> On 22 Sep 2016, at 02:01, Graham Cox <graham....@bigpond.com> wrote:
>
>
>> On 22 Sep 2016, at 9:44 AM, Gabriel Zachmann <z...@tu-clausthal.de> wrote:
>>
>> I have found on the net
>
> That isn’t always a recommendation ;)
>
>
>> if ( context == (__bridge void *) @"mediaLibraryLoaded" )
Gabriel,
this is a pointer comparison, not a string comparison. If the addObserver call
happens in another module (usually unlikely) or Apple changes how its compiler
coalesces string constants, this will break.
> Don’t do this, even if it appears to work. You got lucky, or are taking
> advantage of undocumented implementation details.
>
> This should be: if([(NSString*)context
> isEqualToString:@“mediaLibraryLoaded”])…
>
> I expect the first thing -isEqualToString: does is a pointer comparison, so
> it’s unlikely to be significantly less performant for the case of when the
> pointers are literally identical.
No, Graham, don't do that!
There is no guarantee that the context is a valid object. It is just supposed
to be a unique pointer value so your class can tell a KVO notification from
notifications for other observers on the same object (e.g. if a subclass
observes the same value as a base class).
The best (but a bit clever) way to declare your context is
void* kMediaLibraryLoadedContext = &kMediaLibraryLoadedContext;
this may look invalid, but actually just reserves a bit of memory in your app's
globals section that now has a unique address. As a convenience, it writes this
address into itself. So instead of &kMediaLibraryLoadedContext you can just
write kMediaLibraryLoadedContext.
Cheers,
-- Uli
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