Can't access "findViewById" in AsyncTask

Pass your inflated view to the AsyncTask like this :

public class RecuperarComentarisFoto extends AsyncTask<String, String, String>{
private Context mContext;
private View rootView;
[...]
public RecuperarComentarisFoto(Context context, View rootView){
    this.mContext=context;
    this.rootView=rootView;
}
@Override
protected void onPreExecute() {
    [...]
}

@Override
protected String doInBackground(String... arg0) { 
    params.add(new BasicNameValuePair("pid", "1"));
    JSONObject json = jsonParser.makeHttpRequest(url_get_comentaris, "GET", params);
    JSONArray productObj;
    //HERE I RETRIEVE DATA FROM JSON. ITS WORKING OK.

    return null;
}
@Override
protected void onPostExecute(String result) {
    //super.onPostExecute(result);  <--GET RID OF THIS (it will screw up on 2.1 devices)
    this.pDialog.dismiss();
    // use rootview to findViewById
    TextView comentariEditText = (TextView) rootView.findViewById(R.id.commentsMostrar);
}

Then call like so,

View myFragmentView = inflater.inflate(R.layout.fragment_a, container, false);

cont=getActivity();
new RecuperarComentarisFoto(cont, myFragmentView).execute();

Make your AsyncTask independent of Activity

What if

• Your AsyncTask is a seperate Java class (Outside of Activity) and you still want to access the View elements of any Activity from AsyncTask.
• You want to reuse your AsyncTask with some other Activity without any code change in the your AsyncTask

Answer is. Use an interface

Advantage?
An interface makes things independent (loosely coupled)

You can follow these steps:

1) Create an interface

public interface AsyncTaskListener{

   public void giveProgress(int progress);

}

2) Use listener in your AsyncTask

 DownloadSongTask extends AsyncTask<String,Integer,Void>{

    private AsyncTaskListener listener;

    public DownloadSongTask(Context context)
    {
        listener= (AsyncTaskListener)context;    // Typecast 
    }

    @Override
    public void doInbackGround(String... params)
    {
         // Download code
         int downloadPerc = // calculate that
         publish(downloadPerc);   
    }

    @Override
    public void onProgressUpdate(Integer... params)
    {
         listener.giveProgress(params[0]);  // Use it 
    }

}

3) Implement the interface in the Activity and Override method

 public class YourActivity extends AppcompatActivity implements AsyncTaskListener{

    private TextView textView;

    // Activity code //
    new  DownloadSongTask(this).execute("Paradise.mp3");  // this is how you start Task

    @Override 
    public void giveProgress(int progress){
       textView.setText(progress+"");      
    }

}

Future

You will not realize the beauty of your code until you have to use Your AsyncTask in some other Activity.
Lets say in future you want to use this AsyncTask in some Other Activity which shows progress in a Spinner.
How will you use it ? Simple! implement the interface and override the method.

public class YourActivity extends AppcompatActivity implements AsyncTaskListener{

     private Spinner spinner;

     // Activity code //
     new  DownloadSongTask(this).execute("Paradise.mp3");  // this is how you start Task

    @Override 
    public void giveProgress(int progress){
       // Your Spinner code       
    }

}

try as after passing MyFragmentA context instead of getActivity() :

 cont=this;
 new RecuperarComentarisFoto(cont).execute();

and change RecuperarComentarisFoto as

public class RecuperarComentarisFoto extends AsyncTask<String, String, String>{
public Fragment mContext;
[...]
public RecuperarComentarisFoto(Fragment context){
    this.mContext=context;
}
@Override
protected void onPostExecute(String result) {
    super.onPostExecute(result);
    this.pDialog.dismiss();
    TextView comentariEditText = 
                    (TextView)mContext.findViewById(R.id.commentsMostrar);
}
//...your code..