Re: Heads Up: ABI Change

[Quentin Mathé]

Le 12 févr. 2011 à 09:11, Richard Frith-Macdonald a écrit :

> On 11 Feb 2011, at 23:50, Stefan Bidi wrote:
> 
>> On Fri, Feb 11, 2011 at 5:42 PM, David Chisnall <address@hidden> wrote:
>> Hi Stefan,
>> 
>> I've not looked at the code in detail, but I thought you were creating a 
>> private subclass for the old and new behaviours - would it not be possible 
>> to simply return a private subclass instance from the constructors and not 
>> change the ivar layout of the superclass?
>> 
>> I really haven't got a clue how to go about getting something like that 
>> done.  I'm still at the "getting my feet wet" stage of OOP + ObjC.  I'll 
>> have a look at how NSString and it's subclasses are implemented, but I'll 
>> probably need someone holding my hand, at least part of the way.
> 
> The idea of using a subclass sounds good, and it's quite easy.
> I wouldn't recommend looking at NSString as an example though ... that's a 
> much, much more complex case than would be needed.
> 
> The simple subclassing solution would be to keep the old implementation in 
> the base class, and just modify the +allocWithZone: method.
> eg.
> 
> + (id) allocWithZone: (NSZone*z)
> {
>  if (using_new_api && [NSDateFormatter class] == self)
>    {
>      // If the subclass calls the super implementation, the check for the 
> class will prevent recursion.
>      return [subclass allocWithZone: z];
>    }
>  else
>    {
>      return [super allocWithZone: z];
>    }
> }

The problem I see with this approach is that you can change the formatter 
behavior at runtime with -setFormatterBehavior:.

May be the class cluster approach is still a valid one, if it's possible to use 
isa swizzling. e.g.  Two subclasses NSDateFormatterBehavior10_0 and 
NSDateFormatterBehavior10_4 with the same ivars (or instance size), and 
-setFormatterBehavior implemented as below:

-setFormatterBehavior:
{
        // NOTE: I haven't taken in account the default behavior case.
        if (behavior == NSDateFormatterBehavior10_0)
        {
                behaviorClass = [NSDateFormatterBehavior10_0 class];
        }
        else if (behavior == NSDateFormatterBehavior10_4)
        {
                behaviorClass = [NSDateFormatterBehavior10_4 class];    
        }
        objc_setClass(self, behaviorClass);
}

Cheers,
Quentin.