Real examples where NSProxy class is useful and why?

Question

I have been wondering why is NSProxy class so important. Why does an object need to keep its instance variables inside other objects? I need examples to understand when to use i

Answer 1

NSProxy is useful when there is a need for delegate interception, let's say you have some styled UISearchBar across your app, in which you remove the search icon when user starts typing, it means you need to listen UISearchBarDelegate method -searchBar:textDidChange: but this method is already listened by ViewController which performs searching, to avoid code duplications you don't want to copy-paste hiding icon logic in every your ViewController. To solve this problem you can create NSProxy which will have reference to your ViewController as originalDelegate and your hiding search icon helper as middleMan, then in your NSProxy instance you need implement following methods:

- (void)forwardInvocation:(NSInvocation *)invocation
{
    if ([self.middleMan respondsToSelector:invocation.selector])
    {
        //Note: probably it's better to provide a copy invocation
        [invocation invokeWithTarget:self.middleMan];
    }

    if ([self.originalDelegate respondsToSelector:invocation.selector])
    {
        [invocation invokeWithTarget:self.originalDelegate];
    }
}

- (NSMethodSignature *)methodSignatureForSelector:(SEL)sel
{
    id result = [self.originalDelegate methodSignatureForSelector:sel];
    if (!result) {
        result = [self.middleMan methodSignatureForSelector:sel];
    }

    return result;
} 

And set your proxy instance to searchBar delegate: searchBar.delegate = proxy