Does Objective-C have an equivalent to java annotations?

Question

Does Objective-C have an equivalent to java annotations?

What\'s I\'m trying to do is create a property and be able to somehow access some metadata about it.

Answer 1

No, Objective-C has no annotation or generics support.

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A way to implement such a thing would be to hack Clang to read comments and associate a metadata object to the original object. But, you would be tied to your hacked compiler.

NSString *v1 = [[NSString alloc] init];

// associate
static char key;
NSString *v2 = [[NSString alloc] init];
objc_setAssociatedObject (
    v1,
    &key,
    v2,
    OBJC_ASSOCIATION_RETAIN
);

// retrieve
NSString *associate = (NSString *)objc_getAssociatedObject(v1, &key);

---

Qualifying with a protocol wouldn't be much trouble, and you could test if the collection implements it, but along the way you would need to create a category for each type on the same collection. This would require a different collection at compile time using macros. Overly complicated.

@interface Tomato:NSObject @end
@implementation Tomato @end

@protocol TomatoNSArray <NSObject>
- (Tomato*)objectAtIndexedSubscript:(NSUInteger)index;
- (void)setObject:(Tomato*)tomato atIndexedSubscript:(NSUInteger)index;
@end

// here is the problem, you would need to create one of this for each type
@interface NSMutableArray (TomatoNSArray) <TomatoNSArray>
@end

int main(int argc, char *argv[]) {
    @autoreleasepool {
        NSMutableArray<TomatoNSArray> *tomatoes = [[NSMutableArray alloc] initWithCapacity:2];
        tomatoes[0] = [Tomato new];
        tomatoes[1] = [NSObject new]; // warning: incompatible pointer types 
    }
}

Answer 2

There is no native support of this functionality, but you may to take a look at following solution — https://github.com/epam/lib-obj-c-attr/ It is compile time implementation of attributes. Definition of attributes based on defines but not on comments as in other solutions like ObjectiveCAnnotate.