Check for multiple instances of a Cocoa Application. it is recommended that this be checked for in `applicationWillFinishLaunching:`.

CFIUniqueInstanceCheck.m

static NSMachPort *existancePort = nil;

/**
 * Used by the CFICheckForOtherRunningInstancesOfCurrentApplication() function to
 * create a unique NSMachPort object.  This string *CANNOT CHANGE* in the future,
 * otherwise conflicting versions of an App will run on the same machine and
 * defeat the purpose of this function.
 */
static NSString *const CFIPortNameConstant = @"SomeNon-ArbitraryPortName";

/**
 * Attempts to find out whether the same port has been registered twice with
 * the Mach Bootstrap server.  If so, a message is presented and the other 
 * instance of the app will terminate.
 */
static void CFICheckForOtherRunningInstancesOfCurrentApplication() {
  if (existancePort == nil) {
		existancePort = [[NSMachPort alloc] init];
	}
	BOOL noOtherInstance = [[NSMachBootstrapServer sharedInstance]registerPort:existancePort name:CFIPortNameConstant];
	if (noOtherInstance) return;
	
	NSString *applicationName = NSBundle.mainBundle.infoDictionary[@"CFBundleName"];
	NSString *alertText = [NSString stringWithFormat:@"Another instance of %@ is already running!", applicationName];
	NSString *alertInformativeText = [NSString stringWithFormat:@"Please quit the other instance of %@", applicationName];
	//The `%@, String` redundancy courtesy the Apple engineer who thought that was a good idea at the time.
	NSAlert *instanceAlert = [NSAlert alertWithMessageText:alertText defaultButton:nil alternateButton:nil otherButton:nil informativeTextWithFormat:@"%@", alertInformativeText];
	[instanceAlert runModal];
	[existancePort invalidate];
	[NSRunningApplication.currentApplication terminate];
}