runloopobserver

in interface:
@property(nonatomic) CFRunLoopObserverRef runloopObserver;

in init:
    self.runloopObserver = CFRunLoopObserverCreate(NULL, kCFRunLoopAllActivities, true, 0, &runloopCallback, NULL);
    CFRunLoopAddObserver(CFRunLoopGetMain(), self.runloopObserver, kCFRunLoopCommonModes);
    CFRunLoopRemoveObserver(CFRunLoopGetMain(), self.runloopObserver, kCFRunLoopCommonModes);

void runloopCallback(CFRunLoopObserverRef observer, CFRunLoopActivity activity, void *info) {
    NSLog(@"runloop %lu", activity);
    switch(activity) {
        case kCFRunLoopExit:
            NSLog(@"  kCFRunLoopExit");
            break;
        case kCFRunLoopEntry:
            NSLog(@"  kCFRunLoopEntry");
            break;
        case kCFRunLoopAfterWaiting:
            NSLog(@"  kCFRunLoopAfterWaiting");
            break;
        case kCFRunLoopBeforeTimers:
            NSLog(@"  kCFRunLoopBeforeTimers");
            break;
        case kCFRunLoopAllActivities:
            NSLog(@"  kCFRunLoopAllActivities");
            break;
        case kCFRunLoopBeforeSources:
            NSLog(@"  kCFRunLoopBeforeSources");
            break;
        case kCFRunLoopBeforeWaiting:
            NSLog(@"  kCFRunLoopBeforeWaiting");
            break;
    }
}