Function to test empty objects in Objective-C

Constants.h

static inline BOOL IsEmpty(id thing) {
    return thing == nil
    || thing == [NSNull null]
    || ([thing respondsToSelector:@selector(length)]
        && [(NSData *)thing length] == 0)
    || ([thing respondsToSelector:@selector(count)]
        && [(NSArray *)thing count] == 0);
}