Observer pattern implementation

Question

I had a hard time understand exactly what an observer pattern was, but I produced the following code for my project. It uses SDL. I am using the boost library to implement signals and therefore implementing my observer pattern. Does this look correct?

/* This is setting up our signal for sending observations */ boost::signals2::signal<void (char, int, int)> sig; /* Subjects the Observer will connect with */ sig.connect(&setChest); sig.connect(&setNonTraverse); sig.connect(&setEntry); sig.connect(&setExit); std::cout << "Waiting for user-interaction. Press on the 'X' to quit" << std::endl; while ( !quit ) { status = SDL_WaitEvent(&event); //wait for an event to occur switch (event.type) { //check the event type case SDL_KEYDOWN: //Check if a key was pressed. key = SDL_GetKeyName(event.key.keysym.sym); break; case SDL_MOUSEBUTTONUP: sig(key[0],event.button.x/32,event.button.y/32); break; case SDL_QUIT: // Click on the 'X' to close the window. exit ( 1 ); break; } } //while return true; }

The description from Wikipedia says: "The observer pattern (a subset of the publish/subscribe pattern) is a software design pattern in which an object, called the subject, maintains a list of its dependents, called observers, and notifies them automatically of any state changes, usually by calling one of their methods." Isn't this what I am doing? Thanks for your input though! — Gregorio Di Stefano
– Gregorio Di Stefano, Commented Nov 4, 2011 at 1:29
Your code looks fine, for an initial go. It can certainly be improved upon, but you're on the right track. — Dennis Zickefoose
– Dennis Zickefoose, Commented Nov 4, 2011 at 1:54

score 2 · Accepted Answer · 2011-11-04 01:45:53Z

Your posted code is that of the Observer.

In the Observer pattern, the observer doesn't react directly to the subjects' state changes. Instead, the subject informs the observer of any changes by invoking the observer's callback. This is why the observer must register with the subject, instead of merely polling (checking the state in a while loop) the subject.

I'm not too familiar with C++, but here is some Java-like pseudocode that outlines the basic idea:

class Observer{ public Observer(Subject subject){ subject.register(this); } public void updateFromSubject(Subject subject){ //respond to change } } class Subject{ List<Observer> observers; public void register(Observer observer){ observers.add(observer); } private void notifyObservers(){ for(Observer obs : observers){ obs.updateFromSubject(this); } } public void changeStateToNewState(Object o){ .... //business logic notifyObservers(); }

Notice the lack of a while loop, which means that the observer simply doesn't do any work until an actual event occurs, instead of checking a flag a million times a second just to see if it changed.

Hmm, can you point me into the right direction? How would I change my current code? Aren't the subjects functions that rely on what is being observed?
no, the observers are the classes that are registered to the subject. observers are to subjects what subscribers are to a newspaper. so basically when something important changes in the subject, the observers will know it.

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