Monday, September 27, 2010

Getting the APNS device token

As alluded to in my previous post, this time I'm covering how to get the APNS device token for a given iOS client. Actually, it is pretty straightforward. First, call registerForRemoteNotificationTypes from your application's didFinishLaunchingWithOptions UIApplicationDelegate callback. You need to specify which type of notifications your application will accept. Here is an example:
- (BOOL)application:(UIApplication *)application didFinishLaunchingWithOptions:(NSDictionary *)launchOptions

// Register with the Apple Push Notification Service
// so we can receive notifications from our server
// application. Upon successful registration, our
// didRegisterForRemoteNotificationsWithDeviceToken:
// delegate callback will be invoked with our unique
// device token.
UIRemoteNotificationType allowedNotifications = UIRemoteNotificationTypeAlert
| UIRemoteNotificationTypeSound
| UIRemoteNotificationTypeBadge;
[[UIApplication sharedApplication] registerForRemoteNotificationTypes:allowedNotifications];


return YES;

As mentioned in the code comments, the application will then talk to Apple's Push Notification Service in the background and, when the device's unique token has been issued, your application delegate's didRegisterForRemoteNotificationsWithDeviceToken callback is invoked. This is where you actually get the device token.
- (void)application:(UIApplication *)application
didRegisterForRemoteNotificationsWithDeviceToken:(NSData *)deviceToken
NSLog(@"didRegisterForRemoteNotificationsWithDeviceToken: %@", deviceToken);
// Stash the deviceToken data somewhere to send it to
// your application server.

Once your iOS client application knows its own device token, it needs to send it to your application server so that the application server can push notifications back to the client later. How you do this depends on the architecture of your client-server communication.

At this point, I should add the device tokens can change. So, I recommend repeating the above logic every time your iOS client application starts so your application server always gets the latest device token for the user's terminal.

I would be remiss to not mention error handling. It is possible that the registerForRemoteNotificationTypes call will fail. The most obvious way it could fail is if the user does not have access to the 3G or WiFi networks and, as a result, cannot communicate with Apple's Push Notification Service; for example, when the device is in Airplane Mode with the wireless signals turned off.

In this case, the didFailToRegisterForRemoteNotificationsWithError delegate callback is invoked instead of didRegisterForRemoteNotificationsWithDeviceToken. In which case, you probably want to retry the registration later when network connectivity is restored.

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