> On Sep 21, 2016, at 8:00 PM, Slipp Douglas Thompson 
> <apple+cocoa-...@slippyd.com> wrote:
>> On Sep 21, 2016, at 17:01 , Graham Cox <graham....@bigpond.com> wrote:
>>> This should be: if([(NSString*)context 
>>> isEqualToString:@“mediaLibraryLoaded”])…
>> Actually, this is not a good idea either, because *other* observations — 
>> ones you don’t control — might use a value that’s not an object, or not even 
>> a valid pointer.

I see your point about context plausibly not being neither an NSObject nor nil. 
 While you could check if context is an NSObject beforehand (is there even a 
reliable way to do this?  CocoaWithLove 
 couldn't find a great approach ;-/)— I think coding consistently against all 
other addObserver: calls in your app is a good-enough solution.  I.E. If you 
implement observeValueForKeyPath: with [context isEqual: …] checks, make sure 
all the other addObserver: calls (for those same keys, at least) are using nil 
or NSObjects.


— Slipp

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