Is it possible to create a category of the “Block” object in Objective-C

Question

I would like to add functions by creating a category for Objective-C Blocks.

__block int (^aBlock)(int) = ^int( int n ){
    if( n <= 1 ) return n;
    return         


        

Answer 1

@pwc is correct in that you can't create a category for a class that you can't see.

However...

WHAT I AM ABOUT TO TELL YOU SHOULD BE USED STRICTLY AS AN EXERCISE IN LEARNING, AND NEVER IN ANY SORT OF PRODUCTION SETTING.

1. Some runtime introspection reveals some interesting information. There are a number of classes that contain the word "Block". Some of them look promising: __NSStackBlock, __NSMallocBlock, __NSAutoBlock, and NSBlock.
2. Some more introspection shows that the promising classes inherit from NSBlock

So it looks like any block is going to be some instance or subclass of NSBlock.

You can create a method on an object, like so:

@implementation Foo
- (void) doFoo {
  //do something awesome with self, a block
  //however, you can't do "self()".  
  //You'll have to cast it to a block-type variable and use that
}
@end

Then at runtime, you can move that method to the NSBlock class:

Method m = class_getInstanceMethod([Foo class], @selector(doFoo));
IMP doFoo = method_getImplementation(m);
const char *type = method_getTypeEncoding(m);
Class nsblock = NSClassFromString(@"NSBlock");
class_addMethod(nsblock, @selector(doFoo), doFoo, type);

After this, blocks should respond to the doFoo message.

USE AT YOUR OWN RISK, AND ONLY FOR EXPERIMENTING.