在 Fragment 中显示 RecyclerView

Question

我正在 Android Lollipop 中试用新的 RecyclerView，但我卡住了。

我正在尝试接收一个列表，其中包含一个图标和一个 TextView 图标右侧的 Fragment 内。

I found this great tutorial 了解如何设置RecyclerView。我遵循了每一点，只更改了item_layout.xml 以满足我的需要。

项目构建时没有任何错误，但是当它在我的设备上启动时出现此错误：

java.lang.RuntimeException：无法启动活动 ComponentInfo{com.fredrikaldgard.materialcolors/com.fredrikaldgard.materialcolors.MainActivity}：java.lang.NullPointerException：尝试调用虚拟方法 'void android.support.v7.widget .RecyclerView.setLayoutManager(android.support.v7.widget.RecyclerView$LayoutManager)' 在空对象引用上

我尝试用谷歌搜索这个问题，但我对 Android 开发非常业余。

这是我的MainActivity：

@Override
protected void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    setContentView(R.layout.activity_main);

    // 1. get a reference to recyclerView
    RecyclerView recyclerView = (RecyclerView) findViewById(R.id.list);

    // 2. set layoutManger
    recyclerView.setLayoutManager(new LinearLayoutManager(this));

    // this is data fro recycler view
    ItemData itemsData[] = { new ItemData("Indigo",R.drawable.circle),
            new ItemData("Red",R.drawable.color_ic_launcher),
            new ItemData("Blue",R.drawable.indigo),
            new ItemData("Green",R.drawable.circle),
            new ItemData("Amber",R.drawable.color_ic_launcher),
            new ItemData("Deep Orange",R.drawable.indigo)};


    // 3. create an adapter
    MyAdapter mAdapter = new MyAdapter(itemsData);
    // 4. set adapter
    recyclerView.setAdapter(mAdapter);
    // 5. set item animator to DefaultAnimator
    recyclerView.setItemAnimator(new DefaultItemAnimator());

还有我的MyAdapter：

public class MyAdapter extends RecyclerView.Adapter<MyAdapter.ViewHolder> {
private ItemData[] itemsData;

public MyAdapter(ItemData[] itemsData) {
    this.itemsData = itemsData;
}

// Create new views (invoked by the layout manager)
@Override
public MyAdapter.ViewHolder onCreateViewHolder(ViewGroup parent,
                                               int viewType) {
    // create a new view
    View itemLayoutView = LayoutInflater.from(parent.getContext())
            .inflate(R.layout.item_layout, null);

    // create ViewHolder

    ViewHolder viewHolder = new ViewHolder(itemLayoutView);
    return viewHolder;
}

// Replace the contents of a view (invoked by the layout manager)
@Override
public void onBindViewHolder(ViewHolder viewHolder, int position) {

    // - get data from your itemsData at this position
    // - replace the contents of the view with that itemsData

    viewHolder.txtViewTitle.setText(itemsData[position].getTitle());
    viewHolder.imgViewIcon.setImageResource(itemsData[position].getImageUrl());


}

// inner class to hold a reference to each item of RecyclerView
public static class ViewHolder extends RecyclerView.ViewHolder {

    public TextView txtViewTitle;
    public ImageView imgViewIcon;

    public ViewHolder(View itemLayoutView) {
        super(itemLayoutView);
        txtViewTitle = (TextView) itemLayoutView.findViewById(R.id.item_title);
        imgViewIcon = (ImageView) itemLayoutView.findViewById(R.id.item_icon);
    }
}


// Return the size of your itemsData (invoked by the layout manager)
@Override
public int getItemCount() {
    return itemsData.length;
}
}

编辑：这是Fragment

public class ColorsFragment extends Fragment {

    public ColorsFragment(){}

    @Override
    public View onCreateView(LayoutInflater inflater, ViewGroup container,
                         Bundle savedInstanceState) {

        View rootView = inflater.inflate(R.layout.fragment_colors, container, false);

        return rootView;
    }
}

可能出了什么问题？

Answer 1

这是不久前提出的问题，但根据@nacho_zona3 提供的答案以及以前的片段经验，问题是在您尝试使用@987654321 查找视图时尚未创建视图onCreate() 中的 @ 方法来解决这个问题，移动以下代码：

// 1. get a reference to recyclerView
RecyclerView recyclerView = (RecyclerView) findViewById(R.id.list);

