On Dec 2, 2009, at 12:18, Kyle Sluder wrote:
> On Wed, Dec 2, 2009 at 11:37 AM, Quincey Morris
> <quinceymor...@earthlink.net> wrote:
>> In some cases, you can do the wrong thing -- change the instance variable
>> directly -- and still be KVO compatible if you manually call the
>> willChangeValueForKey:/didChangeValueForKey: methods (or the similar ones
>> designed specifically for collections), but that approach is kind of frowned
>> on, not to mention difficult to get right in general.
>
> I don't know why you call this the "wrong thing." This is very much a
> reasonable thing to do, and in many non-trivial cases you will have to
> do exactly this.
Infelicitous wording on my part. I was trying to say that the "wrong" thing
(changing the instance variable directly, and not causing any notifications)
can be converted into a "right" thing by triggering the notifications manually.
Once you add willChange/didChange, it isn't wrong any more. I still think that
this approach has started to smell a little bit *in general*, now that we have
keyPathsForValuesAffecting<Key>, but if it's necessary to generate the
notifications manually, then I wouldn't call that wrong.
I also need to retract, partially, another thing I said. There is, technically,
a way of getting a immutable array proxy directly: 'valueForKey:', but it has
its pitfalls. For example, consider this implementation:
@interface Year
@property (readonly) NSArray* seasons;
@end
@implementation Year
- (NSUInteger) countOfSeasons {
return 4;
}
- (id) objectInSeasonsAtIndex: (NSUInteger) index {
switch (index) {
case 0: return @"Spring";
case 1: return @"Summer";
case 2: return @"Autumn";
default: return @"Winter";
}
}
- (NSArray*) seasons {
// return the immutable array proxy here
}
Since the only way of getting the immutable array proxy is 'valueForKey:', it
would seem like the correct implementation of that last method would be:
- (NSArray*) seasons {
return [self valueForKey: @"seasons"];
}
However, that will in fact produce an infinite recursive loop, since
'valueForKey:' calls the getter method if it exists. There's no way of having
BOTH the getter AND access to the immutable array proxy via the getter. What's
really needed is:
- (NSArray*) seasons {
return [self immutableArrayValueForKey: @"seasons"];
}
but of course that doesn't exist in the API.
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