pyobjc indexed accessor method with range

I'm trying to implement an indexed accessor method for my model class in Python, as per the KVC guide. I want to use the optional ranged method, to load multiple objects at once for performance reasons. The method takes a pointer to a C-array buffer which my method needs to copy the objects into. I've tried something like the following, which doesn't work. How do I accomplish this?

@objc.accessor    # i've also tried @objc.signature('v@:o^@')
def getFoos_range_(self, range):
    return self._some_array[range.location:range.location + range.length]

Edit: I finally found the type encodings reference after Apple moved all the docs. After reading that, I tried this:

@objc.signature('v@:N^@@')
def getFoos_range_(self, buf, range):

but this didn't appear to work either. The first argument is supposed to be a pointer to a C-array, but the length is unknown until runtime, so I didn't know exactly how to construct the correct type encoding. I tried 'v@:N^[1000@]@' just to see, and that didn't work either.

My model object is bound to the contentArray of an NSArrayController driving a table view. It doesn't appear to be calling this method at all, perhaps because it expects a different signature than the one the bridge is providing. Any suggestions?

You were close. The correct decorator for this method is:

@objc.signature('v@:o^@{_NSRange=QQ}')

NSRange is not an object, but a struct, and can't be specified simply as @; you need to include the members¹.

Unfortunately, this is not the end of it. After a whole lot of experimentation and poring over the PyObjC source, I finally figured out that in order to get this method to work, you also need to specify metadata for the method that is redundant to this signature. (However, I still haven't puzzled out why.)

This is done using the function objc.registerMetaDataForSelector:

objc.registerMetaDataForSelector(b"SUPERCLASSNAME", 
                                 b"getKey:range:",
        dict(retval=dict(type=objc._C_VOID),
             arguments={ 
                  2+0:  dict(type_modifier=objc._C_OUT,
                             c_array_length_in_arg=2+1),
                  2+1:  dict(type=b'{_NSRange=II}',
                             type64=b'{_NSRange=QQ}')
             }
        )
)

Examples and some details of the use of this function can be found in the file test_metadata_py.py (and nearby test_metadata*.py files) in the PyObjC source.

N.B. that the metadata has to be specified on the superclass of whatever class you are interested in implementing get<Key>:range: for, and also that this function needs to be called sometime before the end of your class definition (but either before or inside the class statement itself both seem to work). I haven't yet puzzled these bits out either.

I based this metadata on the metadata for NSArray getObjects:range: in the Foundation PyObjC.bridgesupport file², and was aided by referring to Apple's BridgeSupport manpage.

With this worked out, it's also worth noting that the easiest way to define the method is (at least, IMO):

@objc.signature('v@:o^@{_NSRange=QQ}')
def get<#Key#>_range_(self, buf, inRange):
    #NSLog(u"get<#Key#>")
    return self.<#Key#>.getObjects_range_(buf, inRange)

I.e., using your array's built-in getObjects:range:.

1: On 32-bit Python, the QQ, meaning two unsigned long longs, should become II, meaning two unsigned ints
2: Located (on Snow Leopard) at: /System/Library/Frameworks/Python.framework/Versions/2.6/Extras/lib/python/PyObjC/Foundation/PyObjC.bridgesupport

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