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 aBlock( n - 1 ) + aBlock( n - 2 );
};

Instead of just allowing the normal [aBlock copy], [aBlock retain], [aBlock release], [aBlock autorelease]. I could do thing like:

[aBlock mapTo:anArray];

Possible Category

@interface UnknownBlockClass (map)

- (NSArray *)mapTo:(NSArray *)array_;

@end

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.

Answer 2

A block winds up being an instance of type __NSGlobalBlock__, as seen in the following snippet:

    void (^aBlock)(void) = ^(void) {
        NSLog(@"Hello world");
    };

    // prints "type = __NSGlobalBlock__"
    NSLog(@"type = %@", [aBlock class]);

In order to create a category of a class, the compiler needs to be able to see the original @interface declaration of the class. I can't find the declaration for __NSGlobalBlock__ and probably for good reason.

This article and this article contain some useful information about the implementation of blocks.

To your original point, why not just make a category of NSArray for your mapTo method? It seems like a better place for that sort of functionality.

Updated

Let's say you can add a category to the Block object. How would you invoke the block from the category's method? To the best of my understanding, the only way to invoke a block is via the () operator (e.g., aBlock()). I don't think there's a way to tell from the Block object the number and types of parameters. So, what arguments would you pass in to the block invocation?

I'm not recommending you do this, but the following works...

@interface NSObject (BlockExtension)
- (void)foo;
@end

@implementation NSObject (BlockExtension)
- (void)foo
{
    // not sure how else to determine if self is a Block since neither
    // __NSGlobalBlock__ nor any of its superclasses (except NSObject) 
    // are accessible to the compiler
    if ([[[self class] description] isEqual:@"__NSGlobalBlock__"])
    {
        NSLog(@"foo");
        // now what?
        // can't call self(), it doesn't compile
        // how else can I invoke this block?
    }
}
@end

...

void (^aBlock)(void) = ^(void) {
    NSLog(@"Hello world");
};

// prints "foo"
[aBlock foo];