In Cocoa KVO, why doesn't a change on a NSMutableArray proxy notify observers?

Question

I\'m implementing a DocumentsManager class in iOS, and I want to make a to-many property called documents to be KVO compliant. It seems to mostly w

Answer 1

It turns out that mutableArrayValueForKey: does not always return a notifying array. It only does so when observers have already been registered on the observed object!

So swapping my first two lines fixes the problem:

[[DocumentsManager instance] addObserver:self forKeyPath:@"documents" options:NSKeyValueObservingOptionNew context:NULL];
NSMutableArray *docsProxy = [[DocumentsManager instance] mutableArrayValueForKey:@"documents"];

Can't help thinking how much time we'd save if we could read the source code of those methods…