java - ViewHolder 字段在 Fragment 中被初始化为 null

Question

我正在尝试使用许多“grid_cell”xml 布局在回收站视图中设置 gridLayout，每个布局包含 5 个 ImageView。在我重组代码以适应数据库之前，我的 recyclerView 工作正常。

问题是所有的 ImageViews 在 MapViewHolder 构造函数中都被初始化为空，导致java.lang.NullPointerException: Attempt to invoke virtual method 'void android.widget.ImageView.setImageResource(int)空对象引用”

public class MapFragment extends Fragment
{
    private GameDataStorage gameDataStorage;
    private RecyclerView rv;
    private MapAdapter adapter;
    private GridLayoutManager gridLayoutManager;

    public void onCreate(Bundle savedInstanceState)
    {
        super.onCreate(savedInstanceState);
        gameDataStorage = ((SubApplication)getActivity().getApplication()).gameDataStorage;
    }

    @Override
    public View onCreateView(LayoutInflater inflater, ViewGroup parent, Bundle bundle)
    {
        //Inflate the view from fragment_map.xml layout.
        View view = inflater.inflate(R.layout.fragment_map, parent, false);

        //Crate the recycler view from the map_recycler_view.xml layout
        rv = view.findViewById(R.id.map_recycler_view);

        adapter = new MapAdapter();

        //Initialise the grid layout manager
        gridLayoutManager = new GridLayoutManager(getActivity(), GameData.getInstance().getSettings().getMapHeight(), GridLayoutManager.HORIZONTAL, false);

        //Set the recycler view adapter
        rv.setAdapter(adapter);

        //Set the layout manager to be a horizontal scrolling grid
        rv.setLayoutManager(gridLayoutManager);

        return view;
    }

    public class MapViewHolder extends RecyclerView.ViewHolder
    {
        private ImageView structure;
        private ImageView topLeft;
        private ImageView topRight;
        private ImageView bottomLeft;
        private ImageView bottomRight;

        public MapViewHolder(LayoutInflater li, ViewGroup parent)
        {
            super(li.inflate(R.layout.grid_cell, parent, false));

            int size = parent.getMeasuredHeight() / GameData.getInstance().getSettings().getMapHeight() + 1;
            ViewGroup.LayoutParams lp = parent.getLayoutParams();
            lp.height = size;
            lp.width = size;

            structure = (ImageView) parent.findViewById(R.id.structure);
            topLeft = (ImageView) parent.findViewById(R.id.top_left);
            topRight = (ImageView) parent.findViewById(R.id.top_right);
            bottomLeft = (ImageView) parent.findViewById(R.id.bottom_left);
            bottomRight = (ImageView) parent.findViewById(R.id.bottom_right);

            //DEBUGGING
            Log.d("GAME_DATA", "structure reference is initialised as " + structure);
            Log.d("GAME_DATA", "topLeft reference is initialised as " + topLeft);
            Log.d("GAME_DATA", "topRight reference is initialised as " + topRight);
            Log.d("GAME_DATA", "bottomLeft reference is initialised as " + bottomLeft);
            Log.d("GAME_DATA", "bottomRight reference is initialised as " + bottomRight);
        }

        public void bind(final MapElement element, final int index)
        {
            if(element.getStructure() != null){
                structure.setImageResource(element.getStructure().getImageId());
            }
            else
            {
                if(structure == null)
                {
                    Log.d("GAME_DATA", "structure reference is null for mapElement " + index);
                }

                structure.setImageResource(GameData.getInstance().getEmpty());
            }
            topLeft.setImageResource(element.getTopLeft());
            topRight.setImageResource(element.getTopRight());
            bottomLeft.setImageResource(element.getBottomLeft());
            bottomRight.setImageResource(element.getBottomRight());

            structure.setOnClickListener(new View.OnClickListener() {
                @Override
                public void onClick(View v)
                {
                    int row = index % GameData.getInstance().getSettings().getMapHeight();
                    int col = index / GameData.getInstance().getSettings().getMapHeight();
                    StructureData.setStructureAt(row, col);

                    gameDataStorage.updateMapElement(element, element.getStructure(), index);

                    adapter.notifyItemChanged(index);
                }
            });

            structure.setOnLongClickListener(new View.OnLongClickListener() {
                @Override
                public boolean onLongClick(View v) {
                    return false;
                }
            });
        }
    }
    // END_INCLUDE(recyclerViewSampleViewHolder)

    public class MapAdapter extends RecyclerView.Adapter<MapViewHolder>
    {
        // BEGIN_INCLUDE(recyclerViewOnCreateViewHolder)
        // Create new views (invoked by the layout manager)
        @Override
        public MapViewHolder onCreateViewHolder(ViewGroup container, int viewType)
        {
            Log.d("GAME_DATA", "new MapViewHolder using ViewGroup " + container + " and viewType " + viewType);
            return new MapViewHolder(LayoutInflater.from(getActivity()), container);
        }
        // END_INCLUDE(recyclerViewOnCreateViewHolder)

        // BEGIN_INCLUDE(recyclerViewOnBindViewHolder)
        // Replace the contents of a view (invoked by the layout manager)
        @Override
        public void onBindViewHolder(MapViewHolder vh, int index) {
            // Get element from your dataset at this position and replace the contents of the view
            // with that element
            int row = index % GameData.getInstance().getSettings().getMapHeight();
            int col = index / GameData.getInstance().getSettings().getMapHeight();
            MapElement element = GameData.getInstance().getMapLoc(row, col);

            vh.bind(element, index);
        }
        // END_INCLUDE(recyclerViewOnBindViewHolder)



