[英]My RecyclerView in fragment stays blank even when it's populated
我在Android上使用RecyclerView存在以下問題:
每當我嘗試使用RecyclerView打開片段時,它都保持空白。 我的ArrayList mList不為空,它包含數據(我對其進行了測試)。
我嘗試了很多方法來解決此問題,但是我是一個初學者,所以我真的不知道問題出在哪里。
RecyclerView應該顯示產品列表。 我認為我的適配器有問題,或者xml文件fragment_list無法正確顯示? 我真的不知道
我的ListAdaper類:
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類:
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);
}
}
}
.....和我的Item模型類:
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; }
}
感謝您的幫助,如果我沒有提供重要信息,請告訴我:)
更新:我嘗試添加mListAdapter.notifyDataSetChanged(); 似乎很有效,但我在日志中收到此錯誤:
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
刪除此行:
mRecyclerView.setHasFixedSize(true);
由於要異步加載數據,因此在設置適配器時,數據大小為0。 因為您將其設置為固定大小,所以RecyclerView不會在項目更改時更新其大小。
RecyclerView仍然可以基於其他因素(例如其父級的大小)來更改其大小,但是此大小計算不能取決於其子級的大小或適配器的內容(適配器中的項目數除外)。
您還可以在加載數據后設置適配器。 它也應該工作。
您需要通知適配器有關數據集更改的信息。請在onResponse()
中onResponse()
以下代碼
mListAdapter.notifyDataSetChanged();
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