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

Question

Hello,

I am trying to create a simple Android application that has a ActivityList of information, when the application starts, I plan to start a Service that will be constantly calculating the data (it will be changing) and I want the ActivityList to be in sync with the data that the service is calculating for the life of the app.

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

For example, if you imagine a list of stock prices - the data would be being changed regularly and need to be in sync with the (in my case) Service that is calculating/fetching the data constantly.

Thanks in advance

Answer 1

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.

Answer 2

You will have a background thread running that calculates the changes in the list. This thread now needs a possibility to notify the GUI that the list was updated.

You can use some kind of ArrayAdapter to get the data into the ListView. The ArrayAdapter has a method called adpater.notifyDataSetChanged() every time you call this method the adapter will see that the corresponding data has changed and then notify the listview that it should update at the next possibility.

Answer 3

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.