How can I send a userInfo dict using CFNotificationCenterGetDarwinNotifyCenter()

Question

I need to send an object using CFNotificationCenterGetDarwinNotifyCenter() however, I'm noticing that whenever I send a notification using

const void *tkeys[1] = { @"testKey" };
const void *tvalues[1] = { @"testValue" };
CFDictionaryRef userInfo = CFDictionaryCreate(NULL, tkeys, tvalues, 1, NULL, NULL);
CFNotificationCenterPostNotification(CFNotificationCenterGetDarwinNotifyCenter(),
                                       CFSTR("foo"),
                                       object,
                                       userInfo,
                                       true);

then I find that in the observer callback, the userInfo dict is NULL. Is there something I am missing here? Is there some other way by which I can possibly send the userInfo object? I was looking at http://nshipster.com/inter-process-communication/ and the Distributed Notifications part, however whenever I use :

CFNotificationCenterRef distributedCenter =
CFNotificationCenterGetDistributedCenter();

then I get an error Invalid Use of Function error whenever I try it.

Answer 1

If you read the documentation for CFNotificationCenterGetDarwinNotifyCenter, you'll find this caveat:

Several function parameters are ignored by Darwin notification centers. To ensure future compatibility, you should pass NULL or 0 for all ignored arguments.

userInfo is one of those parameters. CFNotificationCenter.h goes into more detail:

// For this center, there are limitations in the API. There are no notification "objects",
// "userInfo" cannot be passed in the notification, ...
// - CFNotificationCenterPostNotification(): the 'object', 'userInfo', and 'deliverImmediately' arguments are ignored.

Why? Dig a little deeper. The documentation also mentions that this type of notification center is built on the Core OS notify mechanism declared in notify.h. If you look that up, you'll find that there is only one way to post a notification, by calling notify_post(), which takes only a single string argument. There is no way to add additional data to the notification. This is a very old and rudimentary IPC mechanism that doesn't know anything about CoreFoundation data structures.

As for CFNotificationCenterGetDistributedCenter, that isn't available on iOS.