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POAndroid: Implication of using AsyncTask to make repeated Ajax Calls
 

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  1. POAndroid: Implication of using AsyncTask to make repeated Ajax Calls
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    <p>I need my Android app to periodically fetch data from a server using AJAX calls, and update the UI accordingly (just a bunch of <code>TextView</code>s that need to be updated with <code>setText()</code>). Note that this involves 2 tasks:</p> <ol> <li>Making an AJAX call, and updating the UI once I receive a response - I use a simple <code>AsyncTask</code> for this.</li> <li>Doing the above repeatedly, at regular intervals.</li> </ol> <p>I haven't figured out an elegant way to achieve Point 2 above. Currently, I am simply executing the task itself from <code>OnPostExecute()</code>. I read on <a href="https://stackoverflow.com/questions/6400810/android-asynctask-starting-another-asynctask">this thread at SO</a> that I need not worry about garbage collection as far as the AsyncTask objects are concerned.</p> <p>But I'm still unsure as to how I set up a timer that will fire my AsyncTask after it expires. Any pointers will be appreciated. Here is my code:</p> <pre><code>public class MyActivity extends Activity { /** Called when the activity is first created. */ @Override public void onCreate(Bundle savedInstanceState) { super.onCreate(savedInstanceState); setContentView(R.layout.main); new AjaxRequestTask().execute(MY_REST_API_URL); } private void updateReadings(String newReadings) { //Update the UI } class AjaxRequestTask extends AsyncTask&lt;String, Integer, String&gt; { @Override protected String doInBackground(String... restApiUrl) { //Do AJAX Request } @Override protected void onPostExecute(String result) { updateReadings(result); /*Is there a more elegant way to achieve this than create a new AsyncTask object every 10 seconds? Also, How can I update the UI if I create a timer here? */ new AjaxRequestTask().execute(MY_REST_API_URL); } } </code></pre> <p>}</p> <p>Thanks in advance</p> <p>EDIT: I tried posting an answer but couldn't do it since I don't have the reputation to answer within 8 hours.</p> <p>Well, so I found a solution. I'm not convinced however.</p> <pre><code>protected void onPostExecute(String result) { updateReadings(result); // super.onPostExecute(result); new Timer().schedule( new TimerTask() { @Override public void run() { new AjaxRequestTask().execute(MY_REST_API_URL); } }, TIMER_ONE_TIME_EXECUTION_DELAY ); } </code></pre> <ol> <li><p>Are there any flip sides that I should be aware of when I use this? In particular, I am seeing lots of GCs happening in the LogCat. Also, I am wondering how an <code>AsyncTask</code> can be candidate for GC unless the <code>onPostExecute()</code> completes?</p></li> <li><p>How can I "stop" the updates? One way I thought of was to make the very first <code>AsyncTask</code> instance as a member variable of the <code>Activity</code>. That way, I can invoke <code>cancel(true)</code> on it and hope that this will "stop" the tasks.</p></li> </ol> <hr> <h2><strong><em>SOLUTION</em></strong>:</h2> <p>In case anyone is looking for something similar - none of the solutions I mentioned here work satisfactorily. They all suffer from <code>OutOfMemory</code> issues. I did not debug into the details of the OOM, but I suspect it could either be because of the recursion, or because of having HTTP-related objects as member variables in the <code>AsyncTask</code> rather than as members of the <code>Activity</code> (basically because of NOT reusing HTTP and other objects).</p> <p>I discarded this approach for a different one - making my Ajax Calls endlessly in the <code>doInBackground()</code> of my <code>AsyncTask</code>; and updating the UI in <code>onProgressUpdate()</code>. That way I also avoid the overhead of maintaining too many threads or <code>Handler</code>s for updating the UI (remember UI can be updated in <code>onProgressUpdate()</code> ).</p> <p>This approach also eliminates the need for <code>Timer</code>s and <code>TimerTask</code>s, favoring the use of <code>Thread.sleep()</code> instead. <a href="https://stackoverflow.com/questions/6652390/android-timertask-scheduled-for-repetition-getting-fired-only-once">This thread on SO</a> has more details and a code snippet too.</p>
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    1. COThe "solution" I mentioned in my Edit above has serious pitfalls - as suspected, it leads to "infinite loop". Calling `cancel(true)` in `onPause` and `onDestroy()` did not do the trick - it never calls `onCancelled()` which I overrode in my AsyncTask. I cannot think of a way to use the `isCancelled()` method to influence the behavior inside of `doInBackground()`. I guess I'll have to go the AlarmManager way. I'll give it a try.
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