[webkit-dev] JS: Getter/Setter callbacks at Property level

[Brian Barnes]

I'm not sure you get exactly what I'm saying as you put class in places 
where I'd expect property.  For instance:

"
I looked briefly, and it seems like it would be relatively easy to add 
an API for adding C getters and setters to a class individually. So, 
that seems like a reasonable feature request. "

Replacing class with property is what I'd expect here, i.e., each 
property set on an object has it's own getter and setter.  Something like:

void 
JSObjectSetPropertyWithCallback(ctx,object,propertyName,value,getterFunction,setterFunction,attributres,exception);

Also I'm a little worried about the word "dynamic", these are definitely 
static.  I have all the functions created, you just have a pointer to 
them.  If the pointer exist, call it for the value and break out.  If it 
doesn't, go down the chain.  More pseudo code:

void main(...)
{
  obj=JSObjectMake(ctx,NULL,NULL);
  
JSObjectSetPropertyWithCallback(ctx,obj,"red",JSValueMakeNull(),myRedGetter,myRedSetter,attributes,exception);
}

JSValueRef myRedGetter(...ctx...object...exception)
{
  return(JSValueMakeNumber(red_value));
}

bool myRedSetter(..ctx..object..value..exception)
{
  red_value=JSValueToNumber(ctx,value,exception);
}

Here are the benefits (as I see them):

1) I don't need to define classes for objects; right now, the only 
reason I would need to create classes (instead of just passing NULL) is 
to setup the getters & setters.  This reduces workload and generalizes a 
lot of my code

2) The JS engine has already looked up the property by name; with class 
based getters/setters, I also have to lookup the property by name.  With 
property based getters/setters, it's only looked up once and directly 
called to me.  This should be a big savings win and should be more 
simple at the back end (if there's no getter/setter associated with a 
property, just skip forward down the chain.)

I'm not sure how this could be anything but faster then class-based.

[>] Brian

Geoffrey Garen wrote:
> Hi Brian.
>
> I see what you mean now.
>
> In JavaScriptCore, there's no API for defining C getters and setters 
> individually. There's an API for associating a set of C getters and 
> setters with a class, and there's an API for defining a generic 
> fallback getter and setter for a class.
>
> I looked briefly, and it seems like it would be relatively easy to add 
> an API for adding C getters and setters to a class individually. So, 
> that seems like a reasonable feature request.
>
> I'm not sure how important it is to you to be able to add C getters 
> and setters to objects, as opposed to classes, individually. I think 
> that would be a trickier API to get right. The main challenge is that 
> getters and setters of that dynamic nature tend to slow down the 
> object system. So, we would either want to engineer a way to avoid the 
> performance cost in most cases, or we would want to design a way to 
> steer API users away from that particular API in the general case. If 
> you have a good example of how this feature would be generally useful 
> to API clients, it's still a reasonable request, though.
>
> Geoff
>
> On Jul 20, 2009, at 10:21 AM, Brian Barnes wrote:
>
>> In SpiderMonkey, you can create an object, and that object has a 
>> callback to a getter or a setter in C.  You get the name, look it up, 
>> return or set a value.  Nitro has that same functionality.
>>
>> On SpiderMonkey, though, when creating a property, you can also do this:
>>
>> JS_DefineProperty(context,object,name,value,getter,setter,flags);
>>
>> Where "getter" and "setter" are direct calls for the property, for 
>> only that property.  There's no lookup logic in my code.
>>
>> So, in pseudo code, at object level, we'd have a single getter:
>>
>> void getMyObjectValues(.... name ...)
>> {
>>  if (name == 'red') return(red)
>>  if (name == 'green') return(green)
>>  if (name == 'blue') return(blue)
>> }
>>
>> And property level, we'd have 3 getters:
>>
>> void getMyObjectRed(...)
>> {
>> return(red);
>> }
>>
>> void getMyObjectGreen(...)
>> {
>> return(green);
>> }
>>
>> void getMyObjectBlue(...)
>> {
>> return(blue);
>> }
>>
>> [>] Brian
>>
>> Geoffrey Garen wrote:
>>> Hi Brian.
>>>
>>> I don't understand the distinction you're drawing between "the 
>>> property level" and "the object level". Can you explain what those 
>>> mean and give an example of each?
>>>
>>> Thanks,
>>> Geoff
>>>
>>> On Jul 20, 2009, at 9:14 AM, Brian Barnes wrote:
>>>
>>>> I was getting ready to try the first move from SpiderMonkey to 
>>>> Nitro, and ran into a large problem.  Right now, all my getters and 
>>>> setters are at the property level.  In the documentation I have, 
>>>> Nitro only seems to put them at the object level.  This would force 
>>>> a huge refactoring of my code (which I'm willing to do if I have 
>>>> to, I would just like to avoid it as I have hundred or so objects.)
>>>>
>>>> Is my documentation old, did I miss something, or am I stuck?  If 
>>>> I'm stuck, is there any call to have this put into Nitro at some 
>>>> time in the future?
>>>>
>>>> [>] Brian
>>>>
>>>>
>>>> _______________________________________________
>>>> webkit-dev mailing list
>>>> webkit-dev at lists.webkit.org
>>>> http://lists.webkit.org/mailman/listinfo.cgi/webkit-dev
>>>
>>>
>>
>
>