How can code inside an Objective-C block reference the block object itself?

Question

self is merely a captured variable inside a block and doesn't reference the block itself, so how does a block reference itself without having an explicit captured variable for that purpose?

Answer 1

__block void(^strawberryFields)();
strawberryFields = [^{ strawberryFields(); } copy];
dispatch_async(dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT,0),
               strawberryFields);

• you use the __block because the block will make a copy of the value of strawberryFields when the block is created which will be before the assignment.

• you also must copy the block prior to any other copy operation or else you'll end up with a block that references the on-stack original version.

• note that the above code leaks the block. Somewhere, there needs to be a release of that block to balance the copy.

Answer 2

I found this pattern to work and stable for ARC (automatic reference counting), both in Debug and Release builds.

-(void) someMethod
{
    // declare a __block variable to use inside the block itself for its recursive phase.
    void __block (^myBlock_recurse)();

    // define the block
    void (^myBlock)() = ^{
        // ... do stuff ...
        myBlock_recurse(); // looks like calling another block, but not really.
    };

    // kickstart the block
    myBlock_recurse = myBlock; // initialize the alias
    myBlock(); // starts the block
}

Initially I tried just putting a __block modifier to myBlock and use that variable directly to recurse within the block's implementation. That works on the ARC Debug build but breaks with an EXC_BAD_ACCESS on the Release build. On the other hand removing the __block modifier raises a "variable not defined when captured by block" warning (and I was reluctant to run it and test).