On Jul 14, 2008, at 3:16 AM, Bill Bumgarner wrote:
On Jul 13, 2008, at 11:35 PM, Jeff Johnson wrote:
On the other hand, the function method_exchangeImplementations()
just became public API in Leopard, so I don't see how that can
break anytime soon. It's not much different than subclassing or
'categorizing'.
Correct. However, it is what you are using
method_exchangeImplementations() for that is problematic. Namely,
you are editing the method tables for an Apple supplied class.
This is unsupported and may be problematic for any number of
reasons -- KVO, IMP caching, internal implementation details, etc...
It is quite a bit different than either subclassing or using
categories.
For subclassing, you override behavior and, if you are doing it
right, don't break encapsulation -- don't access your super's
instance variables directly and do use API to manipulate super's
state (unless, of course, super is actually a class of your own
design -- then it is all fair game, but you should still consider
carefully how the super/sub relationship should be work).
For categories, all is well until the moment you override an
existing method in a class other than your own. At that point, you
are asking for a world of maintenance headaches and a whole lot of
fun debugging stuff.
b.bum
I can't imagine much use for method_exchangeImplementations() with my
own classes, because I can write the implementations myself. You
certainly need to be careful, but my example seemed pretty innocuous.
Calling at the beginning of main() should avoid IMP caching issues,
should it not?
As I said, I was just doing a proof of concept. I haven't shipped any
apps with that code, though I know plenty of developers who have
shipped much 'worse'. Even published API sometimes break, so
development always involves the assessment of risk and reward, and as
far as I know, Apple provides no official support for developers
apart from paid DTS incidents.
-Jeff
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