doInBackground() method in an AsyncTask doesn't work when the activity is being re-opened

Issue

In my Android app, I have an activity which executes an AsyncTask<Void, Void, Void> named Scan using this code: new Scan().execute();.

In the onPreExecute() method, it starts a progress dialog, on the doInBackground(Void... voids) method it scans a table from DynamoDB, and on the onPostExecute(Void aVoid) method, it dismisses the progress dialog, and views the results of the DB scan in a ListView using a custom BaseAdapter class.

When I open the activity, everything runs great, but when I press the back button, and enter the activity again, then only the onPreExecute() and the onPostExecute(Void aVoid) methods are being executed, while doInBackground(Void... voids) isn't being executed, so it just shows and dismisses the progress dialog, and nothing else is being viewed on the screen.

How can I fix this?

Code:

MessagesListAdapter messages;
ListView messagesLv;
public static ArrayList<Message> arrayList;
public static ProgressDialog progressDialog;

public static DynamoDBScanExpression dbScanExpression;
public static List<Message> messageList;

@Override
public void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    setContentView(R.layout.activity_messages);

    new Scan().execute();
}

private class Scan extends AsyncTask<Void, Void, Void> {
    @Override
    protected void onPreExecute() {
        super.onPreExecute();

        progressDialog = new ProgressDialog(TestActivity.this);
        progressDialog.setTitle(name);
        progressDialog.setMessage("Searching for messages...");
        progressDialog.setIndeterminate(false);
        progressDialog.show();
    }

    @Override
    protected Void doInBackground(Void... voids) {
        dbScanExpression = new DynamoDBScanExpression();

        Condition condition = new Condition()
                .withComparisonOperator(ComparisonOperator.EQ)
                .withAttributeValueList(new AttributeValue().withS(MainActivity.msgId));
        dbScanExpression.addFilterCondition("msgId", condition);

        messageList = MainActivity.mapper.scan(Message.class, dbScanExpression);
        arrayList = new ArrayList<Message>();
        for (Message msg : messageList) {
            if (msg.getUserId() == null || msg.getUserId().equals(MainActivity.userId)) {
                msg.setMsgId(msg.getMsgId());
                msg.setDate(msg.getDate());
                msg.setTime(msg.getTime());
                msg.setMessage(msg.getMessage());
                msg.setUserId(msg.getUserId());
                arrayList.add(msg);
            }
        }
        return null;
    }

    @Override
    protected void onPostExecute(Void aVoid) {
        super.onPostExecute(aVoid);

        if (!messageList.isEmpty()) {
            messagesLv = (ListView) findViewById(R.id.messagesListView);
            messages = new MessagesListAdapter(MinaActivity.this, arrayList);
            messagesLv.setAdapter(messages);

            progressDialog.dismiss();
        } else {
            TextView tv = (TextView) findViewById(R.id.noMessages);
            tv.setVisibility(View.VISIBLE);

            progressDialog.dismiss();
        }
    }
}

Solution

The reason that it couldn't find any messages, was that I checked if the user ID of the message equals to the user ID of the registered user. The problem was that it was taken from MainActivity.java which got it from an Intent extra, therefore, when I have left the activity, the variable has been erased.

What I did is to refer to the user ID from the SharedPreferences and suddenly it worked.

Answered By - Ido Naveh