Adding push notifications to an app

Last week, I’ve built an app that’s quite simple: there are categories, and in the categories are messages. People can subscribe to a category and get a push notification when there’s a new message in the category, or when a significant change was made to the message. The badge icon should update appropriately, showing the number of unread messages. There are lots of tricky things when implementing this, and the point of this article is to explore them.

In a naive approach, you can start by implement the application delegate’s application:didReceiveRemoteNotification. This method is called when you receive a push notification. However, it is only called when your application is running. When your application is not running, this method doesn’t get called. You also need to implement application:didFinishLaunchingWithOptions:, and check if there is a push notification in the options (this dictionary gets set when you open the app by clicking on the notification).

The problem, however, is that not all push notifications arrive. For example, if you get 5 push notifications, but open the app via the home screen, you will not get any notification in the options dictionary, and you won’t get a didReceiveRemoteNotification either.

To do push notifications correctly, you will need to keep a list on the server, and on application startup your app needs to synchronise with the server. That way, you can mark the right and also tell the server that you have seen the push notification.

Now, when you exit the app you can set the badge count based on the number of unread push notification. However, the server needs to know this number as well, so that it can correctly set the badge number in the next push notification that is sent. When you read a message that was marked by the push notification, you need to tell the server as well. (Edge-case: when you read the message in offline mode, you will need to tell the server once you’re back online).

In order to do push notifications right, the client and server need to communicate a lot. Hopefully, in your next project, you can read this article so you don’t have to find these things out while you are programming.

In a next article, I will sketch the application architecture I used for the app above.