android - Setting up async task for loading Json into a listview

Question

I take json object generate from a PHP script on my server and then parse it into a listview using a lazy load for the images. The problem is that the json will load relatively fast or it will hang for second or two depending on the response on the server, which can be frustrating. When I first open the app the hang is espcially annoying because it will hang at a black screen until the object has been loaded and then when I update the list in app it has the same but at least the view has been loaded. Here is my code to get the json:

public void getJson(String selection, String url) {
    JSONObject json = null;
    String formatedcat = selection.toLowerCase();
    ArrayList<HashMap<String, String>> mylist = new ArrayList<HashMap<String, String>>();
    json = JSONfunctions
            .getJSONfromURL(url);
    try {
        //the array title that you parse
        JSONArray category = json.getJSONArray(formatedcat);
        for (int i = 0; i < category.length(); i++) {               
            HashMap<String, String> map = new HashMap<String, String>();
            JSONObject c = category.getJSONObject(i);
            map.put("id", String.valueOf(i));
            map.put("name",
                    c.getString("title"));
            //map.put("text",c.getString("title"));
            map.put("ts",c.getString("run_date") );
            map.put("image","http:"+c.getString("url"));
            mylist.add(map);
        }
    } catch (JSONException e) {

    }
    ListAdapter adapter = new JsonAdapter(this, mylist, R.layout.list,
            new String[] { "name", "text", "ts"}, new int[] { R.id.item_title,
                    R.id.item_subtitle, R.id.timestamp});
    setListAdapter(adapter);        
}

Adapter Code:

public class JsonAdapter extends SimpleAdapter {

    public ImageManager imageManager;
    public ListActivity context;
    public ArrayList<HashMap<String,String>> list;
    public String[] fieldNames;
    public int[] fieldTargetIds;

    public JsonAdapter(ListActivity c, 
            ArrayList<HashMap<String, String>> mylist,
            int textViewResourceId,
            String[] fieldNames,
            int[] fieldTargetIds) {
        super(c, mylist, textViewResourceId, fieldNames, fieldTargetIds );
        this.context = c;
        this.list = mylist;
        this.fieldNames = fieldNames;
        this.fieldTargetIds = fieldTargetIds;
        this.imageManager = new ImageManager(context.getApplicationContext());
    }

    @Override
    public View getView(int position, View convertView, ViewGroup parent) {

        View row = convertView;
        if (row == null) {
            LayoutInflater vi = (LayoutInflater) getSystemService(Context.LAYOUT_INFLATER_SERVICE);
            row = vi.inflate(R.layout.list, null);
        }
        //super.getView(position, convertView, parent);
        ImageView imgView = (ImageView) row.findViewById(R.id.doodlepic);

        try {
            String url = list.get(position).get("image");
            imgView.setTag(url);
            imageManager.displayImage(url, context, imgView);
        } catch (Exception e) {

        }

        for (int i=0; i<fieldNames.length; i++) {
            TextView tv = (TextView) row.findViewById(fieldTargetIds[i]);
            tv.setText(list.get(position).get(fieldNames[i]));              
        }


        return row;
    }

}

How could I implement an async task to show a progress dialog while the json object is being generated and downloaded? I've tried using threads and Async task but I can't quite figure out how to break apart my code into the appropriate parts.

Answer 1

The onPreExecute , onPostExecute of AsyncTask will run in the UI thread, the doInBackground will run in another thread, so below code should just fine for you

public class YourActivity extends Activiy{
   public void getJson(String selection, String url) { 
           new LoadJsonTask().execute( selection, url);

   }
   private class LoadJsonTask extends AsyncTask<String, Void, ArrayList<HashMap<String, String>> > {
       ProgressDialog dialog ;
       protected void onPreExecute (){
            dialog = ProgressDialog.show(YourActivity.this ,"title","message");

       }
       protected ArrayList<HashMap<String, String>> doInBackground (String... params){
           return doGetJson(params[0],params[1]);
       }
       protected void onPostExecute(ArrayList<HashMap<String, String>> mylist){

            ListAdapter adapter = new JsonAdapter(YourActivity.this, mylist, R.layout.list,
              new String[] { "name", "text", "ts"}, new int[] { R.id.item_title,
                R.id.item_subtitle, R.id.timestamp});
            setListAdapter(adapter);
            dialog.dismiss();
       }
    }

 public ArrayList<HashMap<String, String>> doGetJson(String selection, String url) {
     JSONObject json = null;
     String formatedcat = selection.toLowerCase();
     ArrayList<HashMap<String, String>> mylist = new ArrayList<HashMap<String, String>>();
     json = JSONfunctions
        .getJSONfromURL(url);
     try {
    //the array title that you parse
    JSONArray category = json.getJSONArray(formatedcat);
    for (int i = 0; i < category.length(); i++) {               
        HashMap<String, String> map = new HashMap<String, String>();
        JSONObject c = category.getJSONObject(i);
        map.put("id", String.valueOf(i));
        map.put("name",
                c.getString("title"));
        //map.put("text",c.getString("title"));
        map.put("ts",c.getString("run_date") );
        map.put("image","http:"+c.getString("url"));
        mylist.add(map);
    }
} catch (JSONException e) {

}
   return mylist;
  ....   
}