Notification when the system is idle or not on OS X - macos

Question

I know that there is some way to get the system idle time with the IOKit framework on OS X, but I want to know if there's notifications available.
I can create a timer to check if the idle time is more than x, and that's fine. It doesn't matter if I detect the idle mode a few seconds later.
The problem is to detect when the Mac is not idle anymore. I want my app to show a notification as soon as possible, not a few seconds later.
Is there a way to have a notification for that? (iChat seems to have one)

Answer 1

This is from http://developer.apple.com/library/mac/#qa/qa1340/_index.html (from Bill the Lizard comment)
- (void) receiveSleepNote: (NSNotification*) note
{
NSLog(#"receiveSleepNote: %#", [note name]);
}
- (void) receiveWakeNote: (NSNotification*) note
{
NSLog(#"receiveWakeNote: %#", [note name]);
}
- (void) fileNotifications
{
//These notifications are filed on NSWorkspace's notification center, not the default
// notification center. You will not receive sleep/wake notifications if you file
//with the default notification center.
[[[NSWorkspace sharedWorkspace] notificationCenter] addObserver: self
selector: #selector(receiveSleepNote:)
name: NSWorkspaceWillSleepNotification object: NULL];
[[[NSWorkspace sharedWorkspace] notificationCenter] addObserver: self
selector: #selector(receiveWakeNote:)
name: NSWorkspaceDidWakeNotification object: NULL];
}

Answer 2

NSTimeInterval GetIdleTimeInterval() {
io_iterator_t iter = 0;
int64_t nanoseconds = 0;
if (IOServiceGetMatchingServices(kIOMasterPortDefault, IOServiceMatching("IOHIDSystem"), &iter) == KERN_SUCCESS) {
io_registry_entry_t entry = IOIteratorNext(iter);
if (entry) {
CFMutableDictionaryRef dict;
if (IORegistryEntryCreateCFProperties(entry, &dict, kCFAllocatorDefault, 0) == KERN_SUCCESS) {
CFNumberRef obj = CFDictionaryGetValue(dict, CFSTR("HIDIdleTime"));
if (obj)
CFNumberGetValue(obj, kCFNumberSInt64Type, &nanoseconds);
CFRelease(dict);
}
IOObjectRelease(entry);
}
IOObjectRelease(iter);
}
return (double)nanoseconds / 1000000000.0;
}