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.

Answer 3

You have to declare the block variable as __block:

typedef void (^MyBlock)(id);

__block MyBlock block = ^(id param) {
  NSLog(@"%@", param);
  block(param);
};

Answer 4

There is no self for blocks (yet). You can build one like this (assuming ARC):

__block void (__weak ^blockSelf)(void);
void (^block)(void) = [^{
        // Use blockSelf here
} copy];
blockSelf = block;
    // Use block here

The __block is needed so we can set blockSelf to the block after creating the block. The __weak is needed because otherwise the block would hold a strong reference to itself, which would cause a strong reference cycle and therefore a memory leak. The copy is needed to make sure that the block is copied to the heap. That may be unnecessary with newer compiler versions, but it won't do any harm.