Recursive Blocks in Objective-C leaking in ARC

automatic-ref-counting, instruments, objective-c, objective-c-blocks

Solution

Ok, I found the answer on my own...but thanks to those who tried to help.

If you're referencing/using other blocks in a recursive block, you must pass them in as weak variables. Of course, __weak only applies to block pointer types, so you must typedef them first. Here's the final solution:

    typedef void (^IntBlock)(int);

    IntBlock __weak Log = ^(int i){
        NSLog(@"log sub %i", i);
    };

    __block void (^LeakingBlock)(int) = ^(int i){
        Log(i);
        if (i < 100) LeakingBlock(++i);
    };
    LeakingBlock(1);

The above code doesn't leak.

Problem

So I'm using recursive blocks. I understand that for a block to be recursive it needs to be preceded by the __block keyword, and it must be copied so it can be put on the heap. However, when I do this, it is showing up as a leak in Instruments. Does anybody know why or how I can get around it? Please note in the code below I've got references to a lot of other blocks, but none of them are recursive. ``` __block NSDecimalNumber *(^ProcessElementStack)(LinkedList *, NSString *) = [^NSDecimalNumber *(LinkedList *cformula, NSString *function){ LinkedList *list = [[LinkedList alloc] init]; NSDictionary *dict; FormulaType type; while (cformula.count > 0) { dict = cformula.pop; type = [[dict objectForKey:@"type"] intValue]; if (type == formulaOperandOpenParen || type == formulaListOperand || type == formulaOpenParen) [list add:ProcessElementStack(cformula, [dict objectForKey:@"name"])]; else if (type == formulaField || type == formulaConstant) [list add:NumberForDict(dict)]; else if (type == formulaOperand) [list add:[dict objectForKey:@"name"]]; else if (type == formulaCloseParen) { if (function){ if ([function isEqualToString:@"AVG("]) return Average(list); if ([function isEqualToString:@"MIN("]) return Minimum(list); if ([function isEqualToString:@"MAX("]) return Maximum(list); if ([function isEqualToString:@"SQRT("]) return SquareRoot(list); if ([function isEqualToString:@"ABS("]) return EvaluateStack(list).absoluteValue; return EvaluateStack(list); } else break; } } return EvaluateStack(list); } copy]; NSDecimalNumber *number = ProcessElementStack([formula copy], nil); ``` UPDATE So in my own research I've discovered that the problem apparently does have to do with the references to the other blocks this block uses. If I do something simple like this, it doesn't leak: ``` __block void (^LeakingBlock)(int) = [^(int i){ i++; if (i < 100) LeakingBlock(i); } copy]; LeakingBlock(1); ``` However, if I add a another block in this, it does leak: ``` void (^Log)(int) = ^(int i){ NSLog(@"log sub %i", i); }; __block void (^LeakingBlock)(int) = [^(int i){ Log(i); i++; if (i < 100) LeakingBlock(i); } copy]; LeakingBlock(1); ``` I've tried using the __block keyword for Log() and also tried copying it, but it still leaks. Any ideas? UPDATE 2 I found a way to prevent the leak, but it's a bit onerous. If I convert the passed in block to a weak id, and then cast the weak id back into a the block type, I can prevent the leak. ``` void (^Log)(int) = ^(int i){ NSLog(@"log sub %i", i); }; __weak id WeakLogID = Log; __block void (^LeakingBlock)(int) = [^(int i){ void (^WeakLog)(int) = WeakLogID; WeakLog(i); if (i < 100) LeakingBlock(++i); } copy]; LeakingBlock(1); ``` Surely there's a better way?

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