Notifying activity of processed information in Fragment

Question

Whats the recommaned approach to notifying the hosting activity of a fragment that performs some background processing, that its done. Assuming that the fragments are running some threads that are performing work outside of the main looper.

A simple callback won't do since:

• The Activity could be detached due to screen rotation which would lead into a NullPointerException.
• Polling from within the activity is just dumb
• Only calling the activity once if attached and let the activity check after each onCreate call (i.e. due to screen rotation).
• What I currently do but it seems wrong: Whenever the Fragment gets attached, it will check if the work is done and notify the activity via callback. If the fragment finishes the work it will also callback the activity (if attached).

Why I think is is wrong? Because I have some really ugly methods that check if the fragment is attached and also if the work is done in order to maybe call the callback. This becomes very stupid when an exception is raised during performing some work in the fragment and the activity is detached. If android decides to call onSaveInstance in the same moment I will have to put the Exception into the Bundle and deliver it later when the Activity and fragment is recreated from the saved state. Additionally I can run into a situation where a activity will received the same callback twice (once from checking the fragment and the second time when the fragments gets attached; this could happen when the application got saved and restored)

This generates so much code that, in my optinion, could be much more clear if activites won't get detached. That is why I hope I'm doing something wrong and hope someone will provide me with a better solution.

Answer 1

I would say you should use a service for your background processing but if you've chosen a fragment for a specific reason you can stick with that. The way you should notify the activity from either a fragment or a service that might have a different lifecycle from the activity would be through a BroadcastReceiver. With this method the activity can register and unregister a BroadcastReceiver during its own lifecycle callback. From the fragment or service you just send the broadcast and forget about it. If an activity is listening for the broadcast it will receive it, if not nothing will happen.