android - AsyncTask 与无尽的 ListView 滚动在 android

Question

我正在创建一个需要无限滚动 ListView 的应用程序。我不想在我的应用程序中使用任何库。我在网上看到了一些有助于实现此类 ListView 的示例，但我怀疑当我的数据来自服务器并在 Asynctask 中进行解析时，我如何才能拥有无穷无尽的 ListView 。我如何从滚动的异步任务中一次加载 10 个项目？我想知道如何在 asyntask 上实现无尽的 ListView 。我是否在 onScroll() 中调用我的异步任务？？？

public class EndlessScrollExample extends ListActivity  {
    public JSONArray jsonarray,jsondatearray;

    public String url;
    public String selectedvalue;
    public String TAG = "TAG Event Display";
    public String SuggestCity;
    public String SuggestState;
    public String suggestCountry;
    public String event_id,address;


    String lat;

    String lng;

    public String event_name;
    public String dateKey;
    public String datetime,timenew;
    Calendar cal;

    public SharedPreferences prefs;
    public Editor editor;
    public String access_token,id,username;
    public static ArrayList<EventsBean> arrayEventsBeans = new ArrayList<EventsBean>();
    ArrayList<DateBean> sortArray = new ArrayList<DateBean>();
    public SAmpleAdapter adapter;
    public ImageView img_menu,img_calender;
    public ListView listview;
    public  EventsBean eventsbean;
   int counter = 0;
   int currentPage = 0;

   FetchEventValues fetchValues;



    @Override
    protected void onCreate(Bundle savedInstanceState) {
        // TODO Auto-generated method stub
        super.onCreate(savedInstanceState);
        setTheme(android.R.style.Theme);
        setContentView(R.layout.sample_endless);

        listview = (ListView)findViewById(android.R.id.list);   


    try {
        // Preferences values fetched from the preference of FBConnection class.
        prefs = PreferenceManager.getDefaultSharedPreferences(getApplicationContext());
        access_token = prefs.getString("access_token", null);
        id = prefs.getString("uid", null);
        username = prefs.getString("username", null);



        if(access_token == null && id == null && username == null)
        {
            Toast.makeText(getApplicationContext(), "FaceBook Login was not successful" +
                    "/nPlease Relogin.", Toast.LENGTH_SHORT).show();
        }
        else
        {
            Log.i(TAG, "VALUES::" + access_token+ "  " + id + "  " +username);
            url = "my Url";
        }

    } catch (NullPointerException e) {

        Log.i(TAG, "User Not Logged IN " + e.getMessage());
            // TODO Auto-generated catch block
            e.printStackTrace();
    }




        fetchValues = new FetchEventValues();
        fetchValues.execute();
        listview = getListView();
        listview.setOnScrollListener(new EndlessScrollListener());



    }

// AsyncTask Class called in the OnCreate() when the activity is first started.


    public class FetchEventValues extends AsyncTask<Integer, Integer, Integer>
    {
        ProgressDialog progressdialog = new  ProgressDialog(EndlessScrollExample.this);


    @SuppressLint("SimpleDateFormat")
    @SuppressWarnings("unchecked")
    @Override
    protected Integer doInBackground(Integer... params) {

        currentPage++;


         // Creating JSON Parser instance
        JsonParser jParser = new JsonParser();

       // getting JSON string from URL


        //arrayEventsBeans.clear();

        JSONObject jsonobj = jParser.getJSONFromUrl(url);

        Log.i(TAG, "URL VALUES:" + url);

        try{
             // Code to get the auto complete values  Autocomplete Values

            JSONArray jsonAarray = jsonobj.getJSONArray(Constants.LOCATIONS);

            eventsbean = new EventsBean();

            Log.e(TAG, "Location Array Size:" + jsonAarray.length());   

            for(int j = 0 ; j < jsonAarray.length() ; j++)
             {
                if(!jsonAarray.getJSONObject(j).isNull(Constants.LOCATION_CITY) && !jsonAarray.getJSONObject(j).isNull(Constants.LOCATION_STATE) && !jsonAarray.getJSONObject(j).isNull(Constants.LOCATION_COUNTRY))
                {
                    JSONObject job = jsonAarray.getJSONObject(j);

                    if(job.has(Constants.LOCATION_STATE))
                    {
                        SuggestCity = job.getString(Constants.LOCATION_CITY);
                        eventsbean.setLocation_city(job.getString(Constants.LOCATION_CITY));
                        SuggestState = job.getString(Constants.LOCATION_STATE);
                        eventsbean.setLocation_state(job.getString(Constants.LOCATION_STATE));
                        suggestCountry = job.getString(Constants.LOCATION_COUNTRY);
                        eventsbean.setLocation_country(job.getString(Constants.LOCATION_COUNTRY));
                    }       


