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];
}

I feel like I should point you at @mikeash's incredibly clever use of C:

static const void *MyFooContext = &MyFooContext;

MyFooContext is initialized at load time with a pointer to itself.

Yeah, but I really like being able to print out the context in OVFKP while debugging and get something useful.

It looks like you missed a mention ;-)

@ddribin Re http://www.dribin.org/dave/blog/archives/2008/09/24/proper_kvo_usage/ You can correlate the context address with a particular context variable: (gdb) info symbol context

Or with lldb (given that you defined MyFooContext as Kyle described):

(lldb) image lookup -a `context`
      Address: MyApp[0x00026258] (MyApp.__DATA.__data + 4)
      Summary: MyFooContext

Oh… nice! Yeah, I didn’t know about the … syntax. That does work well. Less hacky, too. Thanks!

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…