A quick follow up on http://macoscope.com/blog/mvc-layer-interaction-architecture/ to clarify ModelManager usage.

gistfile1.m

// Since ModelManager is usually a simple facade for many other smaller managers, it can dispatch most method calls to its subcomponents based on which protocol includes the selector.

BOOL ProtocolIncludesSelector (Protocol *protocol, SEL selector)
{
  struct objc_method_description methodDescription = protocol_getMethodDescription(protocol, selector, YES, YES);
  
  return NULL != methodDescription.name;
}


@implementation ModelManager

...

- (instancetype)init
{
  self = [super init];

  if (self) {
    self.dataManager = [[DataManager alloc] initWithUpdatePropagator:self];
    self.remoteConnectionManager = [[RemoteConnectionManager alloc] initWithDataManager:self.dataManager];
  }
  
  return self;
}

- (id)forwardingTargetForSelector:(SEL)selector
{
  if (ProtocolIncludesSelector(@protocol(DataManagerOperations), selector)) {
    return self.dataManager;
    
  } else if (ProtocolIncludesSelector(@protocol(RemoteConnectionManagerOperations), selector)) {
    return self.remoteConnectionManager;
  
  } else if (...) {
    ...

  } else {
    return nil;
  }
}

...

@end


// The submanagers can then answer any concrete data access requests.

@protocol DataManagerOperations <NSObject>

- (UIImage *)avatarImageForEmailAddress:(NSString *)emailAddress;

...

@end


@protocol RemoteConnectionManagerOperations <NSObject>

- (void)requestAvatarImageForEmailAddress:(NSString *)emailAddress;

...

@end


@implementation DataManager

...

- (void)saveAvatarImage:(UIImage *)image forEmailAddress:(NSString *)emailAddress
{
  self.avatarImagesForEmailAddresses[emailAddress] = image;
  
  [self.updatePropagator propagateUpdate:[ModelUpdate avatarImageUpdateForEmailAddress:emailAddress]];
}

- (UIImage *)avatarImageForEmailAddress:(NSString *)emailAddress
{
  return self.avatarImagesForEmailAddresses[emailAddress];
}

...

@end


@implementation RemoteConnectionManager

...

- (void)requestAvatarImageForEmailAddress:(NSString *)emailAddress
{
  if ([self.dataManager hasAvatarImageForEmailAddress:emailAddress]) {
    return;
  }

  [self sendRemoteAvatarImageRequestForEmailAddress:emailAddress withCompletionBlock:^(UIImage *image) {
    [self.dataManager saveAvatarImage:image forEmailAddress:emailAddress];
  }];
}

...

@end


@implementation SomeViewController

...

- (void)viewDidLoad
{
  [super viewDidLoad];
  
  [self setupAvatarView];
  [[ModelManager sharedInstance] requestAvatarImageForEmailAddress:self.emailAddress];
}

- (void)modelUpdated:(ModelUpdate *)update
{
  if (update.type == ModelUpdateTypeAvatarImage && [update.emailAddres isEqualToString:self.emailAddress]) {
    [self setupAvatarView];
  }
}

- (void)setupAvatarView
{
  self.avatarView.image = [[ModelManager sharedInstance] avatarImageForEmailAddress:self.emailAddress];
}

...

@end


// These examples are very simplified, but hopefully enough to clarify the whole concept.