                } 


             }


                   // JSON object to fetch the events in datewise format
                   JSONObject eventobject = jsonobj.getJSONObject("events");

                   arrayEventsBeans = new ArrayList<EventsBean>();


                   // @SuppressWarnings("unchecked")
                    Iterator<Object> keys = eventobject.keys();

                    while (keys.hasNext()) {

                      String datestring = String.valueOf(keys.next());

                      if (datestring.trim().length() > 0) {
                          SimpleDateFormat formatter = new SimpleDateFormat("yyyy-MM-dd");
                          Date date = formatter.parse(datestring);

                          DateBean dateBean = new DateBean(date);
                          sortArray.add(dateBean);
                         } 

                     // JSONArray jsonArray = eventobject.getJSONArray(datestring);
                      //System.out.println(" --"+jsonArray);
                    }

                    System.out.println("size:"+sortArray.size());

                    System.out.println("==========sorting array======");
                    Collections.sort(sortArray,new CompareDate());
                    //reverse order
                    //Collections.reverse(sortArray);

                    for(DateBean d : sortArray){
                     dateKey = new SimpleDateFormat("yyyy-MM-dd").format(d.getDate());
                     System.out.println(dateKey);

                     Date today = new Date();
                     Date alldates = d.getDate();



                  ///  Calendar alldates1 = Calendar.getInstance();

                     JSONArray jsonArray = eventobject.getJSONArray(dateKey);
                     System.out.println(" --"+jsonArray);


                     for (int i = 0 ; i < jsonArray.length() ; i++)
                     {

                         if ((today.compareTo(alldates) < 0  || (today.compareTo(alldates)== 0)))
                            // if (alldates1  > cal) alldates.getTime() >= today.getTime()
                         {

                             String currentTimeStr = "7:04 PM";

                                Date userDate = new Date();
                                String userDateWithoutTime = new SimpleDateFormat("yyyyMMdd").format(userDate);

                                String currentDateStr = userDateWithoutTime + " " + currentTimeStr;
                                Date currentDate = new SimpleDateFormat("yyyyMMdd h:mm a").parse(currentDateStr);

                                if (userDate.compareTo(currentDate) >= 0) {
                                    System.out.println(userDate + " is greater than or equal to " + currentDate);
                                } else {
                                    System.out.println(userDate + " is less than " + currentDate);
                                }


                         JSONObject jsonobjname = jsonArray.getJSONObject(i);

                        EventsBean eventsbean = new EventsBean();

                         JSONObject jobjectpicture = jsonobjname.getJSONObject(Constants.PICTURE);
                         JSONObject jobjeventpicture = jobjectpicture.getJSONObject(Constants.DATA);
                         eventsbean.setUrl(jobjeventpicture.getString(Constants.URL));

                         if(jsonobjname.has(Constants.OWNER))
                         {
                         JSONObject owner_obj = jsonobjname.getJSONObject(Constants.OWNER);
                         eventsbean.setOwner_id(owner_obj.getString(Constants.OWNER_ID));
                         eventsbean.setOwner_name(owner_obj.getString(Constants.OWNER_NAME));
                         String owner_name = owner_obj.getString(Constants.OWNER_NAME);
                         Log.i(TAG, "Owner:" + owner_name);
                         }

                        if(!jsonobjname.isNull(Constants.COVER))
                        {
                         JSONObject objectcover = jsonobjname.getJSONObject(Constants.COVER);
                         eventsbean.setCover_id(objectcover.getString(Constants.COVER_ID));
                         eventsbean.setSource(objectcover.getString(Constants.SOURCE));
                         String cover_url = objectcover.getString(Constants.SOURCE);
                         Log.i(TAG, "Cover Url:" + cover_url);
                         eventsbean.setOffset_y(objectcover.getString(Constants.OFFSET_Y));
                         eventsbean.setOffset_x(objectcover.getString(Constants.OFFSET_X));
                        }


