How to display messages when app is running in the foreground?

In order to display incoming messages (both pushed by APNs and pulled from the server) while your application is running in the foreground, you have to:

1. Subscribe to MMNotificationMessageReceived notification:

   //Objective-C
   [[NSNotificationCenter defaultCenter] addObserver: self
                                            selector: @selector(handleNewMessageReceivedNotification:)
                                                name: MMNotificationMessageReceived
                                              object: nil];

   //Swift
   NotificationCenter.default.addObserver(self,
   			selector: #selector(self.handleNewMessageReceivedNotification(_:)),
   			name: NSNotification.Name(rawValue:  MMNotificationMessageReceived),
   			object: nil)

2. Handle the notifications:

   //Objective-C
   -(void)handleNewMessageReceivedNotification:(NSNotification *)notification {
   	// Display the message only if the application is in the foreground
   	if ([UIApplication sharedApplication].applicationState == UIApplicationStateActive) {
   		id messagePayload = notification.userInfo[MMNotificationKeyMessage];
   		if ([messagePayload isKindOfClass:[MTMessage class]]) {
   			MTMessage * message = messagePayload;
   		
   			// here comes your custom UI, i.e. standard UIAlertController:
   			UIAlertController * alertController = [UIAlertController alertControllerWithTitle:@"New message" message:message.text preferredStyle:UIAlertControllerStyleAlert];
   			[alertController addAction: [UIAlertAction actionWithTitle:@"Dismiss" style:UIAlertActionStyleCancel handler:nil]];
   			UIViewController * presentingVC = [UIApplication sharedApplication].keyWindow.rootViewController;
   			[presentingVC presentViewController:alertController animated:YES completion:nil];
   		}
   	}
   }

   //Swift
   @objc func handleNewMessageReceivedNotification(_ notification: NSNotification) {
   	// Only display the message if the application is in the foreground
   	guard UIApplication.shared.applicationState == .active,
   		let userInfo = notification.userInfo,
   		let message = userInfo[MMNotificationKeyMessage] as? MTMessage
   	else {
   		return
   	}
   	
   	// here comes your custom UI, i.e. standard UIAlertController:
   	let alertController = UIAlertController(title: "New message", message: message.text, preferredStyle: .alert)
   	alertController.addAction(UIAlertAction(title: "Dismiss", style: .cancel, handler: nil))
   	if let presentingVC = UIApplication.shared.keyWindow?.rootViewController {
   		presentingVC.present(alertController, animated: true, completion: nil)
   	}
   }