java - 即使已填充， fragment 中的 RecyclerView 仍保持空白

Question

我在 Android 上使用 RecyclerView 时遇到以下问题:

每当我尝试使用 RecyclerView 打开 fragment 时，它都会保持空白。我的 ArrayList mList 不是空的，它包含数据(我测试过)。

我尝试了很多方法来解决这个问题，但我是初学者，所以我真的不知道问题出在哪里。

RecyclerView 应显示产品列表。我认为我的适配器有问题，或者 xml 文件fragment_list 可能没有正确显示？我真的不知道。

我的ListAdaper类:

import android.content.Context;
import android.view.LayoutInflater;
import android.view.View;
import android.view.ViewGroup;
import android.widget.TextView;
import androidx.annotation.NonNull;
import androidx.cardview.widget.CardView;
import androidx.recyclerview.widget.RecyclerView;
import java.util.ArrayList;

public class ListAdapter extends 
RecyclerView.Adapter<ListAdapter.ListViewHolder> {

private Context mContext;
private ArrayList<Item> mList;

public ListAdapter(Context context, ArrayList<Item> list) {

    mContext = context;
    mList = list;

}

@NonNull
@Override
public ListViewHolder onCreateViewHolder(@NonNull ViewGroup parent, int viewType) {
    View v = LayoutInflater.from(mContext).inflate(R.layout.list_item, parent, false);
    return new ListViewHolder(v);
}

@Override
public void onBindViewHolder(@NonNull ListViewHolder holder, int position) {
    Item currentItem = mList.get(position);

    holder.textViewName.setText(currentItem.getName());
    holder.textViewRegion.setText(currentItem.getRegion());
}

@Override
public void onAttachedToRecyclerView(@NonNull RecyclerView recyclerView) {
    super.onAttachedToRecyclerView(recyclerView);
}

public static class ListViewHolder extends RecyclerView.ViewHolder {

    public CardView cardView;
    public TextView textViewName;
    public TextView textViewRegion;

    public ListViewHolder(@NonNull View itemView) {
        super(itemView);
        cardView = itemView.findViewById(R.id.cardview);
        textViewName = itemView.findViewById(R.id.item_name);
        textViewRegion = itemView.findViewById(R.id.item_region);
    }
}

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

...我的 ListFragment 类:

import android.content.Context;
import android.os.Bundle;
import android.view.LayoutInflater;
import android.view.View;
import android.view.ViewGroup;
import androidx.annotation.NonNull;
import androidx.annotation.Nullable;
import androidx.fragment.app.Fragment;
import androidx.recyclerview.widget.DefaultItemAnimator;
import androidx.recyclerview.widget.LinearLayoutManager;
import androidx.recyclerview.widget.RecyclerView;
import com.android.volley.RequestQueue;
import com.android.volley.Response;
import com.android.volley.VolleyError;
import com.android.volley.toolbox.JsonObjectRequest;
import com.android.volley.toolbox.Volley;
import org.json.JSONException;
import org.json.JSONObject;
import java.util.ArrayList;
import java.util.Iterator;


public class ListFragment extends Fragment {

private RecyclerView mRecyclerView;
private ListAdapter mListAdapter;
private ArrayList<Item> mList;
private RequestQueue mRequestQueue;
private Context mContext;


@Nullable
@Override
public View onCreateView(@NonNull LayoutInflater inflater, @Nullable ViewGroup container, @Nullable Bundle savedInstanceState) {
    View rootview = inflater.inflate(R.layout.fragment_list, container, false);

    mRecyclerView = rootview.findViewById(R.id.recycler_view);
    mRecyclerView.setLayoutManager(new LinearLayoutManager(getActivity()));
    mRecyclerView.setHasFixedSize(true);
    parseJSON();
    mRecyclerView.setAdapter(mListAdapter);
    mRecyclerView.setItemAnimator(new DefaultItemAnimator());
    return rootview;
}

@Override
public void onViewCreated(@NonNull View view, @Nullable Bundle savedInstanceState) {
    super.onViewCreated(view, savedInstanceState);
}

@Override
public void onActivityCreated(@Nullable Bundle savedInstanceState) {
    super.onActivityCreated(savedInstanceState);
}

@Override
public void onAttach(Context context) {
    super.onAttach(context);
    mContext = context;
}

@Override
public void onCreate(@Nullable Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    mList = new ArrayList<>();
    mListAdapter = new ListAdapter(mContext, mList);
    mRequestQueue = Volley.newRequestQueue(mContext);

}


private void parseJSON() {

    JsonObjectRequest request = new JsonObjectRequest("http://my_ip/sestavsisvujsvetweb/api/seznammagnetek", null, new Response.Listener<JSONObject>() {

        @Override
        public void onResponse(JSONObject response) {
            JSONObject magnetky = response;
            try {

                Iterator keys = magnetky.keys();
                while (keys.hasNext()) {
                    Object key = keys.next();
                    JSONObject value = magnetky.getJSONObject((String) key);
                    String monumentnumber = value.getString("monumentid");
                    String monumentname = value.getString("name");
                    String monumentregion = value.getString("region");
                    mList.add(new Item(monumentnumber, monumentname, monumentregion));
                }

            } catch (JSONException e) {
                e.printStackTrace();
            }

        }
    }

            , new Response.ErrorListener() {
        @Override
        public void onErrorResponse(VolleyError error) {
            error.printStackTrace();
        }
    });
    if (request != null) {
        mRequestQueue.add(request);
    }

}

}

......以及我的 Item 模型类:

public class Item {

private String name;
private String number;
private String region;

public Item(String number, String name, String region) {
    this.name = name;
    this.number = number;
    this.region = region;
}
public String getName() {
    return name;
}
public String getRegion() {
    return region;
}
public String getNumber() { return number; }
}

感谢您的帮助，如果我没有包含重要的内容，请告诉我:)

更新:我尝试添加 mListAdapter.notifyDataSetChanged();它看起来很有效，但我在日志中收到此错误:

2019-08-21 19:46:16.947 5542-5542/com.example.app E/AndroidRuntime: FATAL EXCEPTION: main
Process: com.example.app, PID: 5542
java.lang.NullPointerException: Attempt to invoke virtual method 'void android.widget.TextView.setText(java.lang.CharSequence)' on a null object reference
    at com.example.app.ListAdapter.onBindViewHolder(ListAdapter.java:38)
    at com.example.app.ListAdapter.onBindViewHolder(ListAdapter.java:15)

........and some other stuff

Answer 1

您需要通知适配器有关数据集更改的信息。在onResponse()中调用以下代码

mListAdapter.notifyDataSetChanged();