// 2. set layoutManger
recyclerView.setLayoutManager(new LinearLayoutManager(this));

// this is data from recycler view
ItemData itemsData[] = { new ItemData("Indigo",R.drawable.circle),
        new ItemData("Red",R.drawable.color_ic_launcher),
        new ItemData("Blue",R.drawable.indigo),
        new ItemData("Green",R.drawable.circle),
        new ItemData("Amber",R.drawable.color_ic_launcher),
        new ItemData("Deep Orange",R.drawable.indigo)};


// 3. create an adapter
MyAdapter mAdapter = new MyAdapter(itemsData);
// 4. set adapter
recyclerView.setAdapter(mAdapter);
// 5. set item animator to DefaultAnimator
recyclerView.setItemAnimator(new DefaultItemAnimator()); 

您的片段的onCreateView() 调用。需要进行少量重构，因为从此方法调用的所有变量和方法都必须是静态的。最终代码应如下所示：

 public class ColorsFragment extends Fragment {

     public ColorsFragment() {}

     @Override
     public View onCreateView(LayoutInflater inflater, ViewGroup container,
         Bundle savedInstanceState) {

         View rootView = inflater.inflate(R.layout.fragment_colors, container, false);
         // 1. get a reference to recyclerView
         RecyclerView recyclerView = (RecyclerView) rootView.findViewById(R.id.list);

         // 2. set layoutManger
         recyclerView.setLayoutManager(new LinearLayoutManager(getActivity()));

         // this is data from recycler view
         ItemData itemsData[] = {
             new ItemData("Indigo", R.drawable.circle),
                 new ItemData("Red", R.drawable.color_ic_launcher),
                 new ItemData("Blue", R.drawable.indigo),
                 new ItemData("Green", R.drawable.circle),
                 new ItemData("Amber", R.drawable.color_ic_launcher),
                 new ItemData("Deep Orange", R.drawable.indigo)
         };


         // 3. create an adapter
         MyAdapter mAdapter = new MyAdapter(itemsData);
         // 4. set adapter
         recyclerView.setAdapter(mAdapter);
         // 5. set item animator to DefaultAnimator
         recyclerView.setItemAnimator(new DefaultItemAnimator());

         return rootView;
     }
 }

所以这里的主要内容是，无论您在哪里调用findViewById()，都需要使用rootView.findViewById()

Answer 2

您应该在使用该视图膨胀核心视图后在Fragment 中检索RecyclerView。也许它找不到你的 recycler 因为它不是 Activity 的一部分

@Override
public View onCreateView(LayoutInflater inflater, ViewGroup container,
                             Bundle savedInstanceState) {
    final View view = inflater.inflate(R.layout.fragment_artist_tracks, container, false);
    final FragmentActivity c = getActivity();
    final RecyclerView recyclerView = (RecyclerView) view.findViewById(R.id.recyclerView);
    LinearLayoutManager layoutManager = new LinearLayoutManager(c);
    recyclerView.setLayoutManager(layoutManager);

    new Thread(new Runnable() {
        @Override
        public void run() {
            final RecyclerAdapter adapter = new RecyclerAdapter(c);
            c.runOnUiThread(new Runnable() {
                @Override
                public void run() {
                    recyclerView.setAdapter(adapter);
                }
            });
        }
    }).start();

    return view;
}

Answer 3

确保您有正确的布局，并且 RecyclerView id 在布局内。否则，您将收到此错误。我有同样的问题，然后我注意到布局错误。

    public class ColorsFragment extends Fragment {

         public ColorsFragment() {}

         @Override
         public View onCreateView(LayoutInflater inflater, ViewGroup container,
             Bundle savedInstanceState) {

==> make sure you are getting the correct layout here. R.layout...

             View rootView = inflater.inflate(R.layout.fragment_colors, container, false); 

Answer 4

我遇到了同样的问题。当我使用此代码调用上下文时得到了解决方案。我使用网格布局。如果你使用另一个，你可以改变。

   recyclerView.setLayoutManager(new GridLayoutManager(getActivity(),1));

如果您有要设置的适配器。所以你可以按照这个。只需调用 getContext

  adapter = new Adapter(getContext(), myModelList);

如果要显示 Toast，请使用上面相同的内容

   Toast.makeText(getContext(), "Error in "+e, Toast.LENGTH_SHORT).show();

希望这会奏效。

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