execute asynctask from another class in mainactivity

Question

I am trying to execute asynctask from another class and update listview and also toast. But I am unable to do that. Its crashing at:

progressDialog = new ProgressDialog(context);

I really appreciate any help.

fetch.class

 public class fetch extends AsyncTask<Void, Void, JSONObject> {


      Context context;

     Activity activity ;
ListView list;
        LazyAdapter adapter;


        ArrayList<HashMap<String, String>> songsList;
        ProgressDialog progressDialog ;

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

         String ResponseBody = null;

        try {

            String user = "1";

            HttpClient httpclient = new DefaultHttpClient();
            HttpPost httppost = new HttpPost("http://www.example.com/example.php");

            // Add your data
            List<NameValuePair> nameValuePairs = new ArrayList<NameValuePair>(1);

            nameValuePairs.add(new BasicNameValuePair("user",user) );


            httppost.setEntity(new UrlEncodedFormEntity(nameValuePairs));

            // Execute HTTP Post Request
            HttpResponse response = httpclient.execute(httppost);

            BufferedReader reader = new BufferedReader(new InputStreamReader(response.getEntity().getContent(), "iso-8859-1"), 8);
            StringBuilder sb = new StringBuilder();
            sb.append(reader.readLine() + "\n");
            String line = "0";
            while ((line = reader.readLine()) != null) {
                sb.append(line + "\n");
            }
            reader.close();
            String result = sb.toString();


            // parsing data
            return new JSONObject(result);
        } catch (Exception e) {
            e.printStackTrace();
            return null;
        }
    }
    @Override
    protected void onPreExecute() {
        // TODO Auto-generated method stub

        super.onPreExecute();
        runOnUiThread(new Runnable() {
            @Override
            public void run() {
                 progressDialog = new ProgressDialog(MainActivity.this);
                progressDialog.setMessage("loading...");
                progressDialog.setCanceledOnTouchOutside(false);
                progressDialog.show();

            }
        });

    }


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



        if (result != null) {
               JSONArray arr = result.getJSONArray("Array");

         for (int i = 0; i <arr.Length(); i++) {
            // creating new HashMap
            HashMap<String, String> map = new HashMap<String, String>();
            Element e = (Element) nl.item(i);
            // adding each child node to HashMap key =&gt; value
            map.put(KEY_ID, parser.getValue(e, KEY_ID));
            map.put(KEY_TITLE, parser.getValue(e, KEY_TITLE));
            map.put(KEY_ARTIST, parser.getValue(e, KEY_ARTIST));
            map.put(KEY_DURATION, parser.getValue(e, KEY_DURATION));
            map.put(KEY_THUMB_URL, parser.getValue(e, KEY_THUMB_URL));

            // adding HashList to ArrayList
            songsList.add(map);
        }

    }
            list=(ListView)findViewById(R.id.ListView);
            // Getting adapter by passing xml data ArrayList
            adapter=new LazyAdapter(MainActivity.this, songsList);  
            list.setCacheColorHint(0);
            list.setScrollingCacheEnabled(false);
            list.setAdapter(adapter);

         // Click event for single list row
            list.setOnItemClickListener(new OnItemClickListener() {

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

                    Toast.makeText(getBaseContext(), position+"  "+id, 5).show();

                }
            }); 


        } else {
            // error occured
        }


        runOnUiThread(new Runnable() {
            @Override
            public void run() {



                 progressDialog.dismiss();

                 Toast.makeText(getBaseContext(), "done", 5).show();

            }

        });
    }


    public fetch(Context context) {
      activity = (context instanceof Activity) ? (Activity) context : null;
     this.context = context;

     progressDialog = new ProgressDialog(context);

}

}

LazyAdapter.java

package com.example.androidhive;

import java.util.ArrayList;
import java.util.HashMap;

import android.app.Activity;
import android.content.Context;
import android.view.LayoutInflater;
import android.view.View;
import android.view.ViewGroup;
import android.widget.BaseAdapter;
import android.widget.ImageView;
import android.widget.TextView;

public class LazyAdapter extends BaseAdapter {

    private Activity activity;
    private ArrayList<HashMap<String, String>> data;
    private static LayoutInflater inflater=null;
    public ImageLoader imageLoader;

    public LazyAdapter(Activity a, ArrayList<HashMap<String, String>> d) {
        activity = a;
        data=d;
        inflater = (LayoutInflater)activity.getSystemService(Context.LAYOUT_INFLATER_SERVICE);
        imageLoader=new ImageLoader(activity.getApplicationContext());
    }

    public int getCount() {
        return data.size();
    }

    public Object getItem(int position) {
        return position;
    }

    public long getItemId(int position) {
        return position;
    }

    public View getView(int position, View convertView, ViewGroup parent) {
        View vi=convertView;
        if(convertView==null)
            vi = inflater.inflate(R.layout.list_row, null);

        TextView title = (TextView)vi.findViewById(R.id.title); // title
        TextView artist = (TextView)vi.findViewById(R.id.artist); // artist name
        TextView duration = (TextView)vi.findViewById(R.id.duration); // duration
        ImageView thumb_image=(ImageView)vi.findViewById(R.id.list_image); // thumb image

        HashMap<String, String> song = new HashMap<String, String>();
        song = data.get(position);

        // Setting all values in listview
        title.setText(song.get(CustomizedListView.KEY_TITLE));
        artist.setText(song.get(CustomizedListView.KEY_ARTIST));
        duration.setText(song.get(CustomizedListView.KEY_DURATION));
        imageLoader.DisplayImage(song.get(CustomizedListView.KEY_THUMB_URL), thumb_image);
        return vi;
    }
}

MainActivity

public void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.main);

        StrictMode.ThreadPolicy policy = new StrictMode.ThreadPolicy.Builder().permitAll().build();

        StrictMode.setThreadPolicy(policy); 

        Button btn=(Button)findViewById(R.id.button1);





        btn.setOnClickListener(new OnClickListener() {

            @Override
            public void onClick(View v) {
                // TODO Auto-generated method stub

                RelativeLayout rootView = (RelativeLayout)findViewById(R.id.rootLayout);

                PopoverView popoverView = new PopoverView(PopoverViewActivity.this, R.layout.popover_showed_view);
                popoverView.setContentSizeForViewInPopover(new Point(400, 480));
                popoverView.setDelegate(PopoverViewActivity.this);
                popoverView.showPopoverFromRectInViewGroup(rootView, PopoverView.getFrameForView(v), PopoverView.PopoverArrowDirectionUp, true);



                 songsList = new ArrayList<HashMap<String, String>>();


                    fetch load=new fetch(getBaseContext());
                    load.execute();



            }
        });
    }

Answer 1

Problem with this piece of code

progressDialog = new ProgressDialog(context);

is that this needs context of currently displayed activity. And it seems you are passing it the non-displaying activity context (In your case it is null). It needs displayed activity context in order to show the progress bar.

while calling fetch you are passing it as null in main activity. you should change

   fetch(null);

to

   fetch(MainActivity.this);

Hope this helps.