// Return a sequence for photos
    [[[[[[RACObserve(self, event.photos) filter:^BOOL(id value) { return value != nil ; }] flattenMap:^RACStream *(NSDictionary *photos)
        {
        NSLog(@"Got photos: %@" , photos) ;
        return photos.rac_sequence.signal ;
        }]

    // Consider each photo
    filter:^BOOL(NSDictionary *photoDescriptor)
        {
        NSLog(@"Descriptor: %@" , photoDescriptor) ;
        return ((NSNumber *)photoDescriptor[@"primary"]).boolValue ;
        }]

    // Load the selected photo
    map:^id(NSDictionary *selectedPhotoDescriptor)
        {
        NSLog(@"Photo URL: %@" , selectedPhotoDescriptor[@"url"]) ;
        return [[AsyncImageFetcher imageAtURL:[NSURL URLWithString:selectedPhotoDescriptor[@"url"]] cache:YES] firstOrDefault:[UIImage imageNamed:@"detail_placeholder"]] ;
        }]

    // Deliver on main thread
    deliverOn:RACScheduler.mainThreadScheduler]

    subscribeNext:^(id x)
        {
        ((UIImageView *)self.headerView).image = x ;
        }] ;

RAC( ((UIImageView *)self.headerView), image ) =

    // Return a sequence for photos
    [[[[[RACObserve(self, event.photos) filter:^BOOL(id value) { return value != nil ; }] flattenMap:^RACStream *(NSDictionary *photos)
        {
        NSLog(@"Got photos: %@" , photos) ;
        return photos.rac_sequence.signal ;
        }]

    // Consider each photo
    filter:^BOOL(NSDictionary *photoDescriptor)
        {
        NSLog(@"Descriptor: %@" , photoDescriptor) ;
        return ((NSNumber *)photoDescriptor[@"primary"]).boolValue ;
        }]

    // Load the selected photo
    map:^id(NSDictionary *selectedPhotoDescriptor)
        {
        NSLog(@"Photo URL: %@" , selectedPhotoDescriptor[@"url"]) ;
        return [[AsyncImageFetcher imageAtURL:[NSURL URLWithString:selectedPhotoDescriptor[@"url"]] cache:YES] firstOrDefault:[UIImage imageNamed:@"detail_placeholder"]] ;
        }]

    // Deliver on main thread
    deliverOn:RACScheduler.mainThreadScheduler] ;

// When there's a new image, fetch it, and set the headerView (which by default is an UIImageView)
RAC( self, imageView.image ) =

    // Return a sequence for photos
    [[[[RACObserve(self, event.photos) ignore:nil] flattenMap:^RACStream *(NSDictionary *photos)
        {
        NSLog(@"Got photos: %@" , photos) ;
        return photos.rac_sequence.signal ;
        }]

    // Consider each photo
    filter:^BOOL(NSDictionary *photoDescriptor)
        {
        NSLog(@"Descriptor: %@" , photoDescriptor) ;
        return ((NSNumber *)photoDescriptor[@"primary"]).boolValue ;
        }]

    // Load the selected photo
    flattenMap:^RACStream *(NSDictionary *selectedPhotoDescriptor)
        {
        NSLog(@"selected photo desc: %@" , selectedPhotoDescriptor) ;
        return [AsyncImageFetcher imageAtURL:[NSURL URLWithString:selectedPhotoDescriptor[@"url"]] cache:YES] ; // This will -deliverOn: the main thread
        }] ;

@synthesize imageView ;
- (UIImageView *) imageView
{
return (id) self.headerView ;
}

RAC( self.imageView, image ) = …

RAC( self, imageView.image ) = …

RAC( self, (((UIImageView *)headerView).image) ) = …