NSCoder

NSCoder is an abstract class in the Foundation framework that defines the interfaces for encoding and decoding objects, enabling serialization for persistence, interprocess communication, or network transfer. It provides the low-level methods used by concrete archivers, and is central to the supported object graph preservation in Apple's platforms.

In practice, encoding and decoding are performed through a pair of schemes. Keyed archiving uses methods that associate values with string keys, such as encodeObject:forKey: and decodeObjectForKey:. Concrete subclasses like NSKeyedArchiver and NSKeyedUnarchiver implement keyed archiving, which is the most commonly used form today. Non-keyed archivers exist as older alternatives and rely on position-based access. NSCoder itself supplies a broad set of typed methods for primitives and objects to support various data types.

To make a new class archivable, it should adopt the NSCoding protocol and implement two methods: init(coder:) to reconstruct an object from a decoder, and encode(with:) to encode the object's properties into a coder. In Objective-C these are typically -initWithCoder: and -encodeWithCoder:, while in Swift they appear as required init(coder: NSCoder) and func encode(with: NSCoder).

Security considerations are addressed by NSSecureCoding, an extension of NSCoding that enables secure decoding. Classes that adopt NSSecureCoding must implement supportsSecureCoding and use decodeObjectOfClass:forKey: (and related methods) to validate decoded object classes, helping to prevent object substitution attacks during deserialization.

Usage generally involves archiving objects to Data or files via NSKeyedArchiver and later unarchiving with NSKeyedUnarchiver. Although the Codable protocol is widely used for new code, NSCoder remains important for legacy APIs, interoperability with older frameworks, and certain system components.