在 Android Studio 中使用 RecyclerView，但没有任何内容被打印出来。

huangapple
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英文:

Using RecyclerView in android studio but nothing is getting printed

问题

以下是已翻译的内容：

这是一个页面，我想在这里打印出从 Parse 仪表板中提取的所有用户名和价格：

  1. public class HomePage extends AppCompatActivity {
  3.     @Override
  4.     protected void onCreate(Bundle savedInstanceState) {
  5.         super.onCreate(savedInstanceState);
  6.         setContentView(R.layout.activity_home_page);
  7.         final RecyclerView list = (RecyclerView)findViewById(R.id.list);
  8.         list.setLayoutManager(new LinearLayoutManager(this));
  10.         final ArrayList<String> username = new ArrayList<String>();
  11.         final ArrayList<String> netPrice = new ArrayList<String>();
  13.         ParseQuery<ParseObject> query = new ParseQuery<ParseObject>("Image");
  14.         query.findInBackground(new FindCallback<ParseObject>() {
  15.             @Override
  16.             public void done(List<ParseObject> objects, ParseException e) {
  17.                 if (e == null) {
  18.                     if (objects.size() > 0) {
  19.                         for(ParseObject object : objects) {
  20.                             String user = object.getString("username");
  21.                             String price = object.getString("price");
  22.                             username.add(user);
  23.                             netPrice.add(price);
  24.                         }
  25.                     }
  26.                 } else {
  27.                     e.printStackTrace();
  28.                 }
  29.             }
  30.         });
  31.         list.setAdapter(new ListAdapter(username, netPrice));
  32.     }
  33. }

这是我的适配器，名为 ListAdapter：

  1. public class ListAdapter extends RecyclerView.Adapter {
  2.     private ArrayList<String> data1;
  3.     private ArrayList<String> data2;
  5.     public ListAdapter(ArrayList<String> data1, ArrayList<String> data2) {
  6.         this.data1 = data1;
  7.         this.data2 = data2;
  8.     }
  10.     @NonNull
  11.     @Override
  12.     public RecyclerView.ViewHolder onCreateViewHolder(@NonNull ViewGroup parent, int viewType) {
  13.         LayoutInflater inflater = LayoutInflater.from(parent.getContext());
  14.         View view = inflater.inflate(R.layout.list_layout, parent, false);
  15.         return new ListViewHolder(view);
  16.     }
  18.     @Override
  19.     public void onBindViewHolder(@NonNull RecyclerView.ViewHolder holder, int position) {
  20.         String title1 = data1.get(position);
  21.         final TextView viewById1 = holder.itemView.findViewById(R.id.text1);
  22.         viewById1.setText(title1);
  23.         String title2 = data2.get(position);
  24.         final TextView viewById2 = holder.itemView.findViewById(R.id.text2);
  25.         viewById2.setText(title2);
  26.     }
  28.     @Override
  29.     public int getItemCount() {
  30.         return data1.size();
  31.     }
  33.     public class ListViewHolder extends RecyclerView.ViewHolder {
  34.         public ListViewHolder(View itemView) {
  35.             super(itemView);
  36.         }
  37.     }
  38. }

虽然没有报错，但什么都没有被打印出来。我在适配器中传递了 ArrayList，这样做是允许的吗？请帮忙，我已经被困在这里尝试了 8 小时。

英文:

This is the page where i want to print all the usernames and price extracted from Parse dashboard

  1. public class HomePage extends AppCompatActivity {
  3.     @Override
  4.     protected void onCreate(Bundle savedInstanceState) {
  5.         super.onCreate(savedInstanceState);
  6.         setContentView(R.layout.activity_home_page);
  7.         final RecyclerView list=(RecyclerView)findViewById(R.id.list);
  8.         list.setLayoutManager(new LinearLayoutManager(this));
  10.         final ArrayList&lt;String&gt; username = new ArrayList&lt;String&gt;();
  11.         final ArrayList&lt;String&gt; netPrice = new ArrayList&lt;String&gt;();
  13.         ParseQuery&lt;ParseObject&gt; query=new ParseQuery&lt;ParseObject&gt;(&quot;Image&quot;);
  14.         query.findInBackground(new FindCallback&lt;ParseObject&gt;() {
  15.             @Override
  16.             public void done(List&lt;ParseObject&gt; objects, ParseException e) {
  17.                 if (e==null){
  18.                     if (objects.size()&gt;0) {
  19.                         for(ParseObject object : objects){
  20.                             String user=object.getString(&quot;username&quot;);
  21.                             String price=object.getString(&quot;price&quot;);
  22.                             username.add(user);
  23.                             netPrice.add(price);
  24.                         }
  25.                     }
  27.                 }
  28.                 else{
  29.                     e.printStackTrace();
  30.                 }
  32.             }
  33.         });
  34.         list.setAdapter(new ListAdapter(username,netPrice));
  35.     }
  37.    
  38. }

