How to use a stop condition on a block like the enumerateObjectsUsingBlock from the NSDictionary class?

Question

I want to make a method on my class like enumerateObjectsUsingBlock:^(id obj, NSUInteger idx, BOOL *stop) from the NSDictionary class.

I have a little knowledge on blocks usage, but I have not been able to figure out how to make the stop condition that the enumerateObjectsUsingBlock function uses. Any suggestions?

Answer 1

The stop flag is used like this:

[coll enumerateUsingBlock:^(id o, NSUInteger i, BOOL *stop) {
      if (... check for stop ... ) {
           *stop = YES;
           return;
      }
 }];

When the enumeration block returns, the collection checks *stop. If it is YES, it stops enumerating.

It is implemented this way, as opposed to a return value, because this allows for concurrent enumeration without checking the return value of the block (which would incur overhead). I.e. in concurrent enumeration, the collection can dispatch_async() any number of simultaneous iterations and periodically check *stop. Whenever *stop transitions to YES, it stops scheduling more blocks (this is also why the stop flag is not a hard stop; some unspecified number of iterations may still be in flight).

In your iterator, you might do:

 BOOL stop = NO;
 for(...) {
     enumerationBlock(someObj, someIndex, &stop);
     if (stop) break;
 }