Objective-C version of the receipt attributes validation (in a single .m file)

ReceiptAttributesValidation.m

@import Foundation;

#import "ReceiptChecksum.h"
#import "ReceiptPayload.h"
#import "ReceiptValidation.h"

@protocol ReceiptAttributes;

NS_ASSUME_NONNULL_BEGIN

@interface ReceiptAttributesValidation : NSObject <ReceiptValidation>
- (instancetype)initWithOrigin:(id<ReceiptValidation>)origin attributes:(id<ReceiptAttributes>)attributes NS_DESIGNATED_INITIALIZER;
- (instancetype)init NS_UNAVAILABLE;
@end

@protocol ReceiptAttributes <NSObject>
@property(readonly) NSString *identifier;
@property(readonly) NSString *version;
@property(readonly) NSData *guid;
@end

@interface ReceiptAttributesValidation ()
@property(nonatomic, readonly) id<ReceiptValidation> origin;
@property(nonatomic, readonly) id<ReceiptAttributes> attributes;
@end

NS_ASSUME_NONNULL_END

@implementation ReceiptAttributesValidation

- (instancetype)initWithOrigin:(nonnull id<ReceiptValidation>)origin attributes:(nonnull id<ReceiptAttributes>)attributes {
    NSParameterAssert(origin);
    NSParameterAssert(attributes);
    if ((self = [super init])) {
        _origin = origin;
        _attributes = attributes;
    }
    return self;
}

- (instancetype)init {
    @throw [NSException exceptionWithName:NSInternalInconsistencyException reason:@"-init not available" userInfo:nil];
}

#pragma mark - ReceiptValidation

- (void)validateReceipt:(nonnull NSData *)data completion:(void (^ _Nonnull)(Result))completion {
    NSParameterAssert(data);
    NSParameterAssert(completion);
    if ([self validateAttributesWithData:data]) {
        [self.origin validateReceipt:data completion:completion];
    } else {
        completion(ResultReceiptIsInvalid);
    }
}

#pragma mark - Private

- (BOOL)validateAttributesWithData:(nonnull NSData *)data {
    NSParameterAssert(data);
    ReceiptPayload *payload = [[ReceiptPayload alloc] initWithData:data];
    if (payload) {
        return [self validateAttributesWithPayload:payload];
    } else {
        return NO;
    }
}

- (BOOL)validateAttributesWithPayload:(nonnull ReceiptPayload *)payload {
    NSParameterAssert(payload);
    ReceiptChecksum *checksum = [[ReceiptChecksum alloc] initWithGUID:self.attributes.guid opaque:payload.opaque identifier:payload.identifierData];
    return [payload.identifier isEqualToString:self.attributes.identifier] &&
    [payload.version isEqualToString:self.attributes.version] &&
    [payload.checksum isEqualToData:checksum.dataValue];
}

@end