Intercept Objective-C delegate messages within a subclass

Question

I have a subclass of UIScrollView in which I need to internally respond to scrolling behaviour. However, the viewcontroller will still need to listen to scrolling delegate callbacks, so I can't outright steal the delegate within my component.

Is there a way to keep the property named "delegate" and just listen to messages sent along it, or else somehow internally hijack the delegate property and forward messages outward after running some code?

Answer 1

To avoid overriding all of the delegate methods manually, you can use message forwarding. I just implemented the same thing using an intermediate proxy class as follows:

MessageInterceptor.h

@interface MessageInterceptor : NSObject {
    id receiver;
    id middleMan;
}
@property (nonatomic, assign) id receiver;
@property (nonatomic, assign) id middleMan;
@end

MessageInterceptor.m

@implementation MessageInterceptor
@synthesize receiver;
@synthesize middleMan;

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

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

@end

MyScrollView.h

#import "MessageInterceptor.h"

@interface MyScrollView : UIScrollView {
    MessageInterceptor * delegate_interceptor;
    //...
}

//...

@end

MyScrollView.m (Edited, with thanks to jhabbott):

@implementation MyScrollView

- (id)delegate { return delegate_interceptor.receiver; }

- (void)setDelegate:(id)newDelegate {
    [super setDelegate:nil];
    [delegate_interceptor setReceiver:newDelegate];
    [super setDelegate:(id)delegate_interceptor];
}

- (id)init* {
    //...
    delegate_interceptor = [[MessageInterceptor alloc] init];
    [delegate_interceptor setMiddleMan:self];
    [super setDelegate:(id)delegate_interceptor];
    //...
}

- (void)dealloc {
    //...
    [delegate_interceptor release];
    //...
}

// delegate method override:
- (void)scrollViewDidScroll:(UIScrollView *)scrollView {
    // 1. your custom code goes here
    // 2. forward to the delegate as usual
    if ([self.delegate respondsToSelector:@selector(scrollViewDidScroll:)]) {
        [self.delegate scrollViewDidScroll:scrollView];
    }
}

@end

With this approach, the MessageInterceptor object will automatically forward all delegate messages to the regular delegate object, except for the ones that you override in your custom subclass.