What is the BOOL *stop argument for enumerateObjectsUsingBlock: used for?

The stop argument to the Block allows you to stop the enumeration prematurely. It's the equivalent of break from a normal for loop. You can ignore it if you want to go through every object in the array.

for( id obj in arr ){    if( [obj isContagious] ){        break;    // Stop enumerating    }    if( ![obj isKindOfClass:[Perefrigia class]] ){        continue;    // Skip this object    }    [obj immanetizeTheEschaton];}

[arr enumerateObjectsUsingBlock:^(id obj, NSUInteger idx, BOOL *stop) {    if( [obj isContagious] ){        *stop = YES;    // Stop enumerating        return;    }    if( ![obj isKindOfClass:[Perefrigia class]] ){        return;    // Skip this object    }    [obj immanentizeTheEschaton];}];

That is an out parameter because it is a reference to a variable from the calling scope. It needs to be set inside your Block, but read inside of enumerateObjectsUsingBlock:, the same way NSErrors are commonly passed back to your code from framework calls.

- (void)enumerateObjectsUsingBlock:(void (^)(id obj, NSUInteger idx, BOOL *stop))block {    // N.B: This is probably not how this method is actually implemented!    // It is just to demonstrate how the out parameter operates!    NSUInteger idx = 0;    for( id obj in self ){        BOOL stop = NO;        block(obj, idx++, &stop);        if( stop ){            break;        }    }}