This is my Adapter named ListAdapter

  1. public class ListAdapter extends RecyclerView.Adapter {
  2.     private ArrayList&lt;String&gt;data1;
  3.     private ArrayList&lt;String&gt;data2;
  5.     public ListAdapter(ArrayList&lt;String&gt; data1, ArrayList&lt;String&gt; data2){
  6.         this.data1=data1;
  7.         this.data2=data2;
  9.     }
  10.     @NonNull
  11.     @Override
  12.     public RecyclerView.ViewHolder onCreateViewHolder(@NonNull ViewGroup parent, int viewType) {
  13.         LayoutInflater inflater=LayoutInflater.from(parent.getContext());
  14.         View view= inflater.inflate(R.layout.list_layout,parent,false);
  15.         return new ListViewHolder(view);
  16.     }
  18.     @Override
  19.     public void onBindViewHolder(@NonNull RecyclerView.ViewHolder holder, int position) {
  20.           String title1=data1.get(position);
  21.         final TextView viewById1 = holder.itemView.findViewById(R.id.text1);
  22.         viewById1.setText(title1);
  23.         String title2=data2.get(position);
  24.         final TextView viewById2 = holder.itemView.findViewById(R.id.text2);
  25.         viewById2.setText(title2);
  26.     }
  28.     @Override
  29.     public int getItemCount() {
  30.         return data1.size();
  31.     }
  33.     public class ListViewHolder extends RecyclerView.ViewHolder {
  34.         public ListViewHolder(View itemView) {
  35.             super(itemView);
  36.         }
  37.     }
  38.  }

This shows no error but nothing is getting printed.
I have passed ArrayList in my Adapter, am I allowed to that?
please help, I am struck badly trying to do this since 8 hrs.

答案1

得分: 1

  1. 你的项目数量应该与你想展示的项目大小相同
  3. @Override
  4. public int getItemCount() {
  5.     return 1; // 在这里返回项目的数量
  6. }
英文:

your item count should have the size of the items you want to show

  1. @Override
  2. public int getItemCount() {
  3.     return 1; // return here the number of items
  4. }

答案2

得分: 0

问题出在背景查询代码中。

它调用了 query.findInBackground，然后直接调用了 list.setAdapter(...)，而此时 [username, netPrice] 仍然是空列表。所以你需要等待，直到 done 方法完成，然后在 RecyclerView 中设置数据。

你可以这样做：

1 - 创建一个名为 setRecyclerViewData 的方法，用于设置 RecyclerView 的数据。

2 - 在 done 方法中的 for each 循环完成后调用 setRecyclerViewData，这样你就可以确保 [username, netPrice] 有一些数据。

  1. query.findInBackground(new FindCallback&lt;ParseObject&gt;() {
  2.     @Override
  3.     public void done(List&lt;ParseObject&gt; objects, ParseException e) {
  4.         if (e == null) {
  5.             if (objects.size() &gt; 0) {
  6.                 for (ParseObject object : objects) {
  7.                     String user = object.getString(&quot;username&quot;);
  8.                     String price = object.getString(&quot;price&quot;);
  9.                     username.add(user);
  10.                     netPrice.add(price);
  11.                 }
  12.                 setRecyclerViewData(username, netPrice);
  13.             } else {
  14.                 Log.d(&quot;Tag&quot;, &quot;Objects size = 0&quot;);
  15.             }
  16.         } else {
  17.             e.printStackTrace();
  18.         }
  19.     }
  20. });
  1. private void setRecyclerViewData(ArrayList&lt;String&gt; username, ArrayList&lt;String&gt; netPrice) {
  2.     list.setAdapter(new ListAdapter(username, netPrice));
  3. }

此外，通常在 RecyclerView 适配器中不会这样做，这样虽然不会显示错误，但不符合约定。

英文:

problem is with the background word.

It is calling query.findInBackground and directly after that list.setAdapter(...) which at this time, [username, netPrice] are still empty lists.
So you need to wait, until you done method is done and then set data in recycleView.

