LazyMappedArray.m

@interface MappedArraySentinel : NSObject @end
@implementation MappedArraySentinel @end

@interface LazyMappedArray ()

@property (nonatomic) NSArray *originalArray;
@property (nonatomic) NSMutableArray *backingArray;
@property (nonatomic, copy) id (^block)(id);

@end

@implementation LazyMappedArray

- (instancetype)initWithArray:(NSArray *)array transformationBlock:(id (^)(id object))block {
    self = [super init];
    if (!self) return nil;
    
    _originalArray = array;
    _block = [block copy];
    
    return self;
}

- (NSMutableArray *)backingArray {
    if (!_backingArray) {
        NSMutableArray *mappedArray = [NSMutableArray arrayWithCapacity:self.originalArray.count];
        for (NSInteger i = 0; i < self.originalArray.count; i++) {
            [mappedArray addObject:[MappedArraySentinel new]];
        }
        self.backingArray = mappedArray;
    }
    return _backingArray;
}

- (NSUInteger)count {
    return self.backingArray.count;
}

- (id)objectAtIndex:(NSUInteger)index {
    if ([[self.backingArray objectAtIndex:index] isKindOfClass:[MappedArraySentinel class]]) {
        [self.backingArray replaceObjectAtIndex:index withObject:self.block(self.originalArray[index]) ?: [NSNull null]];
    }
    return [self.backingArray objectAtIndex:index];
}

@end

Is there a bug? If customs call - initWithArray:transformationBlock: with an array of type NSMutableArray, and modify that array immediately, later call methods on LazyMappedArray object, he may get unexpected result.