decodeObjectForKey

decodeObjectForKey is a method defined on NSCoder and its concrete subclass NSKeyedUnarchiver. It is used to retrieve an object that was previously encoded with encodeObject:forKey: by supplying the same key. The method reads the encoded data and returns the decoded object, or nil if the data cannot be decoded or the key is not present in the archive.

In practice, decodeObjectForKey takes a key, which is an NSString, and returns an id (Objective-C) or Any? (Swift). The object returned is an instance of the class that was originally encoded, subject to the constraints of the coder’s configuration. If the archived data is corrupted or the expected class cannot be decoded, decoding may fail and return nil or raise an exception, depending on the context and the coding environment.

Common usage examples include decoding objects that were previously encoded with encodeObject:forKey:, such as user records or model objects, during file loading or data transmission. In Swift, the equivalent call from an NSKeyedUnarchiver instance is decodeObject(forKey:), and it is common to pair it with secure coding variants to enforce class constraints.

Security considerations are important when unarchiving. If secure coding is enabled, developers should use methods that verify the class or classes being decoded (for example, decodeObjectOfClass:forKey: or decodeObjectOfClasses:forKey:) to prevent object substitution attacks.

See also: NSCoder, NSKeyedUnarchiver, encodeObject:forKey:, NSSecureCoding, decodeObjectOfClass:forKey:, decodeObjectOfClasses:forKey:.