Changing the argument types on a block callback

Question

I recently saw some example code in a WWDC video from an Apple engineer. He was using NSArray's enumerateObjectsUsingBlock: method, but I noticed the first argument to the block was not id as I'm used to, but had been changed to the concrete type that the developer knew was in the array.

For example, I just gave this a go in Xcode and everything works correctly:

NSArray *test = [NSArray arrayWithObjects:@"one", @"two", @"three", nil];
[test enumerateObjectsUsingBlock:^(NSString *aString, NSUInteger idx, BOOL *stop) {
    // ...
}];

The declaration for enumerateObjectsUsingBlock: declares the first block argument to be of type id:

- (void)enumerateObjectsUsingBlock:(void (^)(id obj, NSUInteger idx, BOOL *stop))block

Now I understand that id can be used to represent any objective-c type, but this little trick with arguments was unknown to me.

Is there any documentation for this feature of the language? Can this be used in regular methods where you have an (id)sender argument too? Is this even safe or fully supported?

Answer 1

Here's a link to apple doc about id type. This trick can be used in any method sending id, and it's safe, as long as you're sure about a type, or check it before you use it.