Objective C Class Method to control all existing instances?

Question

Is it possible to use the class methods of some NSObject I've made to controls all existing instances?

I want a delegate class to be able to send messages to the Object's class methods which can then react appropriately with everything out there.

Answer 1

Not without using some collection of the objects. The simplest way to do it would be to store the objects in an array and send them all a message when you want something to happen. Specifically:

static NSMutableArray *allObjects = nil;
+ (void) initialize {
    if (!allObjects) {
        allObjects = [NSMutableArray new];
    }
}
- (id) init {
    if (self = [super init]) {
        //Each object gets held in an array
        [allObjects addObject:self];
    }
    return self;
}
+ (void) doSomethingToEveryObject {
    [allObjects makeObjectsPerformSelector:@selector(doSomethingToThisObject)];
}
- (void) doSomethingToThisObject {
    NSLog(@"Did something to %@",self];
}
- (void) release {
    [super release];
    //When the retain count reaches 1, then all external references have disappeared,
    //and the only remaining reference is in the objects array.
    if (self.retainCount == 1) {
      [allObjects removeObject:self];
    }
}

Answer 2

Sure. Just have all the objects register for notifications when they're created. For example:

- (id)init {
    self = [super init];
    if (!self) return nil;

    [[NSNotificationCenter defaultCenter] addObserver:self
                                             selector:@selector(announceYourself:)
                                                 name:@"MYCLASS_ANNOUNCE"
                                               object:nil];
    return self;
}

+ (void)identifyInstances {
    [[NSNotificationCenter defaultCenter] postNotificationName:@"MYCLASS_ANNOUNCE" object:self];
}

- (void)announceYourself:(id)notification {
    NSLog(@"Instance %p reporting in!", self);
}

- (void)dealloc {
    [[NSNotificationCenter defaultCenter] removeObserver:self]; // VERY IMPORTANT -- IT WILL CRASH WITHOUT THIS
    [super dealloc];
}