Linked list with an array interface. Can be used for implicit linked lists like superviews or the responder chain.

DHLazyLinkedList.h

@import Foundation;

NS_ASSUME_NONNULL_BEGIN

@interface DHLazyLinkedList<ObjectType> : NSArray<ObjectType>

- (instancetype)initWithRootObject:(ObjectType)rootObject nextObjectBlock:(ObjectType _Nullable (^)(ObjectType))next NS_DESIGNATED_INITIALIZER;

- (nullable ObjectType)objectAfterObject:(ObjectType)object;

@end

NS_ASSUME_NONNULL_END

DHLazyLinkedList.m

#import "DHLazyLinkedList.h"

NS_ASSUME_NONNULL_BEGIN

@interface DHLazyLinkedList<ObjectType> ()

@property (nonatomic, strong, readonly) id rootObject;
@property (nonatomic, copy, readonly) ObjectType _Nullable (^next)(ObjectType);

@end

NS_ASSUME_NONNULL_END

@implementation DHLazyLinkedList

- (instancetype)init {
	return [self initWithRootObject:nil nextObjectBlock:nil];
}

- (instancetype)initWithObjects:(const id _Nonnull __unsafe_unretained *)objects count:(NSUInteger)cnt {
	return [self initWithRootObject:nil nextObjectBlock:nil];
}

- (instancetype)initWithCoder:(NSCoder *)aDecoder {
	return [self initWithRootObject:nil nextObjectBlock:nil];
}

- (instancetype)initWithRootObject:(id)rootObject nextObjectBlock:(id (^)(id))next {
	self = [super init];
	if (self == nil) return nil;

	_rootObject = rootObject;
	_next = [next copy];

	return self;
}

- (id)objectAfterObject:(id)object {
	return [self next](object);
}

- (NSUInteger)count {
	id object = [self rootObject];
	NSUInteger count = 0;
	NSMutableSet *foundObjects = [NSMutableSet set];
	while (object) {
		if ([foundObjects containsObject:object]) {
			return INFINITY;
		}

		++count;
		[foundObjects addObject:object];
		object = [self objectAfterObject:object];
	}
	return count;
}

- (id)objectAtIndex:(NSUInteger)idx {
	id object = [self rootObject];
	for (; idx > 0; --idx) {
		object = [self objectAfterObject:object];
	}
	return object;
}

@end