5

Am not sure how to put this, and I couldn't find the answer because of my inability to find the words to express what am looking for. (!)

In Java, I used to do something like this (I don't remember):

JPanel myButton = new JPanel("Press me"){
    public void add(JComponent component){
        //override add method
    }
};

But, i couldn't find how to do this in Objective-C .. What I found in my search was categories and weird ^{} symbols ...

So, how can I override method(s) in a newly created object?

(For example, override -(BOOL)isEqual; in a newly created NSString* ?)

Am sorry if the question is a bit vague..

EDIT:

Obviously, without subclassing :)

EDIT:

Might as well post my problem in case someone has a better idea:

I have a few CCTransitions in COCOS2D, and I want to be notified when the transition ends .. The thing is, as soon as the transition ends, the -(void)finish; method is invoked (which is part of the CCTransition class structure)

I would really want to avoid subclassing the CCTransition class, and override the finish method to do my logic when the transition ends :)

EDIT:

-(void)onEnterTransitionDidFinish; ... I can't believe something as awesome as that existed and I haven't came across it while searching......

Which means, instead of subclassing CCTransition, override this method in my CCNode subclass :D!

Mazyod
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    Do you want to override a method in a class or just in one object (class instance)? – sergio Jun 02 '11 at 11:54
  • I'm going to answer to use subclass. I dont know other ways.. – Chanok Jun 02 '11 at 11:59
  • It is very possible and convenient in Java .. I thought objective-C was even more versatile ? – Mazyod Jun 02 '11 at 12:01
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    it can be done at runtime, you can even compile the method from a string if you like, but why not use a Subclass? This is exactly what a subclass is for. – hooleyhoop Jun 02 '11 at 12:06
  • I'll end up subclassing around 10 classes.. CCTransitionFadeIn, CCTransitionSlideIn, ...etc. And each one has it's own logic. Maintainability/Readability is a very important requirement for me. – Mazyod Jun 02 '11 at 12:09

3 Answers3

7

It's still not going to be very clean, but assuming you're willing to concentrate the ugliness, you could do something like (untested):

Method methodToReplace =
          [targetClass instanceMethodSignatureForSelector:@selector(methodToReplace)];
IMP implementationToSet =
          [someProxyClass instanceMethodForSelector:@selector(implementationYouWant)];

method_setImplementation(methodToReplace, implementationToSet);

Relevant reference documentation is the Objective-C Runtime Reference and, optionally, the NSObject Class Reference (because it makes a few things slightly neater, though e.g. you could use class_getInstanceMethod from the runtime rather than instanceMethodSigntureForSelector:).

Note that you'll have no way to call the original implementation if you use exactly that recipe. method_setImplementation returns the old implementation, it's generally wise to add that to the class under a brand new selector and call that instead.

For reference, I've had a legitimate reason to do this sort of thing only exactly once: when we implemented printing support in an iOS application with which needed to be compatible with both OS 3.2 and 4.0. You need to subclass a particular class, but the class isn't available in 3.2. So you sort of have to subclass at runtime (though the conceptually neater way would be to use a normal subclass, put that into a framework and weak link, but Apple's iOS SDK terms allow static libraries only, so...).

Tommy
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  • That's an interesting situation for needing runtime-created subclasses. I wonder if a slightly easier way would've been to dynamically create a dummy superclass at runtime early in the program execution so that you can define the subclass normally. Would that even work? – Daniel Dickison Jun 02 '11 at 12:29
  • I'm right on the fringe of my level of knowledge here, but I think the potential problem would be that the metaclass related to my subclass would be instantiated by the system when the binary is loaded. Since the parent class wouldn't exist then, I'd imagine I'd just get an exception and failure to launch at all. – Tommy Jun 02 '11 at 13:58
  • Seems you're right: `+load` runs even before `main()`. But, [according to Mike Ash](http://www.mikeash.com/pyblog/friday-qa-2009-05-22-objective-c-class-loading-and-initialization.html), the `+load` method of categories are executed along with the main class's implementation, so you might be able to put the hacky stuff in a category on `+[NSObject load]`... – Daniel Dickison Jun 02 '11 at 19:18
  • He mentions that frameworks are loaded first and +load is a special case (in that you can have multiple +loads for a class and all get called), I guess the question is whether the category +load will be called after the frameworks are loaded despite NSObject being necessary to load the frameworks. If so then you're right — a neater and better solution that would almost certainly work have been to add a dummy parent class to the runtime then and implement my printing subclass exactly as if I were targeting 4.0 only. It's less to cut out when we drop 3.2 support, if nothing else. – Tommy Jun 03 '11 at 11:36
3

Following Daniel's suggestion, you can implement a method in an NSObject category of the form

[anObject overrideMethod:@selector(foo:) 
          byBlock:^(id self,id super,id originalArg){
          ...
}];

What you need to do is to

  1. objc_allocateClassPair against self's own class, to create a new temporary class
  2. Turn a block into a function pointer, using e.g. this or this
  3. method_setImplementation to set the new implementation to the temporary class
  4. use object_setClass to self to set the class to the new temporary class
  5. I haven't figured out how to provide super to the block :p

It's believed this is basically how the KVO is done by Apple, see e.g. this discussion.

Read Runtime reference.

Community
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Yuji
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  • This would be very cool. I can't think of any way to provide `super`, but you could probably write a method or function that calls a parent's implementation of a method, like `-(id)callSuperMethod:(SEL)selector withObject:(id)arg1 ...`. – Daniel Dickison Jun 02 '11 at 19:08
  • Well, I guess you can write a trampoline method which sends the method to the super, as in: `[[self super] doSomething:...]`. – Yuji Jun 02 '11 at 20:32
2

What you have there in Java is an anonymous subclass. This is not possible in Objective-C (well, it sort of is but you would have to do some pretty involved contortions with the Obj-C runtime library).

But Objective-C as of iOS 4 or OS X 10.6 has "blocks", which is what the ^{} syntax is for. This is Objective-C's notion of a closure. This isn't going to help you directly if the APIs that you're calling don't support block callbacks, but you may be able to create wrapper classes that use blocks instead of subclassed methods to handle callbacks.

There are many resources for learning about blocks in Objective-C.

Abizern
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Daniel Dickison
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  • Aaah, anonymous inner class .. You just made me remember the good old days with Java :) ... Yes, that's the answer I needed "What you have there in Java is an anonymous subclass. This is not possible in Objective-C" .. Thanks! – Mazyod Jun 02 '11 at 12:27