Android Asynctask: wait for entire computation without blocking UI thread

Question

I have almost finished my project, only one thing remain to improve.

My app is a music quiz, that retrieves all the informations about author, song title, previewUrl ecc. from the Itunes Store using the apposite Search API.

When a user choose a genre to play with, I must say to the user to wait for 4-5 seconds because of the computation that fills the List containing all the informations.

I call the Asynctask that retrieve these informations like this:

JsonCanzoni recuperoCanzoni = new JsonCanzoni(arrayGenere,Canzone.this);

    try {
        recuperoCanzoni.execute().get();
    } catch (InterruptedException e) {
        // TODO Auto-generated catch block
        e.printStackTrace();
    } catch (ExecutionException e) {
        // TODO Auto-generated catch block
        e.printStackTrace();
    }

The class (extends Asynctask) that do these operation is the following:

class JsonCanzoni extends AsyncTask<Void, Void, Void>
{
    List<String> canzoni = new ArrayList<String>(5);
    ProgressDialog pDialog;
    int[] arrayGenere;
    Context context;

    public JsonCanzoni(int[] arrayGenere,Context context) 
    {
        this.arrayGenere = arrayGenere;
        this.context = context;
    }



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

        pDialog.dismiss();
    }



    @Override
    protected void onPreExecute() 
    {
        super.onPreExecute();

        pDialog = new ProgressDialog(context);
        pDialog.setMessage("Preparazione round...");
        pDialog.setIndeterminate(false);
        pDialog.setCancelable(false);
        pDialog.show();
    }



    protected Void doInBackground(Void... params) 
    {
        try
        {       
            int randomLookupId = 0;
            JSONObject obj;                         
            JSONArray jsonArray;    

            for(int i = 0; i < 10; i++)
            {
                canzoni = new ArrayList<String>();

                Log.d("GENERE", arrayGenere.toString());

                obj = getJSONObject(scegliClassifica(arrayGenere));
                jsonArray = obj.getJSONArray("resultIds");

                Log.d("dimensione JsonArray", String.valueOf(jsonArray.length()));
                try
                {
                    randomLookupId = new Random().nextInt(jsonArray.length()-1);
                }
                catch(IllegalArgumentException errore)
                {
                    new AlertDialog.Builder(context)
                    .setTitle("Connessione non attiva!")
                    .setMessage("Connessione di rete debole, uscita dal programma!");
                }
                Log.d("randomLookupID", String.valueOf(randomLookupId));

                JSONObject finalObj = getJSONObject("http://itunes.apple.com/lookup?id="+jsonArray.getString(randomLookupId)); 
                Log.d("URL","http://itunes.apple.com/lookup?id="+jsonArray.getString(randomLookupId));

                while(finalObj.getJSONArray("results").length() == 0)
                {
                    Log.d("Array VUOTO!!","Non è possibile!!!!");
                    randomLookupId = new Random().nextInt(jsonArray.length()-1);
                    Log.d("randomID rigenerato", String.valueOf(randomLookupId));

                    finalObj = getJSONObject("http://itunes.apple.com/lookup?id="+jsonArray.getString(randomLookupId));
                    Log.d("URL Rigenerato","http://itunes.apple.com/lookup?id="+jsonArray.getString(randomLookupId));
                }

                JSONArray finalJsonArray = finalObj.getJSONArray("results");

                JSONObject returnObj = finalJsonArray.getJSONObject(0);
                Log.d("returnObj.length",String.valueOf(returnObj.length()));

                canzoni.add(returnObj.getString("previewUrl"));
                canzoni.add(returnObj.getString("artistName"));
                canzoni.add(returnObj.getString("trackName"));
                canzoni.add(returnObj.getString("artistViewUrl"));
                canzoni.add(returnObj.getString("artworkUrl100"));
//              GTTapp app=(GTTapp) ((Activity)context).getApplication();
//              app.dieciCanzoni;
                Canzone.dieciCanzoni.add(i, new ArrayList<String>(canzoni));
            }
        }   
        catch (JSONException ignored)
        {
            ignored.getCause();
        }
        catch (MalformedURLException e) 
        {
            // TODO Auto-generated catch block
            e.printStackTrace();
        }
        catch (IOException e) 
        {
            // TODO Auto-generated catch block
            e.printStackTrace();
        }

        return null;
    }

    private String scegliClassifica(int[] arrayGenere)
    {
        int randomArrayPosition = new Random().nextInt(arrayGenere.length);
        return "http://itunes.apple.com/WebObjects/MZStoreServices.woa/ws/charts?cc=us&g="+arrayGenere[randomArrayPosition]+"&name=Songs&limit=200";
    }

    JSONObject getJSONObject(String url) throws IOException, MalformedURLException, JSONException
    {

        HttpURLConnection conn = (HttpURLConnection) new URL(url).openConnection();

        InputStream in = conn.getInputStream();

        try
        {
            StringBuilder sb = new StringBuilder();
            BufferedReader r = new BufferedReader(new InputStreamReader(new DoneHandlerInputStream(in)));
            for (String line = r.readLine(); line != null; line = r.readLine())
            {
                sb.append(line);
            }
            return new JSONObject(sb.toString());
        }
        finally
        {
            in.close();
        }
    }
}

THE PROBLEM: Using the .get() method make the app waiting for the entire computation of the AsyncTask, but it block the UI thread too!! So the user will remain with a black screen for 5 secs or more, and that's not a good thing!

If I don't use the .get() method, I receive an IndexOutOfBounds Exception, because I begin to play the music stream but the list is has not been filled yet.

Can you suggest me a workarund for this situation?

Thank you!

Answer 1

remove .get() it will block the UI till completion of the task.

Start any task like (playing video) which is dependent on AsycTask in

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

    pDialog.dismiss();
      //You can start music stream here
}

Answer 2

The get() call is blocking. From the docs:

Waits if necessary for the computation to complete, and then retrieves its result.

What you should do is wait asynchronously the computation to finish before you start to play the music.

If you do not want to expose the asynctask outside your class, you can set a callback into your JSonCanzoni class to be called into the onPostExecute method of the asynctask.

Something like

public interface CanzoniDownloadedInterface {
   public void onCanzoniDownloaded();
}


public JsonCanzoni(int[] arrayGenere, CanzoniDownloadedInterface callback, Context context){
    this.arrayGenere = arrayGenere;
    this.context = context;
    this.callback = callback;
}

and in your onPostExecute():

@Override
protected void onPostExecute(Void result)
{
    super.onPostExecute(result);
    this.callback.onCanzoniDownloaded();
    pDialog.dismiss();
}

If you let your activity implement the interface, you can pass it to your class.

The onCanzoniDownloaded() implementation is where you need to start playing.

Answer 3

Finally I solved my problem, some advices were good and helped me a lot!

I simply moved the startActivity instruction to onPostExecute of JsonCanzoni AsyncTask, and changed some code to adapt to the new version and it's all right! ;)