Intercept obj-c delegate messages within a subclass

Question

I have a subclass of UIScrollView that 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

Yes, but you'll have to override every delegate method in the docs. Basically, make a second delegate property and implement the delegate protocol. When your delegate methods are called, take care of your business and then call the same method on your second delegate from the delegate method that was just run. E.g.

- (void)scrollViewDidScroll:(UIScrollView *)scrollView {
    // Do stuff here
    [self.delegate2 scrollViewDidScroll:scrollView];
}

Answer 2

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

@implementation MyScrollView

- (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 ([delegate_interceptor.receiver
         respondsToSelector:@selector(scrollViewDidScroll:)]) {
        [delegate_interceptor.receiver] 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.