main.m

#import <Foundation/Foundation.h>
#import <Cocoa/Cocoa.h>
#import <AppKit/AppKit.h>
#import <SystemConfiguration/SystemConfiguration.h>
#import <Carbon/Carbon.h>
#import <CoreFoundation/CoreFoundation.h>

static CFStringRef CopyCurrentConsoleUsername(SCDynamicStoreRef store)
{
	CFStringRef result;

	result = SCDynamicStoreCopyConsoleUser(store, NULL, NULL);

	if ((result) && CFEqual(result, CFSTR("loginwindow")))
	{
		CFRelease(result);
		result = NULL;
	}

	return result;
}

static CFStringRef gCurrentConsoleUser;

static void onSCConsoleUserChange(SCDynamicStoreRef store, CFArrayRef changedKeys, void* info)
{
	CFStringRef currentConsoleUser;
	Boolean didChange;

	currentConsoleUser = CopyCurrentConsoleUsername(store);

	didChange = (gCurrentConsoleUser != NULL) != (currentConsoleUser != NULL);

	if (!didChange && (gCurrentConsoleUser != NULL) )
	{
		didChange = !CFEqual(gCurrentConsoleUser, currentConsoleUser);
	}

	if (didChange)
	{
		if (gCurrentConsoleUser)
			CFRelease(gCurrentConsoleUser);

		gCurrentConsoleUser = currentConsoleUser;

		if (gCurrentConsoleUser)
			CFRetain(gCurrentConsoleUser);

		NSLog(@"gCurrentConsoleUser: %@", (__bridge NSString*) gCurrentConsoleUser);
	}
}

OSStatus onCarbonNotification(EventHandlerCallRef nextHandler, EventRef switchEvent, void* userData)
{
	if (GetEventKind(switchEvent)== kEventSystemUserSessionActivated)
	{
		NSLog(@"kEventSystemUserSessionActivated");
	}
	else if (GetEventKind(switchEvent)== kEventSystemUserSessionDeactivated)
	{
		NSLog(@"kEventSystemUserSessionDeactivated");
	}

	return noErr;
}

void onDistNotification(CFNotificationCenterRef center, void* observer, CFNotificationName name, const void* object, CFDictionaryRef userInfo)
{
	NSLog(@"onDistNotification: %@", (__bridge NSString*) name);
}

@interface Notifier : NSObject
{

}

@end

@implementation Notifier

- (void)onWorkspaceNotification:(NSNotification*)notification
{
	if ([notification.name isEqualToString:NSWorkspaceSessionDidBecomeActiveNotification])
	{
		NSLog(@"NSWorkspaceSessionDidBecomeActiveNotification");
	}
	else if ([notification.name isEqualToString:NSWorkspaceSessionDidBecomeActiveNotification])
	{
		NSLog(@"NSWorkspaceSessionDidResignActiveNotification");
	}
}

- (instancetype)init
{
	self = [super init];

	if (self)
	{
		//
		[[[NSWorkspace sharedWorkspace] notificationCenter] addObserver:self
								       selector:@selector(onWorkspaceNotification:)
									   name:NSWorkspaceSessionDidBecomeActiveNotification
									 object:nil];
		[[[NSWorkspace sharedWorkspace] notificationCenter] addObserver:self
								       selector:@selector(onWorkspaceNotification:)
									   name:NSWorkspaceSessionDidResignActiveNotification
									 object:nil];

		//
		EventTypeSpec switchEventTypes[2];
		EventHandlerUPP switchEventHandler;

		switchEventTypes[0].eventClass = kEventClassSystem;
		switchEventTypes[0].eventKind = kEventSystemUserSessionDeactivated;
		switchEventTypes[1].eventClass = kEventClassSystem;
		switchEventTypes[1].eventKind = kEventSystemUserSessionActivated;

		switchEventHandler = NewEventHandlerUPP(onCarbonNotification);
		InstallApplicationEventHandler(switchEventHandler, 2, switchEventTypes, NULL, NULL);

		//
		Boolean success;
		SCDynamicStoreRef store;
		CFStringRef key;
		CFArrayRef keys;
		CFRunLoopSourceRef rls;

		store = SCDynamicStoreCreate(NULL, CFSTR("com.apple.dts.ConsoleUser"), onSCConsoleUserChange, NULL);
		key = SCDynamicStoreKeyCreateConsoleUser(NULL);
		keys = CFArrayCreate(NULL, (const void **) &key, 1, &kCFTypeArrayCallBacks);
		success = SCDynamicStoreSetNotificationKeys(store, keys, NULL);
		rls = SCDynamicStoreCreateRunLoopSource(NULL, store, 0);

		CFRunLoopAddSource(CFRunLoopGetCurrent(), rls, kCFRunLoopDefaultMode);

		gCurrentConsoleUser = CopyCurrentConsoleUsername(store);
		NSLog(@"gCurrentConsoleUser: %@", (__bridge NSString*) gCurrentConsoleUser);

		//
		CFNotificationCenterRef distNc = CFNotificationCenterGetDistributedCenter();
		CFNotificationCenterAddObserver(distNc, (__bridge const void *) self, onDistNotification, (__bridge CFStringRef) @"com.apple.screensaver.didlaunch", nil,
						CFNotificationSuspensionBehaviorDeliverImmediately);
		CFNotificationCenterAddObserver(distNc, (__bridge const void *) self, onDistNotification, (__bridge CFStringRef) @"com.apple.screensaver.didstart", nil,
						CFNotificationSuspensionBehaviorDeliverImmediately);
		CFNotificationCenterAddObserver(distNc, NULL, onDistNotification, (__bridge CFStringRef) @"com.apple.screensaver.didstop", nil,
						CFNotificationSuspensionBehaviorDeliverImmediately);
		CFNotificationCenterAddObserver(distNc, NULL, onDistNotification, (__bridge CFStringRef) @"com.apple.shieldWindowRaised", nil,
						CFNotificationSuspensionBehaviorDeliverImmediately);
		CFNotificationCenterAddObserver(distNc, NULL, onDistNotification, (__bridge CFStringRef) @"com.apple.shieldWindowLowered", nil,
						CFNotificationSuspensionBehaviorDeliverImmediately);
		CFNotificationCenterAddObserver(distNc, NULL, onDistNotification, (__bridge CFStringRef) @"com.apple.screenIsLocked", nil,
						CFNotificationSuspensionBehaviorDeliverImmediately);
		CFNotificationCenterAddObserver(distNc, NULL, onDistNotification, (__bridge CFStringRef) @"com.apple.screenIsUnlocked", nil,
						CFNotificationSuspensionBehaviorDeliverImmediately);
		CFNotificationCenterAddObserver(distNc, (__bridge const void *) self, onDistNotification, (__bridge CFStringRef) @"com.apple.sessionDidMoveOffConsole", nil,
						CFNotificationSuspensionBehaviorDeliverImmediately);
		CFNotificationCenterAddObserver(distNc, (__bridge const void *) self, onDistNotification, (__bridge CFStringRef) @"com.apple.sessionDidMoveOnConsole", nil,
						CFNotificationSuspensionBehaviorDeliverImmediately);
		// com.apple.userWillLogOut
	}

	return self;
}

@end

int main(int argc, const char * argv[])
{
	@autoreleasepool
	{
		Notifier* notifier = [[Notifier alloc] init];

		CFRunLoopRun();
	}

	return 0;
}