On Jan 19, 2009, at 15:11, Ken Thomases wrote:
On Jan 19, 2009, at 3:27 PM, Quincey Morris wrote:
2. If some change affects which objects are cached, update the
cache array KVO-compliantly. (Use standard NSMutableArray methods
on [yourCacheOwner mutableArrayValueForKey: @"yourCacheArray"]
instead of yourCacheArray.)
This is common advice, but in my opinion it's misguided.
While directly mutating an array ivar doesn't generate KVO
notifications, and mutating the proxy returned by -
mutableArrayValueForKey: does, that's not the best means to gain KVO
compliance. You should instead implement the indexed to-many
accessors[1] for any to-many property. Then, you should use them to
mutate the property in your own code.
What's wrong with -mutableArrayValueForKey:?
Here's one good guideline: in general, KVC is for accessing a
property based on dynamic data rather than compile-time identifier.
If you find yourself hard-coding string literals with property
names, then You're Doing It Wrong(tm).
More importantly, -mutableArrayValueForKey: doesn't magically allow
for mutations to the property that aren't already allowed by your
class's interface. In particular, if you don't provide the indexed
to-many accessors, then mutating the proxy returned by -
mutableArrayValueForKey: ends up doing the inefficient call to
set<Key>: that you were trying to avoid. (In other words, it has
the same negative implications as will/didChangeValueForKey:.)
Since that's the case, why not just call -set<Key>: yourself rather
than having the proxy do it?
Sorry, you are correct. Assuming you're not depending on direct
instance variable access, my statement should have been:
Use standard NSMutableArray methods on [yourCacheOwner
mutableArrayValueForKey: @"yourCacheArrayProperty"] instead of
yourCacheArray, and implement the 2 or 3 required indexed accessor
methods to make it work.
In that case, using those indexed accessor methods directly *may* be
more convenient than using the mutableArrayValueForKey proxy, but they
are only a limited subset of the methods available to NSArray and
NSMutableArray. If you want to use addObject: or removeAllObjects or
addObjectsFromArray: etc, you're going to have to provide boilerplate
implementations of all of them, which translate them into the few
basic accessors. It's easy, but tedious to do over and over, for each
new property. (I tried that, and decided to go back to using the proxy
before long.)
You can easily hide the ugly compile time string by corralling it into
a single method: define a @property (readonly) NSMutableArray*
mutableYourCacheArrayProperty that returns the proxy object.
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