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?
(There is an exception to the above, but it's hardly worth
mentioning. If your property has neither indexed to-many mutator
methods nor the simple setter method, and
+accessInstanceVariablesDirectly returns YES, then the proxy may
directly access the NSMutableArray-like ivar. In that case, the proxy
can do efficient operations on the property along with efficient KVO
notifications using did/willChange:valuesAtIndexes:forKey:. However,
except for cases of backward compatibility, you should always override
+accessInstanceVariablesDirectly to return NO for your classes.
Allowing KVC to directly modify your internal state without going
through your methods is bad mojo.)
Regards,
Ken
[1] The indexed to-many accessors for array properties are described
here:
http://developer.apple.com/documentation/Cocoa/Conceptual/KeyValueCoding/Concepts/AccessorConventions.html#/
/apple_ref/doc/uid/20002174-178830-BAJEDEFB
See also the documentation for -mutableArrayValueForKey: although some
additional methods are only documented in NSKeyValueCoding.h at the
declaration of -mutableArrayValueForKey:.
The (non-indexed) to-many accessors for set properties are described
in the documentation for -mutableSetValueForKey:.
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