如何在onScroll ListView (Android)中启动onScroll方法

Question

首先，我从服务器获得数据，然后构建了ListView，现在我不想在列表视图滚动到底部时加载更多。我的异步类有名称Fetch，所以我认为需要调用吗？另外，我还需要使用我第一次调用的Fetch对象礼仪的所有值，

感谢大家！

我这里的活动代码

public class InboxActivity extends Activity implements View.OnClickListener {


    ......................


    DatePickerDialog.OnDateSetListener d = new DatePickerDialog.OnDateSetListener() {

        ........

        public void onDateSet(DatePicker view, int year, int monthOfYear,
                int dayOfMonth) {
        ...........     

            this.fetch = new FetchTask();
            this.fetch.Selectedmonth = this.Smonth;
            this.fetch.Selectedyear = this.Syear;
            this.fetch.page = 0;
            this.fetch.sess_id = user_id;
            ListView ll = (ListView)findViewById(R.id.mailList);
            this.fetch.ll = ll;
            this.fetch.execute();
            ll.setOnScrollListener(new OnScrollListener(){


                public void onScrollStateChanged(ListView view, int scrollState)
                {



                }

                @Override
                public void onScroll(AbsListView view, int firstVisibleItem,
                        int visibleItemCount, int totalItemCount) {




                    if(firstVisibleItem+visibleItemCount==totalItemCount)
                    {

                    HERE I NEED TO LAUNCH FETCH object  

                    }                       

                }



        }
    };






public class FetchTask extends AsyncTask<Void, Void, JSONArray> {

    HERE MY PRORPITIES

        @Override

        protected JSONArray doInBackground(Void... params) {
            try {

                HttpClient httpclient = new DefaultHttpClient();
                HttpPost httppost = new HttpPost("my url");

                List<NameValuePair> nameValuePairs = new ArrayList<NameValuePair>(2);


                nameValuePairs.add(new BasicNameValuePair("qw", "er"));
                nameValuePairs.add(new BasicNameValuePair("debug", "1"));
                nameValuePairs.add(new BasicNameValuePair("t", "0"));
                nameValuePairs.add(new BasicNameValuePair("m", Integer.toString(this.Selectedmonth)));
                nameValuePairs.add(new BasicNameValuePair("y", Integer.toString(this.Selectedyear)));
                nameValuePairs.add(new BasicNameValuePair("st", Integer.toString(this.page)));
                nameValuePairs.add(new BasicNameValuePair("sess_id", this.sess_id));


                httppost.setEntity(new UrlEncodedFormEntity(nameValuePairs));


                HttpResponse response = httpclient.execute(httppost);

                BufferedReader reader = new BufferedReader(new InputStreamReader(response.getEntity().getContent(), "utf-8"), 8);
                StringBuilder sb = new StringBuilder();
                sb.append(reader.readLine());
                String line = "0";
                while ((line = reader.readLine()) != null) 
                {
                    sb.append(line);
                }
                reader.close();
                String result11 = sb.toString();
                System.out.println(result11);
                this.result_str = result11;
                // parsing data
                JSONArray arr = new JSONArray(result11);





                return new JSONArray(result11);
            } catch (Exception e) {
                e.printStackTrace();
                return null;
            }
        }



          @Override
          protected void onPreExecute() {
              this.pd = new ProgressDialog(InboxActivity.this);
              this.pd.setMessage("loading");
              this.pd.show();         
          } 


        @Override
        protected void onPostExecute(JSONArray result)
        {

            if (result != null) 
            {
                List<String> subjects = new ArrayList<String>();
                List<String> emails = new ArrayList<String>();

                for(int i = 0; i < result.length(); i++)
                {
                    try 
                    {
                        JSONObject json_data = result.getJSONObject(i);
                        emails.add(json_data.getString("mittente"));
                        subjects.add(json_data.getString("oggetto"));
                    } 
                    catch (JSONException e) 
                    {
                        e.printStackTrace();
                    }

                }
                if(this.page == 0)
                {
                    this.adapter = new ArrayAdapter<String>(
                            InboxActivity.this,
                            R.layout.da_item,
                            emails
                        );
                    this.ll.setAdapter(this.adapter);               
                }
                else
                {
                    for(int i = 0; i < result.length(); i++)
                    { 
                        JSONObject json_data;
                        try 
                        {
                            json_data = result.getJSONObject(i);
                            this.adapter.add(json_data.getString("mittente"));  
                        } 
                        catch (JSONException e) 
                        {
                            e.printStackTrace();
                        }

                    }
                }
            } 
            else 
            {
                System.out.println("Messages not found");

            }    
            this.pd.dismiss();
        }
    }   
}

Answer 1

概括概述：

1. 您需要重新创建一个新的fetch对象。您不能重新执行旧的AsynTask。
2. 一个问题是，您不知道列表视图中有多少元素，因此滚动条将显示为它到达末尾的样子。您的获取可能会及时返回，并且添加更多的元素，并且神奇地有更多的内容要滚动，或者它没有及时返回，并且用户到达了明显的终端。如果可以提前确定总数，请重写GetCount方法。
3. 决定何时获取下一个数据集。如果覆盖适配器的getView方法，则在适配器为每个位置创建视图时，将能够获得回调。当用户滚动时，该位置将越来越接近您的数组的末尾。设置一些阈值以开始获取下一个数据集。 类myAdapter扩展ArrayAdapter{ @重写公共视图getView(int位置，视图convertView，ViewGroup父){返回super.getView(位置，convertView，父)；if(!thresholdReachedBefore & position & position >阈值){ fetchNextDataSet()；} }

Answer 2

要运行AsyncTask，必须调用扩展AsyncTask的类的execute()方法。但是，每次进行抓取时都要小心，因为您将获取一组新的原始数据，这些数据可能会导致重复。解决这一问题的一种方法是只返回一定数量的元素并对其进行计数。下次进行提取时，只从下一个索引中提取元素。