        // Return the size of your dataset (invoked by the layout manager)
        @Override
        public int getItemCount() {
            int rows = GameData.getInstance().getMap().length;
            int cols = GameData.getInstance().getMap()[0].length;
            int length = rows * cols;
            Log.d("GAME_DATA", "getItemCount returns: " + length);
            return length;
        }
    }
}

日志输出：

10-22 19:31:59.177 19926-19926/com.example.mad_assignment_rebuild D/GAME_DATA: 使用 ViewGroup 的新 MapViewHolder androidx.recyclerview.widget.RecyclerView{353fb6f6 VFED.... ......ID 0,0- 1080,1188 #7f07005e app:id/map_recycler_view} 和 viewType 0 10-22 19:31:59.187 19926-19926/com.example.mad_assignment_rebuild D/GAME_DATA：结构引用初始化为空 10-22 19:31:59.187 19926 -19926/com.example.mad_assignment_rebuild D/GAME_DATA：topLeft 参考初始化为空 10-22 19:31:59.187 19926-19926/com.example.mad_assignment_rebuild D/GAME_DATA：topRight 参考初始化为空 10-22 19： 31:59.187 19926-19926/com.example.mad_assignment_rebuild D/GAME_DATA:bottomLeft 参考初始化为空 10-22 19:31:59.187 19926-19926/com.example.mad_assignment_rebuild D/GAME_DATA:bottomRight 引用初始化为 null 10-22 19:31:59.187 19926-19926/com.example.mad_assignment_rebuild D/GAME_DATA：mapElement 0 的结构引用为 null

调试错误输出：

E/AndroidRuntime：致命异常：主进程：com.example.mad_assignment_rebuild，PID：19926 java.lang.NullPointerException：尝试在 com 的空对象引用上调用虚拟方法 'void android.widget.ImageView.setImageResource(int)' .example.mad_assignment_rebuild.MapRecyclerFiles.MapFragment$MapViewHolder.bind(MapFragment.java:101) 在 com.example.mad_assignment_rebuild.MapRecyclerFiles.MapFragment$MapAdapter.onBindViewHolder(MapFragment.java:154) 在 com.example.mad_assignment_rebuild.MapRecyclerFiles.MapFragment $MapAdapter.onBindViewHolder(MapFragment.java:132)

标签： javaandroidandroid-fragments

Answer 1

我认为有一个问题是你想从视图中找到你的观点parent。您应该从您的视图中找到您的item layout视图（在这种情况下R.layout.grid_cell）。

像这样试试。

public MapViewHolder(LayoutInflater li, ViewGroup parent)
    {
        final View = li.inflate(R.layout.grid_cell, parent, false);
        super(view);

        int size = view.getMeasuredHeight() / GameData.getInstance().getSettings().getMapHeight() + 1;
        ViewGroup.LayoutParams lp = view.getLayoutParams();
        lp.height = size;
        lp.width = size;

        structure = (ImageView) view.findViewById(R.id.structure);
        topLeft = (ImageView) view.findViewById(R.id.top_left);
        topRight = (ImageView) view.findViewById(R.id.top_right);
        bottomLeft = (ImageView) view.findViewById(R.id.bottom_left);
        bottomRight = (ImageView) view.findViewById(R.id.bottom_right);

        //DEBUGGING
        Log.d("GAME_DATA", "structure reference is initialised as " + structure);
        Log.d("GAME_DATA", "topLeft reference is initialised as " + topLeft);
        Log.d("GAME_DATA", "topRight reference is initialised as " + topRight);
        Log.d("GAME_DATA", "bottomLeft reference is initialised as " + bottomLeft);
        Log.d("GAME_DATA", "bottomRight reference is initialised as " + bottomRight);
    }

或者你可以做的更清楚更简单，，，像这样。

    public class RecyclerViewAdapter extends RecyclerView.Adapter<RecyclerViewAdapter.Holder> {
    private final List<Model> list;

    RecyclerViewAdapter(List<WorkshopObj> data, EditTextChangeWorkshopListener editTextChangeWorkshopListener) {
        list = data;

        setHasStableIds(true);
    }

    @NotNull
    @Override
    public Holder onCreateViewHolder(ViewGroup parent, int viewType) {
        return new Holder(LayoutInflater.from(parent.getContext()).inflate(R.layout.list_item_workshop_adapter, parent, false));
    }

    @Override
    public void onBindViewHolder(final Holder holder, final int position) {

        final WorkshopObj model = list.get(position);

        holder.titleWorkshop.setText(model.getTitleText());
        holder.requiredWorkshop.setVisibility(model.isRequired() ? View.VISIBLE : View.GONE);
        holder.subTitle.setVisibility(model.isMandatory() ? View.VISIBLE : View.GONE);

    }

    @Override
    public int getItemCount() {
        return list.size();
    }

    class Holder extends RecyclerView.ViewHolder {

        private final TextView requiredWorkshop;
        private final TextView titleWorkshop;
        private final TextView subTitle;
        private final EditText price;

        Holder(View view) {
            super(view);
            requiredWorkshop = view.findViewById(R.id.required_workshop);
            titleWorkshop = view.findViewById(R.id.title_workshop);
            subTitle = view.findViewById(R.id.sub_title);
            price = view.findViewById(R.id.price);
        }
    }
}