Implementation of NSCoder capable of creating sequential archives which must be read in the same order they were written. This class implements methods for saving to and restoring from a serial archive (usually a file on disk, but can be an NSData object) as well as methods that can be used by objects that need to write/restore themselves.

Note, the sibling class NSKeyedArchiver supports a form of archive that is more robust to class changes, and is recommended over this one.

This class reconstructs objects from an archive.

Re-using the archiver

The -resetUnarchiverWithData:atIndex: method lets you re-use the archive to decode a new data object or, in conjunction with the 'cursor' method (which reports the current decoding position in the archive), decode a second archive that exists in the data object after the first one.

Subclassing with different input format.

NSUnarchiver normally reads directly from an NSData object using the methods -

-deserializeTypeTag:andCrossRef:atCursor:

to decode type tags for data items, the tag is the first byte of the character encoding string for the data type (as provided by '@encode(xxx)'), possibly with the top bit set to indicate that what follows is a crossreference to an item already encoded.
Also decode a crossreference number either to identify the following item, or to refer to a previously encoded item. Objects, Classes, Selectors, CStrings and Pointer items have crossreference encoding, other types do not.

[NSData -deserializeDataAt:ofObjCType:atCursor:context:]

to decode all other information.

NSUnarchiver normally uses other NSData methods to read the archive header information from within the method: [-deserializeHeaderAt:version:classes:objects:pointers:] to read a fixed size header including archiver version (obtained by [self systemVersion]) and crossreference table sizes.

To subclass NSUnarchiver, you must implement your own versions of the four methods above, and override the 'directDataAccess' method to return NO so that the archiver knows to use your serialization methods rather than those in the NSData object.