Let’s add achievements to our game – We’ll do so with the Observer pattern.
Observers let one piece of code announce that something interesting happened, and letting listeners subscribe to it.
Let’s implement an observer that notifies anything at all when our character falls off a bridge.
void Physics::updateEntity(Entity& entity) {
bool wasOnSurface = entity.isOnSurface();
entity.accelerate(GRAVITY);
entity.update();
if (wasOnSurface && !entity.isOnSurface()) {
notify(entity, EVENT_START_FALL);
}
}Let’s create an Observer interface:
class Observer {
public:
virtual ~Observer() {}
virtual void onNotify(const Entity& entity, Event event) = 0;
};Now we’ll have our achievements class implement things:
class Achievements : public Observer {
public:
virtual void onNotify(const Entity& entity, Event event) {
switch(event) {
case EVENT_ENTITY_FELL:
if (entity.isHerO() && heroIsOnBridge_) {
unlock(ACHIEVEMENT_FELL_OFF_BRIDGE);
}
break;
// other events here
}
}
private:
void unlock(Achievement achievement) { /* Unlock if not unlcoked */}
bool heroIsOnBridge;
};The notification method is invoked by the object being observed. This is the subject.
We’ll keep a list that lets us store observers.
Next, send notifications.
class Subject {
public:
void addObserver(Observer* observer) { /* Add observer */ }
void removeObserver(Observer* observer) { /* Remove observer */ }
protected:
void notify(const Entity& entity, Event event) {
for (int i = 0; i < numObservers_; i++)
observers_[i]->onNotify(entity, event);
}
private:
Observer* observers_[MAX_OBSERVERS];
int numObservers_;
};Now, for our physics engine, we just have to inherit from Subject.
class Physics : public Subject {
public:
void updateEntity(Entity& entity);
};