mutableArrayValueForValueOfNilTest

Demonstrates a KVC gotcha you might not have guessed

Let's say you're doing something like this:

things = [obj mutableArrayValueForKey:@"foo"];

This (as of 10.9.3) will return non-nil, even if obj.foo is nil.

In fact, the value of obj.foo (nil, non-array, or otherwise) when mutableArrayValueForKey: is called is actually immaterial. mutableArrayValueForKey: simply always succeeds, and returns a mutable array. As of 10.9.3, the class of this array is NSKeyValueSlowMutableArray.

What really matters is the value of obj.foo when you try to use the KVC mutable array.

If obj.foo is not nil, but is not an array, the KVC array will send that object messages it doesn't recognize (assuming that it's an array and treating it as such), and you will get a doesNotRecognizeSelector: exception.

If obj.foo is nil, an assertion inside the KVC array's code will fail, and you will get an NSInternalInconsistencyException.

Either way, when you try to use the array, not when you try to get it.

TL;DR

mutableArrayValueForKey: always returns an array, regardless of the actual value for that key.

That array will throw an exception when used if the actual value for that key at that time is nil.