NSKeyedArchiverNSKeyedUnarchiver

NSKeyedArchiver and NSKeyedUnarchiver are classes in Apple's Foundation framework used to encode and decode objects into and from a keyed archive. They rely on objects conforming to the NSCoding protocol, which requires implementing encodeWithCoder: and initWithCoder:. Through a dictionary-like set of keys, an object graph can be serialized into a binary or XML-style representation and later reconstructed. The archived data can be written to files or kept in memory as NSData.

In typical usage, a root object is encoded by the archiver, which recursively encodes its contained objects. To decode, the unarchiver reads the data and reconstructs objects by querying decodeObjectForKey: for each key. Older APIs included archiveRootObject:toFile: and unarchiveObjectWithFile:, which are deprecated in favor of data-based approaches. Modern APIs offer archivedDataWithRootObject:requiringSecureCoding:error: and unarchiveTopLevelObjectWithData:error:, which promote safer, secure decoding. When using secure coding, the classes of decoded objects should support NSSecureCoding, and you may enable requiringSecureCoding to enforce type checks during decoding.

Security and compatibility considerations are important. Enabling NSSecureCoding on your classes and using the secure variants of the APIs helps prevent object substitution attacks. Designers should manage versioning and class name mappings to handle changes in the object graph over time. The resulting archive can be platform- and architecture-neutral, but exact binary formats may vary between OS versions. For Swift, the same concepts apply with corresponding protocol conformance and bridging to the modern NSKeyedUnarchiver APIs.