You can do that by:

1 - Create a method setRecyclerViewData which will set the recyclerview data.

2 - call setRecyclerViewData inside done after it finishes the for each loop
that way you will be sure that [username, netPrice] have some data.

  1. query.findInBackground(new FindCallback&lt;ParseObject&gt;() {
  2.     @Override
  3.     public void done(List&lt;ParseObject&gt; objects, ParseException e) {
  4.         if (e==null){
  5.             if (objects.size()&gt;0) {
  6.                 for(ParseObject object : objects){
  7.                     String user=object.getString(&quot;username&quot;);
  8.                     String price=object.getString(&quot;price&quot;);
  9.                     username.add(user);
  10.                     netPrice.add(price);
  11.                   }
  12.                   setRecyclerView(username, netPrice);
  13.              }
  14.              else{
  15.                  log.d(&quot;Tag&quot;, &quot;Objects size = 0&quot;);
  16.              }
  17.          }
  18.          else{
  19.              e.printStackTrace();
  20.          }
  21.     }
  22. });
  1. private void setRecyclerView(ArrayList&lt;String&gt; username, ArrayList&lt;String&gt; netPrice){
  2.     list.setAdapter(newListAdapter(username,netPrice));
  3. }

plus thing don't usually go this way in recylerview adapter, it is not showing error OK, but this is not the convention.

答案3

得分: 0

空白屏幕出现的原因是因为您的适配器中没有数据，在获取query.findInBackground中的数据后，您没有通知适配器有新数据。

所以，您需要做的是：

首先，将适配器实例存储到一个变量中：

  1. @Override
  2. protected void onCreate(Bundle savedInstanceState) {
  3.     ...
  4.     final ArrayList<String> username = new ArrayList<String>();
  5.     final ArrayList<String> netPrice = new ArrayList<String>();
  6.     ListAdapter listAdapter = new ListAdapter(username,netPrice); // 添加这行
  7.     ...
  8.     list.setAdapter(listAdapter);
  9. }

然后，在您的query.findInBackground中通知适配器有新数据：

  1. query.findInBackground(new FindCallback<ParseObject>() {
  2.     @Override
  3.     public void done(List<ParseObject> objects, ParseException e) {
  4.         if (e==null){
  5.             if (objects.size()>0) {
  6.                 for(ParseObject object : objects){
  7.                     String user=object.getString("username");
  8.                     String price=object.getString("price");
  9.                     username.add(user);
  10.                     netPrice.add(price);
  11.                 }
  12.                 listAdapter.notifyDataSetChanged(); // 添加这行
  13.             }
  15.         }
  16.         else{
  17.             e.printStackTrace();
  18.         }
  20.     }
  21. });
英文:

The reason you are seeing an empty screen is because your adapter contains no data, after getting the data in query.findInBackground, you don't notify the adapter that there is a new data.

So, what you need to do is

First, store your adapter instance into a variable

  1.     @Override
  2.     protected void onCreate(Bundle savedInstanceState) {
  3.         ...
  4.         final ArrayList&lt;String&gt; username = new ArrayList&lt;String&gt;();
  5.         final ArrayList&lt;String&gt; netPrice = new ArrayList&lt;String&gt;();
  6.         ListAdapter listAdapter = new ListAdapter(username,netPrice); // Add this line
  7.         ...
  8.         list.setAdapter(listAdapter);
  9.     }

Then, in your query.findInBackground notify the adapter that there is a new data

  1.         query.findInBackground(new FindCallback&lt;ParseObject&gt;() {
  2.             @Override
  3.             public void done(List&lt;ParseObject&gt; objects, ParseException e) {
  4.                 if (e==null){
  5.                     if (objects.size()&gt;0) {
  6.                         for(ParseObject object : objects){
  7.                             String user=object.getString(&quot;username&quot;);
  8.                             String price=object.getString(&quot;price&quot;);
  9.                             username.add(user);
  10.                             netPrice.add(price);
  11.                         }
  12.                         listAdapter.notifyDataSetChanged(); // Add this line
  13.                     }
  15.                 }
  16.                 else{
  17.                     e.printStackTrace();
  18.                 }
  20.             }
  21.         });

huangapple
  • 本文由 发表于 2020年7月27日 17:49:40
  • 转载请务必保留本文链接：https://java.coder-hub.com/63112743.html
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