Question

I have a fairly complex problem. I have read some similar posts to my problem but in a slightly different situation.

I have an Activity with a List View. The Activity creates a thread with a long running process. After the process ends, it calls the handler to update the listview and play a sound. It works fine if I’m in the same Activity, but when I click on HOME button while the process is running, then press on the shortcut or application icon to launch the Activity, then when the process ends, I hear the sound but the list view is not updated. Any ideas why this is happening? Some people and me as well are getting the same issues with orientation change.

// thread created to send the reply.
   Thread replyThread = new Thread() {
public void run() {
    **Http POST here to upload an image which takes a while.    
//Once done, send a msg to handler to play a sound and update listview  
Message msg = replyHandler.sendEmptyMessage(0);         
}
   }

Handler:

final Handler replyHandler = new Handler() {
    public void handleMessage(Message msg) {
    fillData();
    **Code to play sound.
    }
}

fillData() method

// refresh the listview.
private void fillData() {
    // Get all of the rows from the database and create the item list

            **I actually have an activity before this one but to simplify the problem I said I have 1 Activity. Bundle is from previous Activity.

            String conversationId = "";

    Bundle extras = getIntent().getExtras();
    if (extras != null) {
        conversationId = extras
                .getString(TestDbAdapter.KEY_CONVERSATION_ID);          
    }

    if (mDbHelper != null) {        

        Cursor notesCursor = mDbHelper
                .fetchAllMessagesByConversationId(conversationId);
        startManagingCursor(notesCursor);

        String[] from = new String[] { TestDbAdapter.KEY_BODY,
                TestDbAdapter.KEY_TIMESTAMP };

        int[] to = new int[] { R.id.text1, R.id.text2 };

        // Now create a simple cursor adapter and set it to display
        MessagesCursorAdapter notes = new MessagesCursorAdapter(this,
                R.layout.messages_row, notesCursor, from, to, mDbHelper);
        setListAdapter(notes);

    }

}

I know some people are gonna say “Use notifyDataSetChanged instead”, done that. The code above works in the same manner as notifyDataSetChanged but maybe not in a very elegant way but in my case it works, but it doesn’t help with the problem I’m having using notifyDataSetChanged.

Answer 1

When the home button is pressed and activity restarted a “new” activity is created.

Your callback from the long running process will be sent to the old activity, not the new activity.

A similar thing happens with orientation changes, each orientation change creates a new activity (that will not receive the callback).

There are a couple of methods I am aware of to deal with this:

• Store the state & results of the long running process in local storage (db, xml, contentprovider etc) and query for state & results when the activity starts. The Google IO REST talk talks about the problem and a pattern you can implement to solve it. (Note: fairly large amount of work to implement this pattern fully)
• Route the callbacks through your Application class and register your activities when they start with the Application so it knows which one to send callbacks to. Droid-fu contains some utilities that can help implement this