Question

For learning (not practical — yet) purposes, I’d like to use the following method on an NSDictionary to give me back a set of keys that have values using a test I’ve defined. Unfortunately have no idea how to specify the predicate.

NSDictionary keysOfEntriesPassingTest:
- (NSSet *)keysOfEntriesPassingTest:(BOOL (^)(id key, id obj, BOOL *stop))predicate

Let’s say for example all my values are NSURLs, and I’d like to get back all the URLs that are on port 8080. Here’s my stab at coding that — though it doesn’t really make sense to me that it’d be correct:

NSSet * mySet = [myDict keysOfEntriesPassingTest:^(id key, id obj, BOOL *stop) {
                 if( [[obj port] isEqual: [NSNumber numberWithInt: 8080]]) {
                     return key;
                 }]

And that’s because I get back the following compiler error:

incompatible block pointer types initializing ‘void (^)(struct objc_object *, struct objc_object *, BOOL *)’, expected ‘BOOL (^)(struct objc_object *, struct objc_object *, BOOL *)’

What am I missing? I’d appreciate a pointer at some docs that go into more detail about the “Block object” that the predicate is supposed to be.

Thanks!

---

And this is the code that works:

NSSet *mySet = [myDict keysOfEntriesPassingTest:^(id key, id obj, BOOL *stop)
{
  if ([[obj port] isEqual:[NSNumber numberWithInt: 8080]])
     return YES;
   else
     return NO;
}];

Answer 1

The error indicates that your block must return a BOOL, rather than id. Check the docs. I suspect you are expected to return YES if the key/obj pair passes the desired test.