Updating Activity UI from Different Activity Method?

Question

I have a tab widget where one of the tabs is a chat-type feature. I want to update the chat data at an interval (variable depending on whether the chat tab is active or not).

The best approach seemed to be using an AsyncTask in my main TabActivity class, as that would avoid any issues of the chat activity being destroyed while in the background, while an AsyncTask was running. I wanted to ensure that the Activity isn't destroyed and recreated, thus causing my AsyncTask to be unable to modify the actual active Activity's data.

However, now that my AsyncTask is in the TabActivity activity, I don't have a direct way to call my Chat's ListAdapter notifyDataSetChanged() from my onPostExecute() method anymore. Is there a way to get a reference to a given Tab's current Activity from the TabHost/TabActivity?

Or, alternatively, can I assume my chat activity will never be destroyed as a child activity of the TabActivity activity (well, never destroyed while the TabActivity is active at least), and then just put the AsyncTask in the Chat Activity?

Answer 1

Really the best way to do this is to have a thread that runs and periodically gets the chat data. Then from your UI you can poll the thread to get new messages. This way you can have the thread running no matter what happens to the Activity.

Answer 2

If you're using the Intent loading mechanism for your tabhost, then you should be safe in assuming the task won't get randomly killed any more than the TabHost itself (only paused), and you can safely modify views in it (as safely as you could from the TabHost, at least).

But beware: Any activity can be killed at any time (example: if a user clicks on a link in the tab that opens a new activity while your task is still running), including the tabhost itself, which can lead to ugly crashes when the task returns and tries to update the UI of the now-dead Activity. So make sure to either cancel your AsyncTasks on destroy, have the asynctasks check that your activity is still active before returning results (see WeakAsyncTask in the Android source tree), or use an Activity-independent (semi)persistent pollable background-thread solution like CaseyB suggests (cwac-bus is a nice premade solution in that vein).

Alternatively you could just have the AsyncTask's UI-updating code catch all exceptions (I've seen a few apps that do this) and fail silently, but that smells funny to me.

Lately I've used a modified version of the WeakAsyncTask in most places that checks that the activity is non-finished on returning, personally.