使用 BaseAdapter 时片段 TabLayout 中的 NullPointerException

Posted 2023-05-07

【中文标题】使用 BaseAdapter 时片段 TabLayout 中的 NullPointerException

【英文标题】：NullPointerException in Fragment TabLayout when using BaseAdapter

【发布时间】：2020-02-23 14:21:56

【问题描述】：

我已经成功解析了一个带有后台 AsyncTask 的简单 rss 提要（XML 提要）。但无法使用 BaseAdapter (MyBaseAdapter) 将接收到的提要从 Fragment (FragmentTOI) 设置为 ListView。由于解析来自片段，我使用“getActivity()”而不是“MainActivity.this”。我可以看到从 rss xml 提要解析的 logcat 中的所有标题。附上代码 sn-ps 和 Logcat 截图。

代码：

    // Fragment
    public static class FragmentTOI extends Fragment 
        ListView listView;
        MyAsyncTask myAsyncTask;
        Context context;
        ArrayAdapter<String> arrayAdapter;

        public FragmentTOI(Context context) 
//            this.context = context;
        

        @Nullable
        @Override
        public View onCreateView(@NonNull LayoutInflater inflater, @Nullable ViewGroup container, @Nullable Bundle savedInstanceState) 
            super.onCreateView (inflater, container, savedInstanceState);
            View view = inflater.inflate (R.layout.fragment_toi, container, false);
            listView = view.findViewById (R.id.list_view_toi);
            startTask ();
            return view;
        

        // Method to start the AsyncTask with new one if null.
        public void startTask() 
            //if the download is empty then cancel, else start the task
            if (myAsyncTask != null) 
                myAsyncTask.cancel (true);
             else 
                myAsyncTask = new MyAsyncTask (getActivity ());
                myAsyncTask.execute ();
            
        

        // Method to receive data from onPostExecute of AsyncTask.
        public void receiveData(ArrayList<HashMap<String, String>> results) 
            Log.d ("XML", "FragmentTOI receiveData Method: " + results);
            listView.setAdapter (new MyBaseAdapter (context, results));
            listView.setAdapter (arrayAdapter);
        
    

    // AsyncTask downloader
    public static class MyAsyncTask extends AsyncTask<Void, Void, ArrayList<HashMap<String, String>>> 

        @Override
        protected void onPostExecute(ArrayList<HashMap<String, String>> hashMaps) 
            super.onPostExecute (hashMaps);
            fragmentTOI.receiveData (hashMaps);
        
    

    public static class MyBaseAdapter extends BaseAdapter 
        ArrayList<HashMap<String, String>> data;
        LayoutInflater layoutInflater;

        public MyBaseAdapter(Context context, ArrayList<HashMap<String, String>> data) 
            this.data = data;
            layoutInflater = (LayoutInflater) context.getSystemService (Context.LAYOUT_INFLATER_SERVICE);
        

        @Override
        public int getCount() 
            return data.size ();
        

        @Override
        public Object getItem(int i) 
            return data.get (i);
        

        @Override
        public long getItemId(int i) 
            return i;
        

        @Override
        public View getView(int i, View view, ViewGroup viewGroup) 
            View view1 = view;
            MyRowHolder myRowHolder = null;
            if (view1 == null) 
                view1 = layoutInflater.inflate (R.layout.single_row, viewGroup, false);
                myRowHolder = new MyRowHolder (view1);
                view1.setTag (myRowHolder);
             else 
                myRowHolder = (MyRowHolder) view1.getTag ();
            
            HashMap<String, String> currentItem = data.get (i);
            myRowHolder.textViewTitle.setText (currentItem.get ("title"));
            return view1;
        
    

    static class MyRowHolder 
        TextView textViewTitle;

        public MyRowHolder(View view) 
            textViewTitle = view.findViewById (R.id.text_view_title);
        
    

Logcat：

【参考方案1】：

当您将适配器设置为您的列表视图时，请使用context，如下所示。

// Method to receive data from onPostExecute of AsyncTask.
public void receiveData(ArrayList<HashMap<String, String>> results) 
    //listView.setAdapter (new MyBaseAdapter (getActivity (), results)); // here you got exception.

    // use context as you initialize it inside the constructor of your fragment.
    listView.setAdapter (new MyBaseAdapter (context, results));
    listView.setAdapter (arrayAdapter);

【讨论】：

请将您的activity 代码贴在您拨打FragmentTOI 的位置。

【参考方案2】：

选项 - 1：

试试requireActivity() 或requireContext()。

而不是

listView.setAdapter (new MyBaseAdapter (getActivity (), results));

使用

listView.setAdapter (new MyBaseAdapter (requireActivity(), results));

选项 - 2： 无需在 Adapter 中传递Context 即可获取LayoutInflater。删除这个：

public MyBaseAdapter(Context context, ArrayList<HashMap<String, String>> data) 
    this.data = data;
    //layoutInflater = (LayoutInflater) context.getSystemService (Context.LAYOUT_INFLATER_SERVICE);

在getView 里面试试这样：

if (view1 == null) 
    view1 = LayoutInflater.from(viewGroup.getContext()).inflate (R.layout.single_row, viewGroup, false);
    myRowHolder = new MyRowHolder (view1);
    view1.setTag (myRowHolder);

【讨论】：

为什么你的片段static 以及为什么你通过Context 来调用片段？如果您需要传递Context，请使用它而不是getActivity()。不是吗？

在我的回答中尝试使用 option-2，然后告诉我

当我删除 static 时发生此异常：java.lang.IllegalStateException: Fragment com.shiva.pillu.headlinessample.MainActivity.FragmentTOI must be a public static class to be properly recreated from instance state. 当我尝试选项 2 时发生此错误：java.lang.IllegalStateException: Fragment FragmentTOIc84840f not attached to an activity.

不要在 AsyncTask 中使用 fragmentTOI = new FragmentTOI (context);，而是使用接口将数据返回到片段。您创建了一个不附加到活动的 FragmentTOI 的新实例

请进一步研究并重构您的代码库。它充满了问题。