java - 在异步任务之后用另一个 fragment 替换一个 fragment

Question

我已经在这上面花了很多时间，我只想在不改变整个项目的情况下找到最快的解决方案。我有一个 Fragment1 调用另一个fragment2 来获取数据。在fragment2 获取所需的数据后，它会用fragment3 替换自己，fragment3 显示收集到的所有数据。我的fragment1包含2个 fragment ，搜索 fragment ，另一个 fragment 用于前面提到的fragment2和fragment3。我的fragment2 显示的是一个对话框和一个 TextView ，其中包含未获取数据时的错误。

我的问题是我无法以某种方式用fragment3替换fragment2，它不断地在FragmentTransaction ft = getFragmentManager().beginTransaction()行上弹出错误空指针异常..请帮忙，我是android编程新手，如果有我错过了一些事情请告诉我..非常感谢..

这是我的fragment1 代码

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

    final View rootView = inflater.inflate(R.layout.fragment_grade_layout, container, false);
     main_spnr = (Spinner) rootView.findViewById(R.id.spinner_gradecategory);
     main_spnr.setSelection(0);
     db = new mycsu_database(rootView.getContext());
     main_spnr.setOnItemSelectedListener(new OnItemSelectedListener() {
         @Override
         public void onItemSelected(AdapterView<?>arg0, View view, int arg2, long arg3) {
             Log.v(TAG, "gradecount= " + db.getgradecount(idnumber));
             if(db.getgradecount(idnumber)!=0){
                 Log.v(TAG, "gradecount= " + db.getgradecount(idnumber));
                 InterfaceMode = main_spnr.getSelectedItemPosition();
                 getfragments();
             }
             else{
                 Fragment fr = new DataGetterClass(idnumber,InterfaceMode);
                 FragmentManager fm = getFragmentManager();
                 FragmentTransaction fragmentTransaction = fm.beginTransaction();
                 fragmentTransaction.replace(R.id.frame_gradeListParent, fr);
                 fragmentTransaction.commit(); 
             }

         }
         @Override
         public void onNothingSelected(AdapterView<?> arg0) {
             // TODO Auto-generated method stub

         }
     });

     InterfaceMode = 0;
     prvInterfaceMode = -1;
     if(db.getgradecount(idnumber)!=0){
         Log.v(TAG, "gradecount= " + db.getgradecount(idnumber));
         InterfaceMode = main_spnr.getSelectedItemPosition();
         getfragments();
     }
     else{
         Fragment fr = new DataGetterClass(idnumber,InterfaceMode);
         FragmentManager fm = getFragmentManager();
         FragmentTransaction fragmentTransaction = fm.beginTransaction();
         fragmentTransaction.replace(R.id.frame_gradeListParent, fr);
         fragmentTransaction.commit(); 
     }
    return rootView;
}


private void getfragments(){

     Fragment fr = null;
     if(prvInterfaceMode != InterfaceMode){
         Log.v(TAG,"InterfaceMode: " + InterfaceMode);
         switch(InterfaceMode){

            case 0:
                Log.v(TAG,"here in 0");
                fr = new ListofGrades();break;
            case 1:
                fr = new fragment_selectsem();break;
            case 2:
                fr = new ListofallGrades();break;
            default:break;
         }

         if(fr!=null){
             Log.v(TAG,"calling " + InterfaceMode);
             FragmentManager fm = getFragmentManager();
             FragmentTransaction fragmentTransaction = fm.beginTransaction();
             fragmentTransaction.setCustomAnimations(R.anim.slide_in_left, R.anim.slide_out_right);
             fragmentTransaction.replace(R.id.frame_gradeListParent, fr);
             fragmentTransaction.commit(); 
         }
         prvInterfaceMode = InterfaceMode;
     }

}

我的 Fragment2 是 DataGetterClass()

public class DataGetterClass extends Fragment {

private static final String TAG = DataGetterClass.class.getSimpleName();
private static String _idnum="";
private static mycsu_database db;
private static ProgressBar progress;
private static TextView tvr;
private static int _callnxt;
FrameLayout frm;
int progressBarStatus = 0;
boolean done = false;

public DataGetterClass(String idnum, int callnxt){
    DataGetterClass._idnum = idnum;
    DataGetterClass._callnxt = callnxt;
}



