android - 当选项卡式 Activity 滑回第一个选项卡时应用程序崩溃

Question

我通过 android studio 内置的标准方法创建了选项卡式 Activity 。每个选项卡包含两个子 fragment - 第一个 fragment 有微调器，具体取决于将哪个选项放在正确的嵌套 fragment 中。选项卡通常从第一个滑到最后一个(4 个选项卡)。第一个和第二个选项卡之间滑动完美，3和4之间也是正常切换。但是，如果我尝试从 3 或 4 滑回到第 1 或第 2 个选项卡，应用程序崩溃，总是归咎于第一个选项卡中的嵌套 fragment ，即使幻灯片是从第 3 个选项卡到第 2 个选项卡。怎么了？

在TabActivity中切换标签

switch(position){
                case 0:
                    Fragment addNewFragment = new AddNew_fragment();
                    Bundle args = new Bundle();
                    args.putString("connectURL", connectURL);
                    args.putString("access_token", access_token);
                    addNewFragment.setArguments(args);
                    return addNewFragment;
                case 1:
                    Fragment toDoListFragment = new ToDoListFragment();
                    Bundle args1 = new Bundle();
                    args1.putString("connectURL", connectURL);
                    args1.putString("access_token", access_token);
                    toDoListFragment.setArguments(args1);
                    return toDoListFragment;
                case 2:
                    Fragment allNewsFragment = new AllNewsFragment();
                    Bundle args2 = new Bundle();
                    args2.putString("connectURL", connectURL);
                    args2.putString("access_token", access_token);
                    allNewsFragment.setArguments(args2);
                    return allNewsFragment;
                case 3:
                    Fragment myPageFragment = new MyPageFragment();
                    Bundle args3 = new Bundle();
                    args3.putString("connectURL", connectURL);
                    args3.putString("access_token", access_token);
                    myPageFragment.setArguments(args3);
                    return myPageFragment;
                default:
                    return null;
            }

第一个选项卡上的嵌套 fragment

public class GoalsListFragment extends ListFragment {

    int goalID;
    int goalExist;

    JSONObject jsonResponseReports = null;
    JSONObject jsonResponse;
    JSONArray items;

    String goalDescriptionText;


    StringBuilder feedInput;

    String connectURL;

    ArrayList<String> allGoalsList;
    ArrayAdapter adapter;

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

        connectURL = getArguments().getString("connectURL");
        View goalFragment = inflater.inflate(R.layout.fragment_goals_list, null);

        allGoalsList = new ArrayList<String>();

        new GetAllGoals().execute();

        return goalFragment;
    }



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

        @Override
        protected Void doInBackground(Void... params) {
            try {

                URL url = new URL(connectURL);

                HttpURLConnection connection = (HttpURLConnection) url.openConnection();
                connection.setRequestMethod("POST");
                connection.setDoOutput(true); // Triggers POST.
                connection.setDoInput(true);


                BufferedReader br = new BufferedReader(new InputStreamReader(connection.getInputStream()));

                feedInput = new StringBuilder();

                String inputBufferSymbol = "";

                while ((inputBufferSymbol = br.readLine()) != null) {
                    feedInput.append(inputBufferSymbol);
                }
                br.close();


                jsonResponseReports = new JSONObject(feedInput.toString());
                jsonResponse = jsonResponseReports.getJSONObject("response");
                if (jsonResponse.getString("count").equals("0")) {
                    goalDescriptionText = "В этой сфере жизни вы не поставили ни одной цели";
                    goalExist = 0;
                } else {
                    goalExist = 1;

                    items = jsonResponse.getJSONArray("items");

                    int count=0;

                    while (count<Integer.parseInt(jsonResponse.getString("count")))
                    {
                        allGoalsList.add(items.getJSONObject(count).getString("title"));
                        count++;
                    }
                }

            } catch (MalformedURLException e) {
                // TODO Auto-generated catch block
                e.printStackTrace();
            } catch (IOException e) {
                // TODO Auto-generated catch block
                e.printStackTrace();
            } catch (JSONException e) {
                e.printStackTrace();
            }
            return null;
        }

