Re: [Pyobjc-dev] NSCoding support for python list & dict

[Ronald O. <ron...@ma...>]

 
On Tuesday, November 27, 2007, at 08:56AM, "Bill Bumgarner" <bb...@ma...> wrote:
>On Nov 26, 2007, at 9:59 PM, Ronald Oussoren wrote:
>> NSCoding support for other objects should also be doable, but is  
>> more work and requires a lot of testing. Pickling Objective-C  
>> objects on the other hand has the outlooks of being a very hard  
>> problem.
>
>NSArchiver and NSKeyedArchiver support arbitrary archival of objects.   
>That is, you can create an archiving session and then archive the sub- 
>graphs of objects of objects in arbitrary order and with arbitrary  
>references between the subgraphs.
>
>Pickling of a graph containing a mix of Python and Objective-C objects  
>could be done by creating a UUID for each object in the graph at the  
>pickle/archiver boundary.  The UUID could be stored in the pickle,  
>could be used to archive the sub-graph of Objective-C objects and  
>store 'em away via -encodeObject:forKey:, and then the resulting BLOB  
>of archived Objective-C data could be stored in the pickle as a  
><string>.
>
>Completely trivial, right?

At first glance that doesn't quite fit into the pickle interface, but could be made to work using a class/function on top of pickle that does the work.

Dealing with cycles and object-identity will be interesting though (e.g. pickle.dumps([O, NSArray.arrayWithArray_([O])]), where O is a pure Python object (or even (O=[]; A=NSArray.arrayWithArray_([O]); O.append(A); pickle.dumps([O,A]).

The other way around (adding NSCoding support to Python objects) is easy: just call __getstate__/__reduce__ to get object state, then use encodeObject:forKey: to write that, more basic, object to the stream.

>
>Only because of my ignorance.   There is a definite chicken-and-egg  
>problem in that archival will produce the archived obj-c objects  
>*last* whereas unarchival will need them to be decoded *first*.   And  
>that is just scratching the surface.

What might work: implement __reduce__ for objc.objc_object using a custom NSCoder subclass that builds a python datastructure containing the object state, then let the pickle machinery serialize that. That is, encodeObject:forKey: would add the object to a dictionary. At some point the object state must be some basic object, such as C integers, which pickle can encode without further recursion.

I know this is a bit vague, I made this up while typing this e-mail. It does look like something that can be made to work though.

Volunteers?

Ronald
>
>b.bum
>
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