Draw();
};
/*
*/
class DigitalClock: public Widget, public Observer {
public:
    DigitalClock(ClockTimer*);
    virtual ~DigitalClock();
/*
*/
    virtual void Update(Subject*);
        // overrides Observer operation
/*
*/
    virtual void Draw();
        // overrides Widget operation;
        // defines how to draw the digital clock
private:
    ClockTimer* _subject;
};
/*
*/
DigitalClock::DigitalClock (ClockTimer* s) {
    _subject = s;
    _subject->Attach(this);
}

DigitalClock::~DigitalClock () {
    _subject->Detach(this);
}
/*
*/
void DigitalClock::Update (Subject* theChangedSubject) {
    if (theChangedSubject == _subject) {
        Draw();
    }
}
/*
*/
void DigitalClock::Draw () {
    // get the new values from the subject

    int hour = _subject->GetHour();
    int minute = _subject->GetMinute();
    // etc.

    // draw the digital clock
}
/*
*/
class AnalogClock : public Widget, public Observer {
public:
    AnalogClock(ClockTimer*);
    virtual void Update(Subject*);
    virtual void Draw();
    // ...
};
/*
*/
ClockTimer* timer = new ClockTimer;
AnalogClock* analogClock = new AnalogClock(timer);
DigitalClock* digitalClock = new DigitalClock(timer);
/*
*/