OSX 10.10 file change event from dispatch queue - macos

Question

I try to monitor file changes on OSX 10.10, starting with a fresh Cocoa application in Xcode, just adding the following code.
If I uncomment the last line in the snippet then I receive the file change events perfectly fine. But I can not make this last call because it should be a Cocoa GUI application.
I digged through a lot of documentation and can't find my error. Do I have to initialize or start this whole dispatch subsystem somehow?
- (void)applicationDidFinishLaunching:(NSNotification *)aNotification {
int fd = open("<FILENAME>", O_EVTONLY);
if (fd == -1) return;
dispatch_queue_t qu = dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0);
if (!qu) {
printf("can not get queue");
return;
}
unsigned long mask =
DISPATCH_VNODE_DELETE |
DISPATCH_VNODE_WRITE |
DISPATCH_VNODE_EXTEND |
DISPATCH_VNODE_ATTRIB |
DISPATCH_VNODE_LINK |
DISPATCH_VNODE_RENAME |
DISPATCH_VNODE_REVOKE;
dispatch_source_t source = dispatch_source_create(DISPATCH_SOURCE_TYPE_VNODE, fd, mask, qu);
printf("source created\n");
if (!source) {
close(fd);
return;
}
printf("source valid\n");
dispatch_source_set_event_handler(source, ^{
printf("FILE CHANGED\n");
});
dispatch_resume(source);
printf("source resumed\n");
// If I call dispatch_main() I will receive the file system events as expected.
// But as a Cocoa application, I must not call this.
// Instead, I was under the impression that NSApplicationMain handles this.
//dispatch_main();
}

Answer 1

Grand Central Dispatch objects, such as dispatch sources, are automatically retained and released by ARC in recent versions of the compiler and frameworks.
At the end of your method, the last strong reference to source is lost and ARC is issuing an automatic dispatch_release(source). (It would also release the queue, but the source has another strong reference to that. So, if the source survived, so would the queue.)
You need to keep a strong reference to the source in an instance variable.