        @Override
        protected void onPostExecute(Void aVoid) {
            adapter = new ArrayAdapter(getActivity(), android.R.layout.simple_list_item_activated_1, allGoalsList);
            setListAdapter(adapter);
        }
    }
}

总是在 onPostExecute 中归咎于适配器

adapter = new ArrayAdapter(getActivity(), android.R.layout.simple_list_item_activated_1, allGoalsList);

09-21 18:07:56.920 1729-1729/im.upgrade.upgradeim D/AndroidRuntime: Shutting down VM 09-21 18:07:56.920 1729-1729/im.upgrade.upgradeim W/dalvikvm: threadid=1: thread exiting with uncaught exception (group=0xb2cd8908) 09-21 18:07:56.920 1729-1729/im.upgrade.upgradeim E/AndroidRuntime: FATAL EXCEPTION: main 09-21 18:07:56.920 1729-1729/im.upgrade.upgradeim E/AndroidRuntime: java.lang.NullPointerException 09-21 18:07:56.920 1729-1729/im.upgrade.upgradeim E/AndroidRuntime: at android.widget.ArrayAdapter.init(ArrayAdapter.java:310) 09-21 18:07:56.920 1729-1729/im.upgrade.upgradeim E/AndroidRuntime: at android.widget.ArrayAdapter.(ArrayAdapter.java:153)

09-21 18:07:56.920 1729-1729/im.upgrade.upgradeim E/AndroidRuntime: at fragments.GoalsListFragment$GetAllGoals.onPostExecute(GoalsListFragment.java:122) 09-21 18:07:56.920 1729-1729/im.upgrade.upgradeim E/AndroidRuntime: at fragments.GoalsListFragment$GetAllGoals.onPostExecute(GoalsListFragment.java:63)

09-21 18:07:56.920 1729-1729/im.upgrade.upgradeim E/AndroidRuntime: at android.os.AsyncTask.finish(AsyncTask.java:631) 09-21 18:07:56.920 1729-1729/im.upgrade.upgradeim E/AndroidRuntime: at android.os.AsyncTask.access$600(AsyncTask.java:177) 09-21 18:07:56.920 1729-1729/im.upgrade.upgradeim E/AndroidRuntime: at android.os.AsyncTask$InternalHandler.handleMessage(AsyncTask.java:644) 09-21 18:07:56.920 1729-1729/im.upgrade.upgradeim E/AndroidRuntime: at android.os.Handler.dispatchMessage(Handler.java:99) 09-21 18:07:56.920 1729-1729/im.upgrade.upgradeim E/AndroidRuntime: at android.os.Looper.loop(Looper.java:137) 09-21 18:07:56.920 1729-1729/im.upgrade.upgradeim E/AndroidRuntime: at android.app.ActivityThread.main(ActivityThread.java:5041) 09-21 18:07:56.920 1729-1729/im.upgrade.upgradeim E/AndroidRuntime: at java.lang.reflect.Method.invokeNative(Native Method) 09-21 18:07:56.920 1729-1729/im.upgrade.upgradeim E/AndroidRuntime: at java.lang.reflect.Method.invoke(Method.java:511) 09-21 18:07:56.920 1729-1729/im.upgrade.upgradeim E/AndroidRuntime: at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:793) 09-21 18:07:56.920 1729-1729/im.upgrade.upgradeim E/AndroidRuntime: at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:560) 09-21 18:07:56.920 1729-1729/im.upgrade.upgradeim E/AndroidRuntime: at dalvik.system.NativeStart.main(Native Method)

Answer 1

尝试记录以查看 getActivity() 是否为 null 或 allGoalList

我的猜测是该 fragment 正在被处理，其中之一为空。

你应该每次在你的应用程序中检查非空值试试这个

 @Override
    protected void onPostExecute(Void aVoid) 
    {
        if(null != getActivity() )
        {
           if(null != allGoalsList){ // log to find error}
           adapter = new ArrayAdapter(getActivity(), android.R.layout.simple_list_item_activated_1, allGoalsList);
           setListAdapter(adapter);
         }
    }