Using Super in an Objective C Category?

Question

I'd like to override a method in an Objective C class that I don't have the source to.

I've looked into it, and it appears that Categories should allow me to do this, but I'd like to use the result of the old method in my new method, using super to get the old methods result.

Whenever I try this though, my method gets called, but "super" is nil... Any idea why? I'm doing iPhone development with the XCode 2.2 SDK. I'm definitely working with an instance of a class, and the method of the class is an instance method.

@implementation SampleClass (filePathResolver)
-(NSString*) fullPathFromRelativePath:(NSString*) relPath
{
    NSString *result = [super fullPathFromRelativePath: relPath];

  ... do some stuff with the old result

    return result;
}

Note and clarification: From what I can see in the Apple Docs, it appears to me that this should be allowed?

Categories docs at developer.apple.com: When a category overrides an inherited method, the method in the category can, as usual, invoke the inherited implementation via a message to super. However, if a category overrides a method that already existed in the category's class, there is no way to invoke the original implementation.

Answer 1

Categories extend the original class, but they don't subclass it, therefore a call to super doesn't find the method.

What you want is called Method Swizzling. But be aware that your code could break something. There's an article on Theocacao written by Scot Stevenson about Method Swizzling in the old Objective-C runtime, Cocoa with Love by Matt Gallagher has an article about Method Swizzling in the new Objective-C 2.0 runtime and a simple replacement for it.

Alternatively, you could subclass the class and then either use the subclass or use + (void)poseAsClass:(Class)aClass to replace the superclass. Apple writes:

A method defined by a posing class can, through a message to super, incorporate the superclass method it overrides.

Be aware that Apple has deprecated poseAsClass: in Mac OS X 10.5.

Answer 2

If you will be coding against this class, simply rename the selector to something your code can use, and call the original selector on self:

@implementation SampleClass (filePathResolver)
-(NSString*) myFullPathFromRelativePath:(NSString*) relPath
{
    NSString *result = [self fullPathFromRelativePath: relPath];

  ... do some stuff with the old result

    return result;
}

If you want to override the default implementation of this selector for that class, you'll need to use the method swizzling approach.

Answer 3

Not exactly in category but there is a workaround by adding the method dynamically at runtime. Samuel Défago in his article describes a neat way to create block IMP implementation calling super, his original article can be found here

The relevant code is:

#import <objc/runtime.h>
#import <objc/message.h>

    const char *types = method_getTypeEncoding(class_getInstanceMethod(clazz, selector));
    class_addMethod(clazz, selector, imp_implementationWithBlock(^(__unsafe_unretained id self, va_list argp) {
        struct objc_super super = {
            .receiver = self,
            .super_class = class_getSuperclass(clazz)
        };

        id (*objc_msgSendSuper_typed)(struct objc_super *, SEL, va_list) = (void *)&objc_msgSendSuper;
        return objc_msgSendSuper_typed(&super, selector, argp);
    }), types);