Evil KVO context hack using macros + C99 compound literal syntax.

gistfile1.m

// Building upon: http://www.dribin.org/dave/blog/archives/2008/09/24/proper_kvo_usage/
// Defines a unique KVO context by taking the address of a 2-element string array.
// Using __FILE__ ensures the address is unique across source files, even with link time optimization.
#define DDDefineContext(_X_) static void * _X_ = &(const char *[] ) {#_X_, __FILE__}

// Example Definition
DDDefineContext(MyFooContext);

// Example Usage. No need for &MyFooContext to make it unique.
- (void)startObserving
{
    [_ivar addObserver:self forKeyPath:@"foo" options:0 context:MyFooContext];
}

- (void)observeValueForKeyPath:(NSString *)keyPath ofObject:(id)object change:(NSDictionary *)change context:(void *)context
{
    // In a debugger, use `p *(char **)context` to dump the context
    if (context == MyFooContext) {
        // foo changed
    } else {
        [super observeValueForKeyPath:keyPath ofObject:object change:change context:context];
    }
}

- (void)stopObserving
{
    [_ivar removeObserver:self forKeyPath:@"foo" context:MyFooContext];
}

Because gist comment markdown sucks, I didn't realize you folks were referring to the shell-like backtick syntax. I was worried vararg macros were somehow involved…