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Re: Can one extend CoreFoundation using Obj-C?

On Sunday, July 29, 2001, at 04:10 PM, Brian Hill wrote:

On Sunday, July 29, 2001, at 01:20 PM, Brian Cave wrote:

I want to have a unique CFTypeID that my class exports that would be unique in the same manner as these already are in the CoreFoundation world, no better. And I want my class to be retainable / releaseable using the CoreFoundation retain / release APIs. I do not desire any more compatability than this.

I looked into this a while back, and even downloaded the portion of the CF source in Darwin so I could see how it's done. Right now, it can't, per se.

There are several limitations to the CF itself (a fairly restrictive limit on the global number of CF types allowed in a single process -- 255 -- for example), and the internal API that you'd need to use (which you could dig out of Darwin) is very likely to change. In fact, I'd almost put money on it having changed in 10.1.

However, someone on this list (I forget who, sorry) had the excellent suggestion of wrapping my class around a CFData. Create some accessors to store the class' private instance data (ie., a struct) in the CFData.

What I ended up doing in the end was to just create a separate set of CF functions for my own class' use that paralleled the standard CF ones (ie., CFXRetain instead of CFRetain). I also created my own set of collection-handling functions for CFDictionaries and CFArrays that did the right thing with both my own CF-like types and standard CF types.

Brian

I have now proved that this can be done and it works. Just a few limitations. Here is what I did and what I can't do.

Create a new Obj-C class inheriting off of NSObject (or I imagine you could do this off of any NS object).

Design and build your class as normal (instance vars, functions, etc).

Then create your CF style interface functions as C APIs. The guts of these are glue code to call the Objc-C class you made. Note, all functions that you want to be external must be declared here. Also, any vars you want access to in the outside world will need Get and/or Set accessor glue functions as C APIs.

Put the prototypes for the C APIs into a standard C header that first includes at least CFBase.h or probably better CoreFoundation.h. This must include appropriate typedefs for mutable and/or immutable (if you need such a difference anyway), at least one CFCreate variant for your specific class, and you should also provide a CFMyTypeGetID() call for consistency sake. Unfortunately, CFAllocatorRefs can't be used here, but I am going to provide them as place holders (for consistency sake) and simply ignore the value. You could hook them up to NSZone, but only the normal default allocator works. kCFAllocatorMalloc or kCFAllocatorSystemDefault will generate an EXC_BAD_ACCESS error.

CFRetain(), CFRelease(), CFGetRetainCount() so far work perfectly. CFGetTypeID() always seems to return 1. This value appears to be an internal CFType type (according to the public source code for CoreFoundation). I can override it by defining the _cfTypeID static member and having it return a different value, but I have *not* yet determined if this is safe. Nor can I determine what would be a non-conflicting number to return (other than the current implementation of this number in CF suggest that negative numbers would never be generated by it). Returning a negative number does not seem to fail the CF functions I have tested at least so far.

I imagine (but have not tested) that CFEqual and CFHash should work through the normal Obj-C equivalent member functions. That is the way that CF works through Obj-C according to both the CF source code and my own tests.

So far my tests have been from a modified Foundation tool project template. I am going to test this via a framework (Mach-O obviously) which is my ultimate goal. I figure CFM can always use CFBundle to load and execute it (that is if I ever need to use these APIs from CFM).

HTH,
brian

_____________________________________________________
Brian Cave <mailto:email@hidden>

References: 
 >Re: Can one extend CoreFoundation using Obj-C? (From: Brian Hill <email@hidden>)



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