how to maintain scroll position of listview when it updates

Question

I have read plenty of examples ,but if I wish to maintain my scroll position after a ListView is updated from JSON ,then can I do that without using an AsyncTask instance ???

the code for my list is

 String wrd;     //ArrayList<HashMap<String,String>> mylist;     @Override     public void onCreate(Bundle savedInstanceState) {         // TODO Auto-generated method stub         super.onCreate(savedInstanceState);         setContentView(R.layout.main);          Intent i2=getIntent();          wrd=i2.getStringExtra("entrd");         Log.v("keyis",wrd);            final Handler handler = new Handler();         Runnable runable = new Runnable() {              @Override             public void run() {                  //call the function                 LoadData();                 //also call the same runnable                 handler.postDelayed(this, 40000);             }         };         handler.postDelayed(runable, 10);      }public void LoadData(){            JSONObject j2=JSONfunctions.getJSONfromURL("/webservice_search.php?keyword="+wrd+"&format=json");          ArrayList<HashMap<String,String>> mylist = new  ArrayList<HashMap<String,String>>();           try{JSONArray jray=j2.getJSONArray("listings");             for(int i=0;i<jray.length();i++){                 Log.v("state","json data being read");                 JSONObject j3= jray.getJSONObject(i);                 String first=j3.getString("listing");                 Log.v("sublist", first);                 JSONObject j4=j3.getJSONObject("listing");                 String sec=j4.getString("links");                  int maxLength = (sec.length() < 30)?sec.length():27;                 sec.substring(0, maxLength);                 String cutsec=sec.substring(0,maxLength);                 Log.v("links are",cutsec);                 String img=j4.getString("image_name");                 Log.v("image name is ",img);                 //Uri dimg=Uri.parse("http://zeesms.info/android_app_images/Koala.jpg");                 HashMap<String,String> map=new HashMap<String,String>();                  map.put("Id",String.valueOf(i));                 map.put(Li_nk,cutsec);                 map.put(Image_name,j4.getString("image_name"));                  map.put(KEY_THUMB_URL,"http://zeesms.info/android_app_images/"+img);                 mylist.add(map);              }              }             catch(JSONException e){                  Log.e("loG_tag","Error parsing"+e.toString());             }          LazyAdapter adapter = new LazyAdapter(this,mylist);          adapter.notifyDataSetChanged();              ListView list=(ListView)findViewById(R.id.lv1);              list.setEmptyView(findViewById(R.id.empty));              list.setAdapter(adapter);                  list.setItemsCanFocus(false); 

and my adapter is

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()); }  @Override public int getCount() {     // TODO Auto-generated method stub     return data.size(); }  @Override public Object getItem(int position) {     // TODO Auto-generated method stub     return position; }  @Override public long getItemId(int position) {     // TODO Auto-generated method stub     return position; }    @Override public View getView(int position, View convertView, ViewGroup parent) {     View vi=convertView;     if(convertView==null)          vi = inflater.inflate(R.layout.custom_row_view1, null);      TextView title = (TextView)vi.findViewById(R.id.linkname); // merchnts name     TextView artist = (TextView)vi.findViewById(R.id.imagename); // address     //TextView duration = (TextView)vi.findViewById(R.id); // distance    ImageView thumb_image=(ImageView)vi.findViewById(R.id.mClogo); // logo      HashMap<String, String> jsn = new HashMap<String, String>();     jsn = data.get(position);      // Setting all values in listview    title.setText(jsn.get(Second.Li_nk));    artist.setText(jsn.get(Second.Image_name));     //duration.setText(song.get(CustomizedListView.KEY_DURATION));     imageLoader.DisplayImage(jsn.get(Second.KEY_THUMB_URL), thumb_image);     return vi; } 

and finally the class being used for json parsing is

    public class JSONfunctions {       public static JSONObject getJSONfromURL(String url){         InputStream is = null;         String result = "";         JSONObject jArray = null;         String  str1="http://zeesms.info"+url;    // ArrayList<NameValuePair> namevaluepairs = new ArrayList<NameValuePair>();         Log.v("url result",url);         //namevaluepairs.add(new BasicNameValuePair("location",str1));         //http post         try{                HttpClient httpclient= new DefaultHttpClient();             HttpGet request = new HttpGet();              request.setURI(new URI(str1));             HttpResponse response = httpclient.execute(request);              is  = response.getEntity().getContent();             if(is==null){                  Log.v("url result","is  null");             }             else             {                 Log.v("url result","is  not null");             }          /*    BufferedReader buf = new BufferedReader(new InputStreamReader(is,"UTF-8"));              StringBuilder sb = new StringBuilder();             String s;             while(true )             {                 s = buf.readLine();                 if(s==null || s.length()==0)                     break;                 sb.append(s);              }             buf.close();             is.close();              sb.toString();  */            //  httppost.setEntity(new UrlEncodedFormEntity(namevaluepairs));             //HttpResponse response=httpclient.execute(httppost);             //HttpEntity entity=response.getEntity();             //is=entity.getContent();               /*                 HttpClient httpclient = new DefaultHttpClient();                 HttpPost httppost = new HttpPost(url);                 HttpResponse response = httpclient.execute(httppost);                 HttpEntity entity = response.getEntity();                 is = entity.getContent(); */         }catch(Exception e){                 Log.v("log_tag", "Error in http connection "+e.toString());                  AlertDialog.Builder alert=new AlertDialog.Builder(null);                 alert.setMessage("Invalid Keyword").setPositiveButton("Ok", new OnClickListener(){                      @Override                     public void onClick(DialogInterface arg0, int arg1) {                         // TODO Auto-generated method stub                      }                  });         }        //convert response to string         try{             Log.v("url result","getting result starts");                  BufferedReader reader = new BufferedReader(new InputStreamReader(is,"iso-8859-1"),8);                  StringBuilder sb = new StringBuilder();                  String line = null;                 Log.v("url result","getting result");                 while ((line = reader.readLine()) != null) {                     Log.v("url result","getting result");                         sb.append(line + "\n");                 }                  is.close();                  result=sb.toString();                 Log.v("url result",result);          }catch(Exception e){                 Log.e("log_tag", "Error converting result "+e.toString());         }          try{              jArray = new JSONObject(result);                     }catch(JSONException e){                 Log.e("log_tag", "Error parsing data "+e.toString());         }          return jArray;     }    } 

along with this if the data is updated from the webpage, what would be the simplest way to show the updated item on top ??

Answer 1

It is easier to maintain scroll position by calling notifydatasetchanged() only. The problem there is that you are creating a new adapter every time the data gets updated... you should do something like this:

if(listView.getAdapter()==null)    listView.setAdapter(myAdapter); else{    myAdapter.updateData(myNewData);  //update adapter's data    myAdapter.notifyDataSetChanged(); //notifies any View reflecting data to refresh } 

This way, your listview will mantain the scrolling position.

In case you want to scroll to a new position, use:

list.smoothScrollToPosition(int position); 

Answer 2

In case for some reason you don't want to call notifyDataSetChanged(), the you can maintain the position by using setSelectionFromTop()

Before updating the adaptor:

lastViewedPosition = listView.getFirstVisiblePosition();  //get offset of first visible view View v = listView.getChildAt(0); topOffset = (v == null) ? 0 : v.getTop(); 

After updating the adaptor:

listView.setSelectionFromTop(lastViewedPosition, topOffset);