@Override
public View onCreateView(LayoutInflater inflater,
          ViewGroup container, Bundle savedInstanceState) {
    final View rootView = inflater.inflate(R.layout.fragment_loading, container, false);
    db = new mycsu_database(rootView.getContext());
    progress = (ProgressBar) rootView.findViewById(R.id.progressBar_getter);
    progress.setIndeterminate(true);
    tvr = (TextView) rootView.findViewById(R.id.textView1_error);
    frm = (FrameLayout) rootView.findViewById(R.id.frame_gradeListParent);
    tvr.setVisibility(View.INVISIBLE);
    new RecieveAsyncTask().execute();
    new Thread(new Runnable() {
        public void run() {
            while(true){
                if(done){
                    Log.v(TAG, "loading done");
                    FragmentTransaction ft = getFragmentManager().beginTransaction();
                    Fragment wvf =  new ListofGrades();
                    ft.replace(frm.getId(), wvf);
                    ft.commit();
                    break;
                }

            }
        }
      }).start();

    return rootView;
}



private class  RecieveAsyncTask extends AsyncTask<Void, Void, Void> {

    String _Response;

    @Override
    protected void onPreExecute() {
        super.onPreExecute();
        progress.setVisibility(View.VISIBLE);
    }

    @Override
    protected Void doInBackground(Void... params) {
        // TODO Auto-generated method stub
        _Response = postData();

        return null;
    }


    @Override
    protected void onPostExecute(Void result) {  
        progress.setVisibility(View.INVISIBLE);
        if(_Response!=""){
            if(ParseJson(_Response))
                done=true;//getfragments();
            else
                tvr.setVisibility(View.VISIBLE);
        }
        else{
            Log.v("json", "unable to connect to server");
            tvr.setVisibility(View.VISIBLE);
        }
    }

}

public String postData() {
    // Create a new HttpClient and Post Header
    HttpClient httpclient = new DefaultHttpClient();
    HttpPost httppost = new HttpPost("http://192.168.5.159/mycsu/mycsu.php");
    String _response = "";
    Log.v("trytry", " 11 ");
    try {
        // Add your data
        List<NameValuePair> nameValuePairs = new ArrayList<NameValuePair>(2);
        nameValuePairs.add(new BasicNameValuePair("studid", _idnum));
        nameValuePairs.add(new BasicNameValuePair("Mode","1"));
        nameValuePairs.add(new BasicNameValuePair("studyr",""));
        nameValuePairs.add(new BasicNameValuePair("studsem",""));
        httppost.setEntity(new UrlEncodedFormEntity(nameValuePairs));

        // Execute HTTP Post Request
        HttpResponse response = httpclient.execute(httppost);
        InputStream inputStream = response.getEntity().getContent();
        InputStreamReader inputStreamReader = new InputStreamReader(inputStream);
        BufferedReader bufferedReader = new BufferedReader(inputStreamReader);
        StringBuilder stringBuilder = new StringBuilder();
        String bufferedStrChunk = null;

        while((bufferedStrChunk = bufferedReader.readLine()) != null){
            stringBuilder.append(bufferedStrChunk);
        }
        _response = stringBuilder.toString();
    } catch (ClientProtocolException e) {
        Log.d(TAG,e.toString());
        // TODO Auto-generated catch block
    } catch (IOException e) {
        Log.d(TAG,e.toString());
        // TODO Auto-generated catch block
    }
    //Log.v(TAG, _response);
    //Log.v("trytry", " 22 ");
   // Log.v("trytry",_response);
    return _response;
} 



boolean ParseJson(String JsonStr){
    boolean returnbool = true;
    String Jsontstr = "{ \"Android\":" + JsonStr + "}";
    GradeClass gc = new GradeClass();
    GpaClass gpac = new GpaClass();
    JSONObject jsonResponse;
    try{
        jsonResponse = new JSONObject(Jsontstr);
        JSONArray jsonMainNode = jsonResponse.optJSONArray("Android");
        int lengthJsonArr = jsonMainNode.length();

        for( int i=0; i< lengthJsonArr; i++){

            JSONObject jsonChildNode = jsonMainNode.getJSONObject(i);
            String coursecode = jsonChildNode.optString("courseno");
            String coursedesc = jsonChildNode.optString("description");
            String grade =jsonChildNode.optString("grade");
            int unit = Integer.parseInt(jsonChildNode.optString("unit"));
            String compgrade = jsonChildNode.optString("gcompl");
            String sem = jsonChildNode.optString("sem");
            String yr = jsonChildNode.optString("sy");
            String idnumber = jsonChildNode.optString("studid");
            float gpa =Float.parseFloat( jsonChildNode.optString("gpa"));
            String standing =  jsonChildNode.optString("standing");

