Android 更新时如何保持列表视图的滚动位置

how to maintain scroll position of listview when it updates

提问by Aalok Sharma

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

我已经阅读了很多示例，但是如果我希望在ListView更新后保持我的滚动位置JSON，那么我可以在不使用AsyncTask实例的情况下做到这一点吗？？？

the code for my list is

我的列表的代码是

 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);

and my adapter is

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;
}

and finally the class being used for json parsing is

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;
    }



}

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

除此之外，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？

回答by Sebastian Breit

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:

仅通过调用 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......你应该做这样的事情：

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.

这样，您的列表视图将保持滚动位置。

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

如果要滚动到新位置，请使用：

list.smoothScrollToPosition(int position);

回答by Sudhasri

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

如果由于某种原因您不想调用 notifyDataSetChanged()，您可以使用 setSelectionFromTop() 保持位置

Before updating the adaptor:

更新适配器之前：

lastViewedPosition = listView.getFirstVisiblePosition();

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

After updating the adaptor:

更新适配器后：

listView.setSelectionFromTop(lastViewedPosition, topOffset);

回答by waqaslam

list.smoothScrollToPosition(int position);     //my favorite :)

It may also help you to scroll nice'n'smooth to a particular item

它还可以帮助您将 nice'n'smooth 滚动到特定项目

回答by MAC

listview.setSelection( i );

this will help you to set particular row at top

这将帮助您在顶部设置特定行

回答by felixwcf

For overall picture:

整体图：

In your API response callback, call this function(example) below:

在您的 API 响应回调中，调用以下函数（示例）：

MyAdapter mAdapter;
ArrayList<Users> mUsers;

private void updateListView(ArrayList<Users> users) {
    mUsers.addAll(users);
    if(mAdapter == null) {
        mAdapter = new MyAdapter(getContext(), mUsers);
        mListView.setAdapter(mAdapter);
    }
    mAdapter.notifyDataSetChanged(); // Add this one
}

回答by Aron Lorincz

If you're using an ArrayAdapter (or a subclass of it), the problem may be caused by that the adapter updates the list when you clean it before adding the new items:

如果您使用的是 ArrayAdapter（或其子类），则问题可能是由于在添加新项目之前清理列表时适配器更新了列表：

adapter.clear();
adapter.addAll(...);

You can fix it by wrapping the code that modifies the adapter like this:

您可以通过包装修改适配器的代码来修复它，如下所示：

adapter.setNotifyOnChange(false); // Disable calling notifyDatasetChanged() on modification
adapter.clear();
adapter.addAll(...); // Notify the adapter about that data has changed. Note: it will re-enable notifyOnChange
adapter.notifyDataSetChanged();