How to subclass UIScrollView and make the delegate property private

Question

Here is what I want to achieve:

I want to subclass an UIScrollView to have additional functionality. This subclass should be able to react on scrolling, so i have to

Answer 1

Thanks @robmayoff I encapsulated to be a more generic delegate interceptor: Having original MessageInterceptor class:

MessageInterceptor.h

@interface MessageInterceptor : NSObject

@property (nonatomic, assign) id receiver;
@property (nonatomic, assign) id middleMan;

@end

MessageInterceptor.m

@implementation MessageInterceptor

- (id)forwardingTargetForSelector:(SEL)aSelector {
    if ([self.middleMan respondsToSelector:aSelector]) { return    self.middleMan; }
    if ([self.receiver respondsToSelector:aSelector]) { return self.receiver; }
    return [super forwardingTargetForSelector:aSelector];
}

- (BOOL)respondsToSelector:(SEL)aSelector {
    if ([self.middleMan respondsToSelector:aSelector]) { return YES; }
    if ([self.receiver respondsToSelector:aSelector]) { return YES; }
    return [super respondsToSelector:aSelector];
}

@end

It is used in your generic delegate class:

GenericDelegate.h

@interface GenericDelegate : NSObject

@property (nonatomic, strong) MessageInterceptor * delegate_interceptor;

- (id)initWithDelegate:(id)delegate;

@end

GenericDelegate.m

@implementation GenericDelegate

- (id)initWithDelegate:(id)delegate {
    self = [super init];
    if (self) {
        self.delegate_interceptor = [[MessageInterceptor alloc] init];
        [self.delegate_interceptor setMiddleMan:self];
        [self.delegate_interceptor setReceiver:delegate];
    }
    return self;
}

// delegate methods I wanna override:
- (void)scrollViewDidScroll:(UIScrollView *)scrollView {
    // 1. your custom code goes here
    NSLog(@"Intercepting scrollViewDidScroll: %f %f", scrollView.contentOffset.x, scrollView.contentOffset.y);

    // 2. forward to the delegate as usual
    if ([self.delegate_interceptor.receiver respondsToSelector:@selector(scrollViewDidScroll:)]) {
        [self.delegate_interceptor.receiver scrollViewDidScroll:scrollView];
    }
 }
//other delegate functions you want to intercept
...

So I can intercept any delegate I need to on any UITableView, UICollectionView, UIScrollView... :

@property (strong, nonatomic) GenericDelegate *genericDelegate;
@property (nonatomic, strong) UICollectionView* collectionView;

//intercepting delegate in order to add separator line functionality on this scrollView
self.genericDelegate = [[GenericDelegate alloc]initWithDelegate:self];
self.collectionView.delegate = (id)self.genericDelegate.delegate_interceptor;

- (void)scrollViewDidScroll:(UIScrollView *)scrollView {
    NSLog(@"Original scrollViewDidScroll: %f %f", scrollView.contentOffset.x, scrollView.contentOffset.y);
}

In this case UICollectionViewDelegate scrollViewDidScroll: function will be executed on our GenericDelegate (with any code we want to add) and on in the implementation of our own class

Thats my 5 cents, thanks to @robmayoff and @jhabbott previous answer