Question

I’m totally stumped with this one and hope someone could help me out here:

Class A:

- (void)setBlock:(BOOL(^)(id sender))block {
    myBlock = Block_copy(block);
}

- (BOOL)runBlock:(id)sender {
    myBlock(sender);
}

Class B:

- (void)applicationDidFinishLaunching:(NSNotification *)aNotificationx {
    //The outer block provides behaviour according to strategy pattern:
    [classAInstance setBlock:^BOOL(id sender) {
        NSOperationQueue *queue = [[[NSOperationQueue alloc] init] autorelease];
        //The inner block is a special case of behaviour where I want the task to run asynchronously:
        [queue addOperationWithBlock:^(void) {
            NSLog(@"sender: %@", [sender class]);
            [sender doSomething];
        }];
        return YES;
    }];
}

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Then later when a GUI event causes classAInstance to call - (BOOL)runBlock;
(which it is supposed to be) I get the following crash stack:

    0 objc_exception_throw
    3 __forwarding_prep_0___
    4 __58-[ClassB applicationDidFinishLaunching:]_block_invoke_037
    5 -[NSBlockOperation main]
    11 start_wqthread

And the very last debug log that I get is this:

    sender: __NSMallocBlock__
    -[__NSMallocBlock__ doSomething]: unrecognized selector sent to instance 0x10043f2a0

Now why did the block’s argument suddenly turn into __NSMallocBlock__ in first place?
I was clearly passing something else (namely sender) to it, wasn’t I?

Answer 1

Found the reason. Duh, I feel stupid now.

The mistake was attaching the block (as well as a second one) to my object as associated objects via:

static NSString * const FirstBlockKey;
static NSString * const SecondBlockKey;

objc_setAssociatedObject(self, FirstBlockKey, blockA, OBJC_ASSOCIATION_COPY);
objc_setAssociatedObject(self, SecondBlockKey, blockB, OBJC_ASSOCIATION_COPY);

while I should clearly have used this:

objc_setAssociatedObject(self, (void *)&FirstBlockKey, block, OBJC_ASSOCIATION_COPY);

FirstBlockKey and SecondBlockKey themselves are both 0x0 obviously, while their own pointers are not.

This way it simply called the wrong block (as they both were assigned to the same `0x0′ key). The blocks had different return and argument types, which seem to have caused the strange exchange of passed block arguments. Working fine now.

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That being said: Joe, Ryan & wbyoung, thanks for your efforts guys!