在获取视图时在 AsyscTask posEcecute 方法中获取 NPE

Question

我是 android 开发新手，这是我的第一个应用程序。 我正在使用片段和适配器来显示选项卡。这是 Tab2（Downoad 详细信息）片段代码，在选择 Tab 时，我正在调用一个网络服务器，它从数据库中获取数据并将其设置在视图中。一切正常，我收到了来自 Jason 的 WS 的回复。我正在解析这个 Jason 数据并希望在 tabLayout 中显示。显示时我得到 NPE。

很少有帖子建议使用setUserVisibleHint，但我无法通过它获得所需的输出。

请建议我应该怎么做，以便当我选择 Tab2（下载详细信息）或滑动到此选项卡时，我的 WS 被调用并且数据将显示在选项卡页面上。

下面是我的活动

 @SuppressLint("NewApi")
public class DownloadStatusActivity extends FragmentActivity  implements ActionBar.TabListener  {


ProgressDialog prgDialog1;
TextView errorMsg1;
private ViewPager viewPager;
private TabsPagerAdapter mAdapter;
private ActionBar actionBar;
// Tab titles
private String[] tabs = { "Download Overview", "Downoad Details"  };


@Override
protected void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    setContentView(R.layout.dwnld_status_activity);

    String value1= getIntent().getStringExtra("loccodeselected");
    String value2= getIntent().getStringExtra("bccodeselected");
    String value3= getIntent().getStringExtra("bmselected");
    TextView dwndActlbl = (TextView) findViewById(R.id.dwndActlbl);
    dwndActlbl.setText(Html.fromHtml("Download Status for " + value1 +","+value2 +","+value3+ "is as below: </b>"));

     // Initilization
    viewPager = (ViewPager) findViewById(R.id.pager);
    actionBar = getActionBar();
    mAdapter = new TabsPagerAdapter(getSupportFragmentManager());

    viewPager.setAdapter(mAdapter);
    actionBar.setHomeButtonEnabled(false);
    actionBar.setNavigationMode(ActionBar.NAVIGATION_MODE_TABS);        


    ////passing data to fragement start//
    //bundle
    Bundle bundle =new Bundle();
    bundle.putString("value1",value1);
    bundle.putString("value2",value2);
    bundle.putString("value3",value3);

    DSFragment frag_in = new DSFragment();
    DDFragement frag_out = new DDFragement();

    frag_in.setArguments(bundle);
    frag_out.setArguments(bundle);
    mAdapter.addFrag(frag_in," ");
    mAdapter.addFrag(frag_out," ");
    viewPager.setAdapter(mAdapter);
    ///////////////

    ///passing data to fragement end///


    // Adding Tabs
    for (String tab_name : tabs) {
        actionBar.addTab(actionBar.newTab().setText(tab_name)
                .setTabListener(this));
    }

    /**
     * on swiping the viewpager make respective tab selected
     * */
    viewPager.setOnPageChangeListener(new ViewPager.OnPageChangeListener() {

        @Override
        public void onPageSelected(int position) {
            // on changing the page
            // make respected tab selected
            actionBar.setSelectedNavigationItem(position);
        }

        @Override
        public void onPageScrolled(int arg0, float arg1, int arg2) {
        }

        @Override
        public void onPageScrollStateChanged(int arg0) {
        }
    });


    }

    @Override
    public void onTabReselected(Tab tab, FragmentTransaction ft) {
    }

    @Override
    public void onTabSelected(Tab tab, FragmentTransaction ft) {
    // on tab selected
    // show respected fragment view
    viewPager.setCurrentItem(tab.getPosition());
    }

    @Override
    public void onTabUnselected(Tab tab, FragmentTransaction ft) {
    }


}

下面是分片代码。

public class DDFragement extends Fragment {
JsonArray arrayFromString=new JsonArray(); 
private ProgressDialog    prgDialog1;
View rootView=null;

private boolean isViewShown = false;


@Override

public void setUserVisibleHint(boolean isVisibleToUser) {
    super.setUserVisibleHint(isVisibleToUser);


    if (getView() != null ) {
        isViewShown = true;
        // fetchdata() contains logic to show data when page is selected mostly asynctask to fill the data

    } else {
        isViewShown = false;
    }


}



@Override
public View onCreateView(LayoutInflater inflater, ViewGroup container,
        Bundle savedInstanceState) {






    View rootView = inflater.inflate(R.layout.dd_fragement, container, false);
    String loc="";
    String bc="";String bm="";

    if(!isViewShown)
    {
    if(getArguments()!=null)
    {
      loc = getArguments().getString("value1");    
      bc = getArguments().getString("value2");  
      bm = getArguments().getString("value3");  
    }


        new invokegetDwnldDetailDataAsynTask().execute(loc,bc,bm);
    }
    return rootView;
}




/*******Implmentation of Async Task******************/
class invokegetDwnldDetailDataAsynTask extends AsyncTask<String, String, JsonArray> {

    /**
     * Before starting background thread Show Progress Dialog
     * */
    @Override
    protected void onPreExecute() {
        super.onPreExecute();
            prgDialog1 = new ProgressDialog(getActivity());
        prgDialog1.setMessage("Loading Download Details. Please wait...");
        prgDialog1.setIndeterminate(false);
        prgDialog1.setCancelable(false);
        prgDialog1.show();
    }

