Is there a SELF pointer for blocks?

Question

I'd like to recursively call a block from within itself. In an obj-c object, we get to use "self", is there something like this to refer to a block instance from inside itself?

Answer 1

Fun story! Blocks actually are Objective-C objects. That said, there is no exposed API to get the self pointer of blocks.

However, if you declare blocks before using them, you can use them recursively. In a non-garbage-collected environment, you would do something like this:

__weak __block int (^block_self)(int);
int (^fibonacci)(int) = [^(int n) {
    if (n < 2) { return 1; }
    return block_self(n - 1) + block_self(n - 2);
} copy];

block_self = fibonacci;

It is necessary to apply the __block modifier to block_self, because otherwise, the block_self reference inside fibonacci would refer to it before it is assigned (crashing your program on the first recursive call). The __weak is to ensure that the block doesn't capture a strong reference to itself, which would cause a memory leak.

Answer 2

The following recursive block code will compile and run using ARC, GC, or manual memory management, without crashing, leaking, or issuing warnings (analyzer or regular):

typedef void (^CountdownBlock)(int currentValue);

- (CountdownBlock) makeRecursiveBlock
{
    CountdownBlock aBlock;
    __block __unsafe_unretained CountdownBlock aBlock_recursive;
    aBlock_recursive = aBlock = [^(int currentValue)
    {
        if(currentValue >= 0)
        {
            NSLog(@"Current value = %d", currentValue);
            aBlock_recursive(currentValue-1);
        }
    } copy];
#if !__has_feature(objc_arc)
    [aBlock autorelease];
#endif

    return aBlock;
}

- (void) callRecursiveBlock
{
    CountdownBlock aBlock = [self makeRecursiveBlock];

    // You don't need to dispatch; I'm doing this to demonstrate
    // calling from beyond the current autorelease pool.
    dispatch_async(dispatch_get_main_queue(), ^
                   {
                       aBlock(10);
                   });
}

Important considerations:

• You must copy the block onto the heap manually or else it will try to access a nonexistent stack when you call it from another context (ARC usually does this for you, but not in all cases. Better to play it safe).
• You need TWO references: One to hold the strong reference to the block, and one to hold a weak reference for the recursive block to call (technically, this is only needed for ARC).
• You must use the __block qualifier so that the block doesn't capture the as-yet unassigned value of the block reference.
• If you're doing manual memory management, you'll need to autorelease the copied block yourself.