多重 delegate，有別於一般的一對一 delegate，使用 weak 避免 reference cycle。大家都不熟悉所以採用 NSNotifications

GeneralService.h

@protocol ServiceDelegate <NSObject>

@optional
- (void)generalService:(GeneralService *)service didRetrieveEntries:(NSArray *)entries;

@end

@interface GeneralService : NSObject

- (void)registerDelegate:(id<ServiceDelegate>)delegate;
- (void)deregisterDelegate:(id<ServiceDelegate>)delegate;

@end

GeneralService.m

@interface GeneralService ()

@property (nonatomic, strong) NSMutableSet *delegates;

@end

@implementation ZOCGeneralService
- (void)registerDelegate:(id<ServiceDelegate>)delegate {
    if ([delegate conformsToProtocol:@protocol(ServiceDelegate)]) {
        [self.delegates addObject:[[WeakObject alloc] initWithObject:delegate]];
    }
}

- (void)deregisterDelegate:(id<ServiceDelegate>)delegate {
    if ([delegate conformsToProtocol:@protocol(ServiceDelegate)]) {
        [self.delegates removeObject:[[WeakObject alloc] initWithObject:delegate]];
    }
}

- (void)_notifyDelegates {
    ...
    for (WeakObject *object in self.delegates) {
        if (object.object) {
            if ([object.object respondsToSelector:@selector(generalService:didRetrieveEntries:)]) {
                [object.object generalService:self didRetrieveEntries:entries];
            }
        }
    }
}

@end

WeakObject.h

@interfact WeakObject : NSObject

@property (nonatomic, readonly, weak) id object;

+ (instancetype)weakObjectWithObject:(id)object;
- (instancetype)initWithObject:(id)object;

@end

WeakObject.m

@interface ZOCWeakObject ()

@property (nonatomic, weak) id object;

@end

@implementation ZOCWeakObject

+ (instancetype)weakObjectWithObject:(id)object {
    return [[[self class] alloc] initWithObject:object];
}

- (instancetype)initWithObject:(id)object {
    if ((self = [super init])) {
        _object = object;
    }
    return self;
}

- (BOOL)isEqual:(id)object {
    if (self == object) {
        return YES;
    }

    if (![object isKindOfClass:[object class]]) {
        return NO;
    }

    return [self isEqualToWeakObject:(WeakObject *)object];
}

- (BOOL)isEqualToWeakObject:(WeakObject *)object {
    if (!object) {
        return NO;
    }

    BOOL objectsMatch = [self.object isEqual:object.object];
    return objectsMatch;
}

- (NSUInteger)hash {
    return [self.object hash];
}

@end