Question

I have a map that i want to update from a separate thread. Im having some issues when i try to update the UI from my class that extends asynctask. And when i move the code to a handler in the main thread i get NetworkOnMainThreadException. Below is my asynctask. Is there any way to modify it to make it work?

@Override
    protected Integer doInBackground(Void... params) {

        try {

            HttpClient httpclient = new DefaultHttpClient();
            HttpPost httppost = new HttpPost("http://83.xx.xx:8080/android/service.php");

            List<NameValuePair> nameValuePairs = new ArrayList<NameValuePair>(2);
            nameValuePairs.add(new BasicNameValuePair("action", "getusers"));
            nameValuePairs.add(new BasicNameValuePair("interval", Integer.toString(interval)));

            httppost.setEntity(new UrlEncodedFormEntity(nameValuePairs));

            HttpResponse response = httpclient.execute(httppost);

            String jsonString = EntityUtils.toString(response.getEntity());

            Gson gson = new GsonBuilder().create();

            Type listType = new TypeToken<ArrayList<User>>() {}.getType();
            ArrayList<User> userList = gson.fromJson(jsonString, listType);

            ((GoogleMapsActivity) activity).removeOverlayItems();

            for(User user : userList ){

                ((GoogleMapsActivity) activity).addOverlayItem(Double.parseDouble(user.last_lat), Double.parseDouble(user.last_lng), user.bluetooth_name, "test desc");
            }


            return 1;

        } catch (Exception e) {
            e.printStackTrace();
        }
        return null;

Answer 1

Rather than touching the map in your doInBackground, return your data into onPostExecute. Then update the map from onPostExecute.

Please note that you should never retain a reference to your Activity from inside your AsyncTask or you could cause a memory leak.

Ideally, you should create an observer that can return data to your Activity.

interface Observer {
    public void onMyDataAvailable(MyDataClass data);
}

Then, you instantiate your AsyncTask with a reference to this observer.

class MyAsyncTask extends AsyncTask ...
    private Observer observer;

    public MyAsyncTask(Observer observer) {
        mObserver = observer;
    }

    public synchronized void onActivityDestroyed() {
        mObserver = null;
    }

    public synchronized void onPostExecute(MyDataClass result) {
        if (mObserver != null) {
            mObserver.onMyDataAvailable(result);
        }
    }

Just make sure you call onActivityDestroyed() from your activity’s onDestroy method. Otherwise your Activity will leak.