数组和适配器不能调用虚空片段

Posted 2023-04-15

【中文标题】数组和适配器不能调用虚空片段

【英文标题】：Array and Adapter cant called on Void Fragment

【发布时间】：2016-11-09 18:47:49

【问题描述】：

请帮助我。我的家庭片段是空白的。我无法将 JSON 数组打印到我的主片段。我希望 ListView 显示来自 API 的 Json 数组的数据，我如何使用片段放置来自 Json 的数组数据。对不起我的英语。

我正在使用片段：

public class HomeFragment extends Fragment implements OnFeedListener
ListView listView;
FeedAdapter adapter;
ArrayList<Post> posts;


 View myView;



 @Override

   public View onCreateView(LayoutInflater inflater,  ViewGroup container,  Bundle savedInstanceState) 
        myView = inflater.inflate(R.layout.home, container, false);
       return myView;
    
    @Override
    public void onActivityCreated(Bundle savedInstanceState) 
        super.onActivityCreated(savedInstanceState);
    

@Override
public void onFeed(JSONArray array) 
    posts = new ArrayList<>();
    int lenght = array.length();
    for(int i = 0; i < lenght; i++)
    
        JSONObject object = array.optJSONObject(i);
        Post post = new Post(object.optString("title"), object.optString("excerpt"), object.optString("thumbnail"));

        posts.addAll(posts);
    
    adapter.addAll(posts);
    adapter.notifyDataSetChanged();
    listView.setAdapter(adapter);
    FeedTask task = new FeedTask(this);
    task.execute("http://indo-coc.com/api/get_recent_posts/");



public class FeedTask extends AsyncTask<String, Void, JSONArray>

    private OnFeedListener listener;
    public FeedTask(OnFeedListener listener)
    
        this.listener = listener;
    
    @Override
    protected JSONArray doInBackground(String... params)
    
        String url = params[0];
        OkHttpClient client = new OkHttpClient();
        Request.Builder builder = new Request.Builder();
        Request request = builder.url(url).build();
        try 
            Response response = client.newCall(request).execute();
            String json = response.body().string();

            try
            
                JSONObject object = new JSONObject(json);
                JSONArray array = object.optJSONArray("posts");
                return array;
            
            catch (JSONException e)
            
                e.printStackTrace();
            

         catch (IOException e) 
            e.printStackTrace();
        
        return null;
    

    @Override
    protected void onPostExecute(JSONArray array) 
        super.onPostExecute(array);
        if(null == array)
            return;
        if(null != listener )
            listener.onFeed(array);
    


public class FeedAdapter extends ArrayAdapter<Post>

    private int resource;

    public FeedAdapter(Context context, int resource) 
        super(context, resource);
       this.resource = resource;
    

    @Override
    public View getView(int position, View convertView, ViewGroup parent) 
        // Convert View -> Reuse
        if (null==convertView)
        

            LayoutInflater inflater = LayoutInflater.from(getContext());
            convertView = inflater.inflate(resource, null);
        
        // Binding Data
        Post post = getItem(position);
        TextView title = (TextView) convertView.findViewById(R.id.title);
        TextView desc = (TextView) convertView.findViewById(R.id.description);

        title.setText(post.title);
        desc.setText(post.description);



        return convertView;
    

public class Post

    public String title;
    public String description;
    public String thumbnail; //URL

    public Post(String title, String desc, String thumbnail)
    
        this.title = title;
        this.description = desc;
        this.thumbnail = thumbnail;
    

【参考方案1】：

将FeedAdapter adapter 和ArrayList<Post> posts 声明为公共类中的全局变量。它们必须在public void onActivityCreated(Bundle savedInstanceState)之外声明

【参考方案2】：

这里发生了什么：

在 onActivityCreated 中，您声明局部变量而不为它们分配值。因为它们是本地的，所以一旦方法终止，它们就会消失。将方法上方的变量声明为类变量，以便稍后在onFeed 中访问它们 您没有为变量listView 和adapter 赋值。我假设在其他地方，通常的做法是在onViewCreated 中执行此操作，您为片段设置主视图。如果您不知道如何执行此操作，请参阅 this question。然后，您可以使用findViewById 加载 listView，假设 listView 位于您在片段中膨胀的布局中。然后你必须创建一个新的FeedAdapter 并使用listView.setAdapter(adapter) 将它分配给listView

祝你好运！

【参考方案3】：

试试这个：

class MyFragment extends Fragment
ListView listView;
FeedAdapter adapter;
ArrayList<Post> posts;


public void onCreate(Bundle b)
   super.onCreate(b);
   //My bad listview will only be assigned properly in onCreateView once the view is inflated
   posts = new ArrayList<>();
   adapter=new FeedAdapter(getActivity(),R.id.yourresouceid,posts); // which ever way your adapter is defined
   listView.setAdapter(adapter);



public void onActivityCreated(Bundle savedInstanceState) 
super.onActivityCreated(savedInstanceState);



public void onFeed(JSONArray array) 

int lenght = array.length();
for (int i = 0; i < lenght; i++)

    JSONObject object = array.optJSONObject(i);
    posts.add(post);

//adapter.addAll(posts); //you don't need to do this, already done when creating your adapter
adapter.notifyDataSetChanged();

 

说明：onActivityCreated 的范围只到它的右括号。在内部声明的任何内容对任何外部函数都是不可见的。有关其工作原理的更多详细信息，请参阅 java 中的范围解析。

您还必须在使用它们之前对其进行初始化。只是声明它并尝试使用它会给出 NullPointerExceptions

【讨论】：

很遗憾，抽屉停止了