Message Handling, unexpected behavior with worker threads

I'm taking a stab at a large program with a background Service and I'm implementing a (rather poorly thought out) Message handling procedure using basic Handler objects. The application has a main menu with buttons which start 6 different activities.

Problem is this: if i start a worker thread which kicks off a query to the database and retrieves some data, and I close the Activity that started the aforementioned worker thread开发者_JS百科, the Handler in the Activity still tries to run and show a dialog, even though the Activity that created it is now finished (or not in focus). How can I tell whether the current Activity is in focus before committing any (UI) changes?

I ended up solving the issue by simply putting the 'showDialog()' call in a try statement, but i'd like a more sophisticated solution, as this just seems like the wrong way to do things.

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Use sendBroadcast(), with the Activity registering a BroadcastReceiver for the broadcast via registerReceiver() in onResume() and unregistering it in onPause(). Then, it will only process the event if it is in the foreground.

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Put some flag in onPause() method of activity that starts the thread to indicate that it is not foreground anymore. In onStart() reverse the flag.

When it is a time to display dialog, check this flag and only show dialog if activity is running.