NSKeyValueObserving's use of a context pointer for identifying interested observations

gistfile1.md

The NSKeyValueObserving addition to NSObject adds an observe message that takes an identifying context pointer – don't use NULL in its place.

Your superclass may observe the same key path for the same object. You need to pass along its observations, and only take actions on yours. The context pointer distinguishes which observations are yours.

static void *kContext = &kContext;

// …

// alternatively, you could just use self as the context pointer
void *context = (__bridge void *) self;

[obj addObserver:self
forKeyPath:key
       options:0
         context:kContext]; // <-- context pointer

// …

- (void)observeValueForKeyPath:(NSString *)keyPath
                      ofObject:(id)object
                        change:(NSDictionary *)change
                       context:(void *)context
{
    if (context == kContext)
        // do stuff
    else
        [super observeValueForKeyPath:keyPath
                             ofObject:object
                               change:change
                              context:context];
}

via Mike Ash's Defensive Programming in Cocoa talk – PDF.

Alternatively, you could use self as the context pointer:

void *context = (__bridge void *) self;

[obj addObserver:self
      forKeyPath:key
         options:0
         context:context];

// …

- (void)observeValueForKeyPath:(NSString *)keyPath
                      ofObject:(id)object
                        change:(NSDictionary *)change
                       context:(void *)context
{
    if (context != (__bridge void *)self) {
        [super observeValueForKeyPath:keyPath
                             ofObject:object
                               change:change
                              context:context];
        return;
    }

    // do stuff
}