android - 更新时如何保持 ListView 的滚动位置

Question

我已经阅读了很多示例，但是如果我希望在从 JSON 更新 ListView 后保持滚动位置，那么我可以不使用 AsyncTask 实例？？？

我的列表代码是

 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);

而我的适配器是

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;
}

最后用于 json 解析的类是

    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;
    }



}

如果数据是从网页更新的，那么在顶部显示更新项目的最简单方法是什么？？

Answer 1

只调用 notifydatasetchanged() 更容易保持滚动位置。问题是每次数据更新时你都在创建一个新的适配器......你应该这样做:

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

这样，您的 ListView 将保持滚动位置。

如果您想滚动到新位置，请使用:

list.smoothScrollToPosition(int position);