Add Methods at Runtime in Objective-C

Andy Finnel posted some ideas for Objective-C features beyond those slated for Leopard. One of them is adding methods at runtime. It turns out this has been possible in Mac OS X since day one, it's just not obvious how to do so.

Here's what Andy says:

My previous post about Key-Value Coding and being able dynamically add transient attributes to NSManagedObject got me thinking. Why can’t I dynamically add methods? I don’t mean on just NSManagedObjects, I mean on NSObjects. I realize that categories allow that somewhat, but only as a group of functions and only at compile time.

First thing to clear up here is that categories can be loaded at runtime. Quoth the Bundle Loading documentation:

The executable code files in loadable bundles hold class (and category) definitions that the NSBundle object can dynamically load while the application runs.

But, come on, that's the easy way out. Let's dig into the Objective-C runtime. There are two structures we need to deal with:

struct objc_method {
  SEL method_name;
  char *method_types;
  IMP method_imp;
};

struct objc_method_list {
  struct objc_method_list *obsolete;
  int method_count;
  struct objc_method method_list[1];
};

This looks kinda scary, but it's actually not too bad. The steps are basically:

0. Make a function
1. Create an objc_method instance
2. Register the function name
3. Give the objc_method a pointer to the function
4. Add the objc_method to a objc_method_list
5. Pass the objc_method_list to class_addMethods
6. There is no step 6!

So here's all the code to do that:

#import <objc/objc-class.h>

// create a class with no methods
@interface EmptyClass : NSObject { }
@end

@implementation EmptyClass
@end

// define the function to add as a method
id sayHello ( id self, SEL _cmd,... )
{
  NSLog (@"Hello");
}

void addMethod ()
{
  // create the method

  struct objc_method myMethod;
  myMethod.method_name = sel_registerName("sayHello");
  myMethod.method_imp  = sayHello;
  
  // build the method list.
  // this memory needs to stick around as long as the
  // methods belong to the class.

  struct objc_method_list * myMethodList;
  myMethodList = malloc (sizeof(struct objc_method_list));
  myMethodList->method_count = 1;
  myMethodList->method_list[0] = myMethod;
  
  // add method to the class
  class_addMethods ( [EmptyClass class], myMethodList );
  
  // try it out
  EmptyClass * instance = [[EmptyClass alloc] init];
  [instance sayHello];
  [instance release];
}

Now let me be clear that I don't think is fun or easy, but it is possible. It also wouldn't be too hard to create some sort of wrapper API to do this. In any case, there are only about seven lines of code that involve actually adding the method.

Also, please don't use the above in the production code. It's a proof of concept only. It uses local variables, imperfect error checking, and so on. If you need real examples, check out the source code for PyObjC.

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