Checking Objective-C block type?

Question

This is primarily a curiosity, I'm not really sure what's the practical use of this but here goes.

Since blocks are also Objective-C objects, is it possible to check their type? That is, does it respond to the isKindOfClass: message and how to use that message with respect to blocks?

My naive thought that it's probably like this:

-(void) aMethod {     typedef int (^BlockA)(int x, int y);     id blockVar = ...; // get a block from somewhere     if([blockVar isKindOfClass:BlockA]) {         BlockA blockVarA = blockVar;         int result = blockVarA(1,2);     } } 

The code above probably won't work. But if it is possible to check a block's type, what is the correct way to do it?

Answer 1

Can do, kinda sorta.

But first, let's disambiguate. -[NSObject isKindOfClass:] can tell you it's a block, and that's about it. E.g. I believe this line of code -- ostensibly & unfortunately A BAD IDEA -- will return YES for blocks on present Lion & iOS 5.x:

[myBlock isKindOfClass:NSClassFromString(@"NSBlock")] 

That won't help you distinguish the block's function signature.

But it can be done, by snagging the signature from the block's documented internal struct. Code follows for an example OS X command-line app, much of which ripped from Mike Ash's MABlockClosure (great detailed explanation). (UPDATE: Github project CTObjectiveCRuntimeAdditions also apparently provides library code for just this purpose.)

#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 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]; }  int main(int argc, const char * argv[]) {     @autoreleasepool {          int (^block)(NSNumber *) = ^(NSNumber *num) {              NSLog(@"%@ %@", NSStringFromClass([num class]), num);              return [num intValue];          };         NSLog(@"signature %s", BlockSig(block));         NSLog(@"retval %d", (int)block([NSNumber numberWithInt:42]));     }     return 0; } 

Run this and you should get something like:

[58003:403] signature i16@?0@8 [58003:403] __NSCFNumber 42 [58003:403] retval 42 

The numbers in the signature (I'm told they are offsets) can be stripped for simpler i@?@.

The signature is in the @encode format, which isn't perfect (e.g. most objects map to same @), but should afford you some ability to distinguish blocks with different signatures at runtime.

While it's not documented in the Apple link, my testing points to @? being the code for a block type, which makes sense of the signature above. I found a clang-developers discussion on this issue which seems to back this up.

Answer 2

The "BlockA" in (^BlockA) is the variable name (in this case a typedef), not its class.
Blocks are objects, but not regular subclasses of NSObject. They only implement a subset of the methods. -isKindOfClass: will probably just crash.
Blocks are of the type NSMallocBlock or NSConcreteGlobalBlock, ... depending on where they were created (heap, stack, ...).