NSInvocation works for blocks too!

blockforward.m

#import <dlfcn.h>
#import <Foundation/Foundation.h>


struct BlockDescriptor
{
    unsigned long reserved;
    unsigned long size;
    void *rest[1];
};

struct Block
{
    void *isa;
    int flags;
    int reserved;
    void *invoke;
    struct BlockDescriptor *descriptor;
};

static void *BlockImpl(id block)
{
    return ((struct Block *)block)->invoke;
}

static const char *BlockSig(id blockObj)
{
    struct Block *block = (void *)blockObj;
    struct BlockDescriptor *descriptor = block->descriptor;
    
    int copyDisposeFlag = 1 << 25;
    int signatureFlag = 1 << 30;
    
    assert(block->flags & signatureFlag);
    
    int index = 0;
    if(block->flags & copyDisposeFlag)
        index += 2;
    
    return descriptor->rest[index];
}

@interface NSInvocation (PrivateHack)
- (void)invokeUsingIMP: (IMP)imp;
@end

typedef void (^BlockInterposer)(NSInvocation *inv, void (^call)(void));

@interface FakeBlock : NSObject
{
    int _flags;
    int _reserved;
    IMP _invoke;
    struct BlockDescriptor *_descriptor;
    
    id _forwardingBlock;
    BlockInterposer _interposer;
}

- (id)initWithBlock: (id)block interposer: (BlockInterposer)interposer;

@end

@implementation FakeBlock

- (id)initWithBlock: (id)block interposer: (BlockInterposer)interposer
{
    if((self = [super init]))
    {
        _forwardingBlock = [block copy];
        _interposer = [interposer copy];
        _invoke = [self methodForSelector: @selector(thisDoesNotExistOrAtLeastItReallyShouldnt)];
    }
    return self;
}

- (void)dealloc
{
    [_forwardingBlock release];
    [_interposer release];
    
    [super dealloc];
}

- (NSMethodSignature *)methodSignatureForSelector: (SEL)sel
{
    const char *types = BlockSig(_forwardingBlock);
    NSMethodSignature *sig = [NSMethodSignature signatureWithObjCTypes: types];
    while([sig numberOfArguments] < 2)
    {
        types = [[NSString stringWithFormat: @"%s%s", types, @encode(void *)] UTF8String];
        sig = [NSMethodSignature signatureWithObjCTypes: types];
    }
    return sig;
}

- (void)forwardInvocation: (NSInvocation *)inv
{
    [inv setTarget: _forwardingBlock];
    _interposer(inv, ^{
        [inv invokeUsingIMP: BlockImpl(_forwardingBlock)];
    });
}

@end

id ForwardingBlock(BlockInterposer interposer, id block)
{
    return [[[FakeBlock alloc] initWithBlock: block interposer: interposer] autorelease];
}

int main(int argc, char **argv)
{
    @autoreleasepool
    {
        void (^block)(int) = ForwardingBlock(^(NSInvocation *inv, void (^call)(void)) {
            [inv setArgument: &(int){ 4242 } atIndex: 1];
            call();
        }, ^(int testarg){
            NSLog(@"%d %d", argc, testarg);
        });
        block(42);
    }
}