CoreFoundation: Receive/process notifications in background thread

Question

I'm writing a simple application that should be able to receive and process notifications in a background thread using Apple's CoreFoundation framework. Here is what I'm trying to accomplish:

static void DummyCallback(CFNotificationCenterRef center,
              void *observer,
              CFStringRef name,
              const void *object,
              CFDictionaryRef userInfo) {
    printf("RECEIVED NOTIFICATION\n");
}

void *ThreadStart(void *arg) {
    CFNotificationCenterAddObserver(CFNotificationCenterGetDistributedCenter(),
                    NULL, 
                    &DummyCallback, 
                    NULL,
                    CFSTR("TEST_OBJECT"),
                    CFNotificationSuspensionBehaviorDeliverImmediately);

    printf("background thread: run run loop (should take 5 sec to exit)\n");
    int retval = CFRunLoopRunInMode(kCFRunLoopDefaultMode, 5, true);
    printf("background thread: exited from run loop (retval: %d)\n", retval);
    return NULL;
}

int main(int argc, char** argv) {
    pthread_t thread;
    int rc = pthread_create(&thread, NULL, &ThreadStart, NULL);
    assert(rc == 0);

    printf("main: sleep\n");
    sleep(10);
    printf("main: done sleeping\n");

    return 0;
}

If I run the program I just get

main: sleep
background thread: run run loop (should take 5 sec to exit)
background thread: exited from run loop (retval: 1)
main: done sleeping

The problem is that the background thread's run loop exits immediately (return code kCFRunLoopRunFinished instead of kCFRunLoopRunTimedOut) because there is no source/observer/timer. CFNotificationCenterAddObserver registers itself only with the run loop of the main thread but not the one of my background thread.

I need the main thread for some other stuff and can't use it to run it's run loop. Is there any way to get this working? Maybe by registering CFNotificationCenter with the run loop of the background thread?

Thanks in advance!

score 1 · Answer 1 · answered Jul 06 '11 at 19:17

1

As stated in http://developer.apple.com/library/mac/#documentation/CoreFoundation/Reference/CFNotificationCenterRef/Reference/reference.html

The first time an observer is registered with a distributed notification center, the notification center creates a connection with the system-wide notification server and places a listening port into the common modes of the current thread’s run loop. When a notification is delivered, it is processed on this initial thread, even if the observer that is receiving the notification registered for the notification on a different thread.

Because loaded frameworks may potentially spawn threads and add their own observers before your code executes, you cannot know for certain which thread will receive distributed notifications. If you need to control which thread processes a notification, your callback function must be able to forward the notification to the proper thread. You can use a CFMessagePort object or a custom CFRunLoopSource object to send notifications to the correct thread’s run loop.

answered Jul 06 '11 at 19:17

Andrew

89
1
2

Thanks for your answer. I read that passage too, but this does not solve my problem. In order to receive notifications a CFRunLoop is needed in the main thread. There should be a way around that. – Benjamin Jul 06 '11 at 23:19
Implement your own notification center that will care about thread's run loop. – Andrew Jul 08 '11 at 11:27
Thought about that too, but I could not figure out how to connect it to the distnoted daemon. This part of Core Foundation seems to be closed source. And I'm not sure how future proof that solution would be. Apple might change some of the internals at some point – Benjamin Jul 08 '11 at 12:30
Yeap. the only solution would probably be to implement your own "notification dispatcher" that will receive dstributed notifications in main tread and dispatch the corresponding messages to the dedicated thread. Main thread will be used anyway.... – Andrew Jul 08 '11 at 18:09

CoreFoundation: Receive/process notifications in background thread

1 Answers1