    /**
     * getting All products from url
     * @return 
     * */
    protected JsonArray doInBackground(String...  dwnparams) {

        String loc = dwnparams[0];
        String bc = dwnparams[1];
        String bm = dwnparams[2];


        RequestParams reqparams = new RequestParams();
        reqparams.put("loccodeselected", loc);
        reqparams.put("bccodeselected", bc);
        reqparams.put("bmselected", bm);

        // Show Progress Dialog 
        //prgDialog1.show();

        // Make RESTful webservice call using AsyncHttpClient object
        AsyncHttpClient client = new AsyncHttpClient();
        client.get("http://localhost:8081/useraccount/queries/fetchdwnDetailStatus",reqparams ,new AsyncHttpResponseHandler() {
            // When the response returned by REST has Http response code '200'
             @SuppressWarnings("unused")
            @Override
             public void onSuccess(String response) {
                // Hide Progress Dialog
                // prgDialog1.hide();
                 try {
                         // JSON Object
                         JSONObject jObj = new JSONObject(response);
                       if(jObj!=null)
                       {
                           System.out.println("RESPONSE1===>"+jObj.getString("downloadDetails"));


                           JsonParser jsonParser = new JsonParser();
                           arrayFromString = jsonParser.parse((String) jObj.getString("downloadDetails")).getAsJsonArray();
                           System.out.println("RESPONSE2===>"+arrayFromString);
                         }

                         // Else display error message
                         else{
                            // errorMsg1.setText(jObj.getString("error_msg"));
                             Toast.makeText(getActivity().getApplicationContext(), "ERROR SOME", Toast.LENGTH_LONG).show();
                         }




                 } catch (JSONException e) {
                     // TODO Auto-generated catch block
                     Toast.makeText(getActivity().getApplicationContext(), "Error Occured [Server's JSON response might be invalid]!", Toast.LENGTH_LONG).show();
                     e.printStackTrace();

                 }
             }
             // When the response returned by REST has Http response code other than '200'
             @Override
             public void onFailure(int statusCode, Throwable error,
                 String content) {
                 // Hide Progress Dialog
                // prgDialog1.hide();
                 // When Http response code is '404'
                 if(statusCode == 404){
                     Toast.makeText(getActivity().getApplicationContext(), "Requested resource not found", Toast.LENGTH_LONG).show();
                 } 
                 // When Http response code is '500'
                 else if(statusCode == 500){
                     Toast.makeText(getActivity().getApplicationContext(), "Something went wrong at server end", Toast.LENGTH_LONG).show();
                 } 
                 // When Http response code other than 404, 500
                 else{
                     Toast.makeText(getActivity().getApplicationContext(), "Unexpected Error occcured! [Most common Error: Device might not be connected to Internet or remote server is not up and running]", Toast.LENGTH_LONG).show();
                 }
             }
         });
        return arrayFromString;





    }

    /**
     * After completing background task Dismiss the progress dialog
     * **/
    protected void onPostExecute(JsonArray arrayFromString1) {
        // dismiss the dialog after getting all products
        prgDialog1.dismiss();

                /**
                 * Updating parsed JSON data into ListView
                 * */


               if (arrayFromString1 != null) { 
                   int count=0;
                     TableLayout tl;
                        tl = (TableLayout) rootView.findViewById(R.id.ddfragment_table);

                     for (int i=0;i<arrayFromString.size();i++){ 
                      // System.out.println("array==>"+arrayFromString.get(i).toString());


                       TableRow tr1 = new TableRow(getActivity());
                        if(count%2!=0) tr1.setBackgroundColor(Color.LTGRAY);
                        tr1.setId(i);

                        // tr.setBackgroundResource(Color.MAGENTA);
                         tr1.setLayoutParams(new LayoutParams(LayoutParams.MATCH_PARENT, LayoutParams.WRAP_CONTENT));

                                     //TEXTVIEWS********
                         TextView tv1 = new TextView(getActivity());
                         String textLabel="";
                         tv1.setText(textLabel );  //seetting first response in cell

                         tv1.setId(i);
                         tv1.setTextColor(Color.BLACK);
                         tv1.setTextSize(15);
                         tv1.setPadding(5, 5, 5, 5);
                         tr1.addView(tv1);

                        tl.addView(tr1, new TableLayout.LayoutParams(LayoutParams.MATCH_PARENT, LayoutParams.WRAP_CONTENT));
                         count++;  
                   } 
                } 

           }

}

}

错误是在tl = (TableLayout) rootView.findViewById(R.id.ddfragment_table) 行的onpostExecute()； 错误：NullPointerException.. 尝试在空 obj 引用上调用虚拟方法..findViewbyId(int)

Answer 1

片段中您的 oncreateView 中的 rootView 与作为类成员的 rootview 不同。替换：

View rootView = inflater.inflate(R.layout.dd_fragement, container, false);

与：

rootView = inflater.inflate(R.layout.dd_fragement, container, false);

顺便说一句，这就是为什么最好这样命名类成员：View mRootView;