Issue when setting a ListView in a AsyncTask class

Question

Hi,

I'd like to set a ListView to data I get from a web service. I get the data in a AsyncTask instance, but when I try to set some of my ListView attributes, it crashes (on line "lv.setVisibility(View.VISIBLE);"). Anybody can help?

thanks

public class Atable extends Activity {

    private EditText mSearch;
    private static final int ACTIVITY_EDIT=0;
    private Button mSearchButton;
    private TextView mNoresults;
    private ListView lv;
    private CheckBox checkBox;
    private LocationManager locationManager;
    private RestaurantList restaurantList;
    private Criteria criteria;

    public void onCreate(Bundle savedInstanceState) {

        super.onCreate(savedInstanceState);

        setContentView(R.layout.main);

        lv= (ListView)findViewById(R.id.listview);

        mNoresults = (TextView) findViewById(R.id.noresults);  
        mNoresults.setVisibility(View.GONE);

        mSearchButton = (Button)this.findViewById(R.id.button);
        checkBox = (CheckBox) findViewById(R.id.local_check);        

        locationManager = (LocationManager)getSystemService(Context.LOCATION_SERVICE);
        criteria = new Criteria();
        criteria.setAccuracy(Criteria.ACCURACY_FINE);

        mSearchButton.setOnClickListener(new OnClickListener() {
            @Override
            public void onClick(View v) {
                mNoresults.setVisibility(View.GONE);
                mSearch = (EditText) findViewById(R.id.search);  
                String tmp_str = mSearch.getText().toString().replace(" ","+");  
                String url = "http://www.atable.org/getRestaurantByQuery/?query=" + tmp_str;

                if (checkBox.isChecked()) {

                    //get location
                    String provider = locationManager.getBestProvider(criteria, true);                
                    Location location = locationManager.getLastKnownLocation(provider);

                    if (location!=null) {

                        String lat = String.valueOf(location.getLatitude());
                        String lng = String.valueOf(location.getLongitude());

                        url += "&lat="+lat+"&lng="+lng;                   
                    }
                }
                new GetRestaurantData().execute(url);               
            }
        });

    };

    private class GetRestaurantData extends AsyncTask<String, Boolean, RestaurantList> {

        private HttpClient httpclient = new DefaultHttpClient();

        @Override
        protected RestaurantList doInBackground(String... url) {            

            publishProgress(true);         

            HttpGet httpget = new HttpGet(url[0]);          

            // Execute the request
            HttpResponse response;
            try {
                response = httpclient.execute(httpget);
                HttpEntity entity = response.getEntity();

                if (entity != null) {

                    InputStream instream = entity.getContent();

                    Reader r = new InputStreamReader(instream);                      

                    Gson gson = new Gson();
                    restaurantList = gson.fromJson(r, RestaurantList.class);

                    int nResults = restaurantList.getSize();

                    if (nResults>0) {                       

                        lv.setVisibility(View.VISIBLE); //app crashes here                      

                        lv.setAdapter( new ArrayAdapter<String>(Atable.this ,android.R.layout.simple_list_item_1,restaurantList.getRestaurantNames()));

                        lv.setOnItemClickListener(new OnItemClickListener() {

                            public void onItemClick(AdapterView<?> parent, View view, int position, long id) {

                                Intent intent = new Intent(Atable.this, RestaurantDescription.class);
                                Restaurant tmp_resto = restaurantList.getRestaurant((int)id);

                                String tmp_categories = tmp_resto.getCategories().get(0);
                                for (int i=1; i<tmp_resto.getCategories().size(); i++) {
                                    tmp_categories+=", "+tmp_resto.getCategories().get(i);
                                }

                                String address = tmp_resto.getStreet()+", "+tmp_resto.getStreetNumber()+"\n"+tmp_resto.getCity()+
                                                " "+tmp_resto.getPostalCode()+"\n"+tmp_resto.getCountry();

                                intent.putExtra("name", tmp_resto.getName());
                                intent.putExtra("address", address);                                
                                intent.putExtra("rating", tmp_resto.getRating());
                                intent.putExtra("price_range", tmp_resto.getPriceRange());
                                intent.putExtra("categories", tmp_categories);
                                intent.putExtra("latitude", tmp_resto.getLatitude());
                                intent.putExtra("longitude", tmp_resto.getLongitude());
                                startActivityForResult(intent, ACTIVITY_EDIT);
                            }
                        });


                    }

                    else {
                        lv.setVisibility(View.GONE);
                        mNoresults.setVisibility(View.VISIBLE);
                    }

                    //Closing the input stream will trigger connection release
                    instream.close();
                }   


            } catch (ClientProtocolException e) {                   
                e.printStackTrace();
            } catch (IOException e) {                   
                e.printStackTrace();
            }

            return restaurantList;
        }

        @Override
        protected void onProgressUpdate(Boolean... progress) {
            // line below coupled with 
            //    getWindow().requestFeature(Window.FEATURE_INDETERMINATE_PROGRESS) 
            //    before setContentView 
            // will show the wait animation on the top-right corner
            Atable.this.setProgressBarIndeterminateVisibility(progress[0]);
        }

        @Override
        protected void onPostExecute(RestaurantList result) {
            publishProgress(false);
            // Do something with result in your activity
        }




    }

Answer 1

You can't access the ui thread from the doinbachground method.. you need to do that from the post execute, pre execute or on progress update methods...

Answer 2

In Android, all UI updates are done on the main thread (also called UI thread). It's good when you spawn a new thread to do time consuming tasks in order not to block the UI.

But in order to avoid all sorts of indeterminate behaviors, UI updates always have to be done on the UI thread. If you ever attempt to do so from a different thread, an exception will be thrown.

In your example, I see you're performing different updatings to ListView based on the value of nResults. You can try returning nResults for doInBackground(). It will be passed to onPostExecute(), which will be executed on the UI thread.

You wanna check out this article for some useful information about threadings in Android: http://android-developers.blogspot.com/2009/05/painless-threading.html

HTH