使用 BaseAdapter 时片段 TabLayout 中的 NullPointerException

Question

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：

Answer 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);
}

Answer 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);
}