Sample (preferably simple) subclass of NSCoder?

Question

I'm trying to create a subclass of NSCoder, but I really don't know where to start. Apple's documentation lists which methods are required, but not much else. Maybe my Google-fu is weak, but I can't find any examples of an implementation of, e.g. encodeValueOfObjCType:at:, anywhere. (Though I assume it involves a lot of cases.)

Anyone know of a sample subclass of NSCoder I can look at, or have an idea of what a case or two of encodeValueOfObjCType:at: and decodeValueOfObjCType:at: should look like?

Answer 1

I've also been wanting to (ab)use NSCoder to generate simpler XML than what NSKeyedArchiver produces and have implemented some classes for it. The classes are called RWPlainXMLTreeEncoder and RWPlainXMLTreeDecoder, and I've written some test code for them too.

RWPlainXMLTreeEncoder assumes that the object graph you're encoding is a tree (in case the same object is encoded twice, the decoded tree will contain two different copies instead of one shared copy; if you try to encode a cyclic graph it raises an exception). Per encoded object it generates an XML element that looks roughly like the one for this example, an encoding of an array containing the string "A string":

<ROOT type="@NSArray"><NS.object.0 type="@NSString"><NS.bytes>4120737472696E67</NS.bytes></NS.object.0></ROOT>

I wanted to further improve the above by using a different method instead of the object's own encodeWithCoder: for objects such as arrays and strings, so that the above would become:

<ROOT type="array"><item.0 type="string">A string</item.0></array></ROOT>

I'm however not sure if I will continue working on this. My overall goal was to have a fairly generic, simple way of saving an object tree to a file that leverages of the encodeWithCoder: methods I've already written, while producing a file that is not as Cocoa-dependent as when using NSKeyedArchiver. This would allow others to write applications that open those files on other platforms.

But I've now come to understand there have been similar efforts which may already be more advanced anyway, and furthermore, with XML being a document markup language it may not be the best target format and some non-markup language might be better suited.

Nevertheless, if you want to continue with this or have some other reason to look at a fairly simple NSCoder subclass, feel free to use my code. You could also take a look at MAKeyedArchiver. Oh, and my code is covered by a BSD-style license (at least the version that is in SVN revision 424 is, I might change this for future versions). Improvements and feedback are welcomed.