将ListView添加到具有崩溃的片段

Question

I'm trying to convert my entire ListFragment class into a Fragment class with a ListView added to it.

Call this class HomeworkListFragment. I changed the extension from ListFragment to Fragment:

As a result I have created the following:

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;
    }
}

}

As far as I can tell there's an error in the onCreateView method with some null pointer error but I can't make heads or tails of this issue.

I recently picked up Android after moving from iOS so I'm pretty new to this. Any explanation could help a lot!

Answer 1

Use this instead:

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

    View v = inflater.inflate(R.layout.my_fragment_layout, parent, false);

    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;
}

Answer 2

problem:

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

The android.R.id.list is only bounded to the ListFragment not in Fragment and that is why it is null.

as the documentation for 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)

Solution:

You need to inflate a view in your fragment and create a layout with ListView and use the created ID of the ListView instead of android.R.id.list