[Pyobjc-dev] Bridging NS and CF classes

[<bb...@ma...>]

On Wednesday, Jan 8, 2003, at 18:03 US/Eastern, Jack Jansen wrote:
> I think I would like the bridging to be done one-way, i.e. if 
> something wants an NSArray and the argument isn't an NSArray it should 
> try coercing the argument to an NSArray. One of the things that could 
> be coerced is CF.CFArray.
>
> I think that for going the other way (from NS to CF) manual conversion 
> is probably good enough initially.

Won't work as there is no class identification on the arguments to 
methods.   As such, there is no way to implement a system as you 
described -- no way to identify [short of parsing the headers-- but 
that won't be completely correct] that a particular argument should be 
an instance of NSArray vs. an NSDictionary vs. a DeveloperDefined.

ObjC just doesn't care.

Why can't we add enough stuff to both the CF* module(s) and the PyObjC 
bridge such that both can transparently consume objects from the other, 
where appropriate.   This is basically what Apple did to ObjC and CF to 
create the no-cost bridge -- it would seem that we need a parallel 
mechanism within the "two" Python modules.

That is, make the following work:

 >>> import _CF  #this is framework build, why do I do this and not 
"CF"??
 >>> x = _CF.CFArrayCreateMutable(10)
 >>> from Foundation import NSMutableArray
 >>> NSMutableArray.addObject_(x, 'y')
Traceback (most recent call last):
   File "<stdin>", line 1, in ?
TypeError: First argument must be an objective-C object, got 
<CFMutableArrayRef object at 0x0012c0e0 for 0x00160000>

And so should this:

 >>> import _CF
 >>> from Foundation import NSMutableArray
 >>> x = NSMutableArray.array()
 >>> _CF.CFArrayAppendValue(x, 'y')
Traceback (most recent call last):
   File "<stdin>", line 1, in ?
AttributeError: 'module' object has no attribute 'CFArrayAppendValue'

--

In the first case (CF type being messaged via PyObjC), a simple [I 
think] fix would be to add a method *on the C side* that can return a 
reference to the ObjC representation of the CF type.  In the handler 
that produces the TypeError as shown above, the PyObjC bridge could 
check to see if the object has a method/attribute/function/identifier 
that can be used to retrieve/identify what part of the object could be 
used directly as the ObjC instance to be messaged.

Something similar could be done for the second case, as well -- but I 
have *no* idea what the implementation issues would be in that context.

(I couldn't actually figure out how to call CFArrayAppendValue() on 
CFMutableArrayRef.)

b.bum