            /*
            Log.d(TAG, "coursecode: " + coursecode  
                        +" coursedesc: " + coursedesc
                        +" grade: " + grade
                        +" unit: " + unit
                        +" compgrade: " + compgrade
                        +" sem: " + sem
                        +" yr: " + yr
                        +" idnumber: " + idnumber
                        +" gpa: " + gpa
                        +" standing: " + standing);
            */
            float compgradef, gradeint;
            if(grade.equals("INC"))
                gradeint = 4;
            else if(grade.equals("PASSED"))
                gradeint = (float) 4.5;
            else if(grade.equals("FAILED"))
                gradeint = (float) 6;
            else if(grade.equals("IN PROG"))
                gradeint = (float) 7;
            else
                gradeint =  Float.parseFloat(grade);

            if(compgrade.equals("null")){
                compgradef = 0;
            }else
                compgradef = Float.parseFloat(compgrade);




            gc.setcoursecode(coursecode);
            gc.setcoursedesc(coursedesc);
            gc.setgrade(gradeint);
            gc.setunits(unit);
            gc.setcompgrade(compgradef);
            gc.setcoursesem(sem);
            gc.setcourseyr(yr);
            gc.setidnumber(idnumber);
            gpac.setcoursesem(sem);
            gpac.setcourseyr(yr);
            gpac.setgpa(gpa);
            gpac.setidnumber(idnumber);
            gpac.setstanding(standing);



            if(db.findifgradeexist(gc)==false){
                db.add_grade(gc);
                //Log.v(TAG,"data GRADE inserted in db");
            }else;
                //Log.v(TAG,"data GRADE  exist NOT inserted in db");

            if(!db.findifgpaexist(gpac)==false){
                db.add_gpa(gpac);
                //Log.v(TAG,"data GPA inserted in db");
            }else;
                //Log.v(TAG,"data GPA exist NOT inserted in db");

        }


    }catch (JSONException e) {
        returnbool = false;
        e.printStackTrace();
    }
    return returnbool;
}

我在我的fragment2中做了很多事情，只是为了让它按照我想要的方式运行，以至于我创建了一个线程来监听异步任务是否完成..但它再次不起作用..请帮忙如果您心中有解决方案..我们将不胜感激..

哦!!，这是 LogCat

 02-17 18:59:38.064: E/AndroidRuntime(3069): FATAL EXCEPTION: Thread-3298
02-17 18:59:38.064: E/AndroidRuntime(3069): java.lang.NullPointerException
02-17 18:59:38.064: E/AndroidRuntime(3069):     at com.csu.mycsu_proto1_DataGetter.DataGetterClass$1.run(DataGetterClass.java:82)
02-17 18:59:38.064: E/AndroidRuntime(3069):     at java.lang.Thread.run(Thread.java:856)

Answer 1

坂田雄鸡，

我认为，这里的问题是您在 AsyncTask 中使用局部变量 _Response。我不相信它是线程安全的。 doInBackground 在异步线程中操作，onPostExecute 在主线程中操作。 AsyncTask 类使您能够通过使用其 header 中的 Void 在线程之间安全地传递信息。第三个Void用于将信息从doInBackground传递到onPostExecute。

尝试将 doInBackground 的最后两条语句更改为

返回postData();

并将 doInBackground 更改为 protected 字符串

并将 header 更改为 onPostExecute

protected void onPostExecute(String _Response){

并将 header 更改为 ReceiveAsyncTask ，以便将第三个 Void 更改为 String (抱歉，我无法在答案中输入尖括号或我会发表声明)。

并消除 String _Response 声明。