Observer
Sample Code
An abstract class defines the Observer interface:
class Subject;
class Observer {
public:
virtual ~ Observer();
virtual void Update(Subject* theChangedSubject) = 0;
protected:
Observer();
};
This implementation supports multiple subjects for each observer. The
subject passed to the Update operation lets the observer
determine which subject changed when it observes more than one.
Similarly, an abstract class defines the Subject interface:
class Subject {
public:
virtual ~Subject();
virtual void Attach(Observer*);
virtual void Detach(Observer*);
virtual void Notify();
protected:
Subject();
private:
List *_observers;
};
void Subject::Attach (Observer* o) {
_observers->Append(o);
}
void Subject::Detach (Observer* o) {
_observers->Remove(o);
}
void Subject::Notify () {
ListIterator i(_observers);
for (i.First(); !i.IsDone(); i.Next()) {
i.CurrentItem()->Update(this);
}
}
ClockTimer is a concrete subject for storing and
maintaining the time of day. It notifies its observers every second.
ClockTimer provides the interface for retrieving individual
time units such as the hour, minute, and second.
class ClockTimer : public Subject {
public:
ClockTimer();
virtual int GetHour();
virtual int GetMinute();
virtual int GetSecond();
void Tick();
};
The Tick operation gets called by an internal timer at
regular intervals to provide an accurate time base. Tick
updates the ClockTimer's internal state and calls
Notify to inform observers of the change:
void ClockTimer::Tick () {
// update internal time-keeping state
// ...
Notify();
}
Now we can define a class DigitalClock that displays the
time. It inherits its graphical functionality from a Widget
class provided by a user interface toolkit. The Observer interface is
mixed into the DigitalClock interface by inheriting from
Observer.
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);
}
Before the Update operation draws the clock face, it checks
to make sure the notifying subject is the clock's subject:
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
}
An AnalogClock class can be defined in the same way.
class AnalogClock : public Widget, public Observer {
public:
AnalogClock(ClockTimer*);
virtual void Update(Subject*);
virtual void Draw();
// ...
};
The following code creates an AnalogClock and a
DigitalClock that always show the same time:
ClockTimer* timer = new ClockTimer;
AnalogClock* analogClock = new AnalogClock(timer);
DigitalClock* digitalClock = new DigitalClock(timer);
Whenever the timer ticks, the two clocks will be updated
and will redisplay themselves appropriately.