How can I update information in an Android Activity from a background Service

How can I set up my Activity to be listening to the Service? Is this the best way to approach this problem?

You have three major options, as I see it:

1. Polling. The Activity periodically asks the Service for the latest data. IMHO, this option sucks, but it's certainly possible.

2. Callbacks. Per jax's answer, the Activity registers a callback object ("observer") with the Service. The Service invokes a method on the callback when the data changes, which in turn updates the UI. You can see an example of using that with a Service here.

3. Broadcast Intents. The Service broadcasts an Intent via sendBroadcast() on a data change. The Activity registers a BroadcastReceiver using registerReceiver(), and that BroadcastReceiver is notified of an incoming broadcast. This triggers the Activity to load the latest data from the Service, or possibly just to get the latest data out of extras in the broadcast Intent. You can see an example of using that technique with a Service here.

This sound like a good candidate for the Observer Pattern. Basically your activity (The Observer) will register itself with the background service (The Observable) and you can push or pull data from your Activity. Your Observer in this case will be your Activity and the Observable will be your Service.

If you know nothing about Design Patterns buy "Head First Design Patterns", it is easy to read and is full of great information.

PS: I am reading it now.

I am really, really wondering why no one mentioned a simple approach using an EventBus by whatever library. This is of course if you are not using RX. My personal favorite is EventBus by GreenRobot.https://github.com/greenrobot/EventBus

With just a couple of lines of code, and no interfaces. Fire an event, and listen for it wherever you want. It is decoupled, it is thread safe, and it will not crash your app.