[Openmcl-devel] wrapping foreign objects in lisp classes

Cyrus Harmon ch-openmcl at bobobeach.com
Tue Aug 17 00:52:38 PDT 2004

Anyone have a simple example of a working instance of 
slot-value-using-class? AMOP is on the way, but in the meantime, an 
example would help. The Cocoa stuff is a bit hairy and my simple 
example doesn't work. Not sure where I'm going wrong...

As for Gary's other approach, this definitely works and is simpler. I'm 
bound and determined to figure out why the slot-value-using-class isn't 
working for me though...



On Aug 16, 2004, at 4:29 PM, Gary Byers wrote:

> On Mon, 16 Aug 2004, Dan Knapp wrote:
>>    Using the metaobject protocol doesn't avoid having to write the
>> custom readers and
>> writers; you still need to do that.  However, you should be able to do
>> it in such a way
>> that you only need to do it once instead of for every slot of every
>> class.
> The methods that a custom metaclass would have to override are
> writer methods do the equivalent of calls to these methods in general
> (in a case where a user-defined [SETF ]SLOT-VALUE-USING-CLASS method
> could be applicable, the user-defined method will be invoked.)
> The Cocoa bridge implements SLOT-VALUE-USING-CLASS (and SETF thereof)
> by storing getter/setter functions (defined in terms of low-level
> memory accessors) in specialized SLOT-DEFINITION objects.
> In the bridge's case, it seemed natural to treat ObjC instance 
> variables
> as CLOS slots.  It may be simpler (in some ways) to simply define 
> methods
> on the fields of an encapsulated foreign structure, without trying to
> make those methods be accessor methods.  As a poor example:
> ;; This definition might be encoded in a .cdb file
> (def-foreign-type nil (:struct :point  ; this is a hypothetical example
>                          (:x :float)
>                          (:y :float)))
> (defclass point ()
>   ((real-point :initarg :real-point
>                :accessor real-point
>                :documentation "a pointer to a real point")))
> (defmethod point-x ((p point))
>   (let* ((real-point (real-point p)))
>     (pref real-point :point.x)))
> (defmethod (setf point-x) (new (p point))
>   (let* ((real-point (real-point p)))
>     (setf (pref real-point :point.x) new)))
> You might want these methods to do a little more type- and 
> sanity-checking
> than is shown here, but it seems like most/all of that is boilerplate:
> given a foreign structure definition, it's probably not too hard to
> generate an encapsulating class and methods to access that structure's
> fields.
> (You wouldn't have the ability to use SLOT-VALUE or WITH-SLOTS on the
> foreign field, but you'd be able to deal with a foreign structure and
> its fields at a fairly high level.)
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