                         eventsbean.setName(jsonobjname.getString(Constants.NAME));
                         eventsbean.setEvent_id(jsonobjname.getString(Constants.EVENT_ID));
                         eventsbean.setStart_time(jsonobjname.getString(Constants.START_TIME));
                         eventsbean.setDescription(jsonobjname.getString(Constants.DESCRIPTION));
                         eventsbean.setLocation(jsonobjname.getString(Constants.LOCATION));

                         if(!jsonobjname.isNull(Constants.IS_SILHOUETTE))
                         {
                            eventsbean.setIs_silhouette(jsonobjname.getString(Constants.IS_SILHOUETTE)); 
                         }

                         eventsbean.setPrivacy(jsonobjname.getString(Constants.PRIVACY));
                         datetime = jsonobjname.getString(Constants.START_TIME);


                         if(!jsonobjname.isNull(Constants.VENUE))
                         {

                         JSONObject objectvenue = jsonobjname.getJSONObject(Constants.VENUE);

                         if(objectvenue.has(Constants.VENUE_NAME))
                         {
                             eventsbean.setVenue_name(objectvenue.getString(Constants.VENUE_NAME));
                             event_name = objectvenue.getString(Constants.VENUE_NAME); 
                             Log.i(TAG, "Event Venue Name:" + event_name);
                         }
                         else
                         {   

                         eventsbean.setLatitude(objectvenue.getString(Constants.LATITUDE));
                         eventsbean.setLongitude(objectvenue.getString(Constants.LONGITUDE));
                         eventsbean.setCity(objectvenue.getString(Constants.CITY));
                         eventsbean.setState(objectvenue.getString(Constants.STATE));
                         eventsbean.setCountry(objectvenue.getString(Constants.COUNTRY));
                         eventsbean.setVenue_id(objectvenue.getString(Constants.VENUE_ID));
                         eventsbean.setStreet(objectvenue.getString(Constants.STREET));
                         address = objectvenue.getString(Constants.STREET);
                         eventsbean.setZip(objectvenue.getString(Constants.ZIP));

                }
            }
                         arrayEventsBeans.add(eventsbean);

                         Log.i(TAG, "arry list values:" + arrayEventsBeans.size());


                        }
                    }
                 }

               }catch(Exception e){

                   Log.e(TAG , "Exception Occured:" + e.getMessage());
               }


            return null;
        }


        class CompareDate implements Comparator<DateBean>{

           @Override
           public int compare(DateBean d1, DateBean d2) {

            return d1.getDate().compareTo(d2.getDate());
           }   
    }


        @Override
        protected void onProgressUpdate(Integer... values) {
        // TODO Auto-generated method stub
        super.onProgressUpdate(values);
        }


        @Override
        protected void onPostExecute(Integer result) 
        {
            // TODO Auto-generated method stub
            super.onPostExecute(result);

                if(this.progressdialog.isShowing()) 
                {
                    this.progressdialog.dismiss();

                }


                if(adapter == null)
                {
                    adapter = new SAmpleAdapter(EndlessScrollExample.this, 0, arrayEventsBeans);
                    listview.setAdapter(adapter);
                }
                else
                {
                    adapter.notifyDataSetChanged();
                }

                    //currentPage++;




        }


        @Override
        protected void onPreExecute() {

            super.onPreExecute();

            this.progressdialog.setMessage("Loading....");
            this.progressdialog.setCanceledOnTouchOutside(false);
            this.progressdialog.show();
        }


        public int setPage(int currentPage) {
            return currentPage;
            // TODO Auto-generated method stub



        }       
    }


    public class EndlessScrollListener implements OnScrollListener {

        private int visibleThreshold = 0;
        private int currentPage = 0;


        public EndlessScrollListener() {
        }
        public EndlessScrollListener(int visibleThreshold) {
            this.visibleThreshold = visibleThreshold;
        }

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

        }

        @Override
        public void onScrollStateChanged(AbsListView view, int scrollState) {

            if (scrollState == SCROLL_STATE_IDLE) {
                if (listview.getLastVisiblePosition() >= listview.getCount() - visibleThreshold) {
                    currentPage++;
                    fetchValues.setPage(currentPage);
                    fetchValues.execute();
                }
            }

        }
    }



    }

提前致谢。

Answer 1

ListView 已经支持OnScrollListener，所以你必须覆盖它并检查条件(在 onScroll() 中)，是否到达列表的末尾或不是。如果是，则添加页脚(可选)并触发异步任务。收到结果后通知适配器。您可以查看此 link 上的解决方案，它基于相同的概念。