java - 将 ListView 添加到 Fragment 时发生崩溃

Question

我正在尝试将整个 ListFragment 类转换为添加了 ListView 的 Fragment 类。

将此类命名为 HomeworkListFragment。我将扩展名从 ListFragment 更改为 Fragment:

因此我创建了以下内容:

public class HomeworkListFragment extends Fragment{

public static final String TAG = "HomeworkListFragment";

public ListView mListView;
public HomeworkAdapter mAdapter;

private ArrayList<HomeworkObject> mCrimes;


@Override
public void onCreate(Bundle savedInstanceState){
    super.onCreate(savedInstanceState);
    setHasOptionsMenu(true);

    getActivity().setTitle(R.string.homework_title);
    mCrimes = HomeworkLab.get(getActivity()).getHomework();

    mAdapter = new HomeworkAdapter(mCrimes);
    mListView.setAdapter(mAdapter);

    setRetainInstance(true);

}

@TargetApi(11)
@Override
public View onCreateView(LayoutInflater inflater, ViewGroup parent, Bundle savedInstanceState){
    View v = super.onCreateView(inflater,parent,savedInstanceState);

    mListView = (ListView)v.findViewById(android.R.id.list);
    mListView.setChoiceMode(ListView.CHOICE_MODE_MULTIPLE_MODAL);

    mListView.setMultiChoiceModeListener(new MultiChoiceModeListener(){
        @Override
        public boolean onActionItemClicked(ActionMode mode, MenuItem item) {
            switch (item.getItemId()){
                case R.id.menu_item_delete_homework:
                    HomeworkLab homeworkLab = HomeworkLab.get(getActivity());
                    for (int i = mAdapter.getCount() - 1;i>=0;i--){
                        //if (mListView.getListView().isItemChecked(i)){
                            //homeworkLab.deleteHomework(mAdapter.getItem(i));
                        //}
                    }
                    mode.finish();
                    mAdapter.notifyDataSetChanged();
                    return true;
                default:
                    return false;
            }
        }

        @Override
        public boolean onCreateActionMode(ActionMode mode, Menu menu) {
            //ActionMode.callback Methods
            MenuInflater inflater = mode.getMenuInflater();
            inflater.inflate(R.menu.homework_list_item_context, menu);
            return true;
        }

        @Override
        public void onDestroyActionMode(ActionMode arg0) {
            //not used here
        }

        @Override
        public boolean onPrepareActionMode(ActionMode arg0, Menu arg1) {
            //not used here
            return false;
        }

        @Override
        public void onItemCheckedStateChanged(ActionMode arg0,
                int arg1, long arg2, boolean arg3) {
            //not used here
        }

    });
    mListView.setOnItemClickListener(new AdapterView.OnItemClickListener() {

        @Override
        public void onItemClick(AdapterView<?> parent, View view, int position,
                long id) {
            HomeworkObject c = mAdapter.getItem(position);
            //Start CrimeActivity
            //Intent i = new Intent(getActivity(), CrimeActivity.class);
            Intent i = new Intent(getActivity(), HomeworkPagerActivity.class);
            i.putExtra(HomeworkFragment.EXTRA_HOMEWORK_ID, c.getId());
            startActivity(i);               
        }

    });     
    return v;
}

@Override
public void onCreateOptionsMenu(Menu menu, MenuInflater inflater){
    super.onCreateOptionsMenu(menu,inflater);
    inflater.inflate(R.menu.fragment_homework_list, menu);
}

@TargetApi(11)
@Override
public boolean onOptionsItemSelected(MenuItem item){
    switch(item.getItemId()){
        case R.id.menu_item_new_homework:
            HomeworkObject homework = new HomeworkObject();
            homework.setTitle("");
            HomeworkLab.get(getActivity()).addHomework(homework);
            Intent i = new Intent(getActivity(),HomeworkPagerActivity.class);
            i.putExtra(HomeworkFragment.EXTRA_HOMEWORK_ID, homework.getId());
            startActivityForResult(i,0);
            return true;
        default:
            return super.onOptionsItemSelected(item);
    }
}

@Override
public void onCreateContextMenu(ContextMenu menu, View v, ContextMenuInfo menuInfo) {
    getActivity().getMenuInflater().inflate(R.menu.homework_list_item_context, menu);
}

@Override
public boolean onContextItemSelected(MenuItem item) {
    AdapterContextMenuInfo info = (AdapterContextMenuInfo)item.getMenuInfo();
    int position = info.position;
    HomeworkObject h = mAdapter.getItem(position);

    switch (item.getItemId()) {
        case R.id.menu_item_delete_homework:
            HomeworkLab.get(getActivity()).deleteHomework(h);
            mAdapter.notifyDataSetChanged();
            return true;
    }
    return super.onContextItemSelected(item);
}


@Override
public void onResume() {
    super.onResume();
    mAdapter.notifyDataSetChanged();
    mCrimes = HomeworkLab.get(getActivity()).getHomework();//update list
}

@Override
public void onPause(){
    super.onPause();
    HomeworkLab.get(getActivity()).saveHomework();
}

/*@Override
public void onListItemClick(ListView l, View v, int position, long id){
    HomeworkObject c = ((HomeworkAdapter)getListAdapter()).getItem(position);
    //Start CrimeActivity
    //Intent i = new Intent(getActivity(), CrimeActivity.class);
    Intent i = new Intent(getActivity(), HomeworkPagerActivity.class);
    i.putExtra(HomeworkFragment.EXTRA_HOMEWORK_ID, c.getId());
    startActivity(i);
}*/

private class HomeworkAdapter extends ArrayAdapter<HomeworkObject>{
    public HomeworkAdapter(ArrayList<HomeworkObject> crimes){
        super(getActivity(), 0, crimes);
    }

    @SuppressLint("SimpleDateFormat")
    @Override
    public View getView(int position, View convertView, ViewGroup parent){
        if (convertView == null){
            convertView = getActivity().getLayoutInflater().inflate(R.layout.list_item_homework, null);
        }
        HomeworkObject c = getItem(position);

        TextView titleTextView = (TextView)convertView.findViewById(R.id.homework_list_item_titleTextView);
        titleTextView.setText(c.getTitle());
        TextView dateTextView = (TextView)convertView.findViewById(R.id.homework_list_item_dateTextView);

        DateFormat df = new SimpleDateFormat("EEEE, d MMMM yyyy");     
        String reportDate = df.format(c.getDate());
        dateTextView.setText(reportDate);
        //dateTextView.setText(c.getDate().toString());

        CheckBox solvedCheckBox = (CheckBox)convertView.findViewById(R.id.homework_list_item_solvedCheckBox);
        solvedCheckBox.setChecked(c.isSolved());

        return convertView;
    }
}

}

据我所知，onCreateView 方法中存在一个错误，其中包含一些空指针错误，但我无法弄清楚这个问题。

我最近从 iOS 迁移到 Android，所以对此还很陌生。任何解释都会有很大帮助!

Answer 1

问题:

 mListView = (ListView)v.findViewById(android.R.id.list);

android.R.id.list 仅绑定(bind)到 ListFragment，不在 Fragment 中，这就是它为 null 的原因。

作为documentation对于ListFragment:

ListFragment has a default layout that consists of a single list view. 
However, if you desire, you can customize the fragment layout by returning 
your own view hierarchy from onCreateView(LayoutInflater, ViewGroup, Bundle). 
To do this, your view hierarchy must contain a ListView object with the 
id "@android:id/list" (or list if it's in code)

解决方案:

您需要在 fragment 中膨胀 View 并使用 ListView 创建布局，并使用 ListView 创建的 ID 而不是 android.R.id.list