Android ViewPager 框架 Listview 崩溃

Question

我正在尝试制作一个包含三个片段的 TabLayout (ViewPager) 应用程序。在其中一个选项卡中，我想显示一个 json 解析的自定义列表视图。自定义列表视图和选项卡布局可以单独工作（教程中的代码）。当我将这两者结合起来时，应用程序立即崩溃，我无法弄清楚出了什么问题。希望大家帮忙

谢谢 Sam_0829....你救了我

问题是 1.Getmessages必须在Fragment中，而不是MainActivity

1. 调用 onViewCreated 时视图为空，所以我声明了一个私有视图变量

   私人视图 v;

   //然后onCreate

   查看 v = inflater.inflate(R.layout.message_tab, container, false);

   并将其传递给

onViewCreated public void onViewCreated(View v, @Nullable Bundle savedInstanceState) { super.onViewCreated(v, savedInstanceState); contactList = new ArrayList();

    lv = (ListView) v.findViewById(R.id.list);

    new Getmessages().execute();
}

生活变得美好

感谢山姆，你教我如何通过日志排除故障……这是我以前从未信任过的东西

消息片段（已解决的代码）->

    package com.example.user.newton;

 public class MessageTab extends Fragment {
    private String TAG = MainActivity.class.getSimpleName();

    private ProgressDialog pDialog;
    private ListView lv;
    private View v;
    // URL to get messages JSON

    private static String url = "http://myjson.com/17oyz5";

    ArrayList<HashMap<String, String>> contactList;
    private MainActivity.SectionsPagerAdapter mSectionsPagerAdapter;
    @Override
    public View onCreateView(LayoutInflater inflater, ViewGroup container,
                             Bundle savedInstanceState) {
        // Inflate the layout for this fragment

       View v =  inflater.inflate(R.layout.message_tab, container, false);
        return v;
    }
    private class Getmessages extends AsyncTask<Void, Void, Void> {

        @Override
        protected void onPreExecute() {
            super.onPreExecute();
            // Showing progress dialog
            pDialog = new ProgressDialog(getActivity());
            pDialog.setMessage("Please wait...");
            pDialog.setCancelable(false);
            pDialog.show();

        }

        @Override
        protected Void doInBackground(Void... arg0) {
            HttpHandler sh = new HttpHandler();

            // Making a request to url and getting response
            String jsonStr = sh.makeServiceCall(url);

            Log.e(TAG, "Response from url: " + jsonStr);

            if (jsonStr != null) {
                try {
                    JSONObject jsonObj = new JSONObject(jsonStr);

                    // Getting JSON Array node
                    JSONArray messages = jsonObj.getJSONArray("messages");

                    // looping through All messages
                    for (int i = 0; i < messages.length(); i++) {
                        JSONObject c = messages.getJSONObject(i);

                        String message= c.getString("message");
                        String sender= c.getString("sender");
                        String mtime= c.getString("time");

                        HashMap<String, String> contact = new HashMap<>();

                        // adding each child node to HashMap key => value
                        contact.put("message", message);
                        contact.put("sender", sender);
                        contact.put("mtime", mtime);

                        contactList.add(contact);
                    }
                } catch (final JSONException e) {
                    Log.e(TAG, "Json parsing error: " + e.getMessage());
                    getActivity().runOnUiThread(new Runnable() {
                        @Override
                        public void run() {
                            Toast.makeText(getActivity(),
                                    "Json parsing error: " + e.getMessage(),
                                    Toast.LENGTH_LONG)
                                    .show();
                        }
                    });

                }
            } else {
                Log.e(TAG, "Couldn't get json from server.");
                getActivity().runOnUiThread(new Runnable() {
                    @Override
                    public void run() {
                        Toast.makeText(getActivity(),
                                "Couldn't get json from server. Check LogCat for possible errors!",
                                Toast.LENGTH_LONG)
                                .show();
                    }
                });

            }

            return null;
        }

        @Override
        protected void onPostExecute(Void result) {
            super.onPostExecute(result);
            // Dismiss the progress dialog
            if (pDialog.isShowing())
                pDialog.dismiss();
            /**
             * Updating parsed JSON data into ListView
             * */
            ListAdapter adapter = new SimpleAdapter(
                    getActivity(), contactList,
                    R.layout.custom_layout, new String[]{"message", "sender",
                    "mtime"}, new int[]{R.id.name,
                    R.id.email, R.id.mobile});

            lv.setAdapter(adapter);
        }

    }

    @Override
    public void onViewCreated(View v, @Nullable Bundle savedInstanceState) {
        super.onViewCreated(v, savedInstanceState);
        contactList = new ArrayList<>();

        lv = (ListView) v.findViewById(R.id.list);

        new Getmessages().execute();
    }
}

消息片段布局

<?xml version="1.0" encoding="utf-8"?>
<LinearLayout xmlns:android="http://schemas.android.com/apk/res/android"
    android:orientation="vertical" android:layout_width="match_parent"
    android:layout_height="match_parent">

    <ListView
        android:id="@+id/list"
        android:layout_width="match_parent"
        android:layout_height="match_parent" />
</LinearLayout>

Answer 1

请更改

pDialog = new ProgressDialog(getApplicationContext());

到

pDialog = new ProgressDialog(MainActivity.this);

并尝试。

Answer 2

您似乎试图在错误的位置更新 ListView 内容：您在 MainActivity 中将 ListView 引用为 lv，但需要在您的 MessageFragment 中更新它.您应该将您的 Getmessages 移动到（例如）您的 MessageFragment 的 onCreateView。

Answer 3

如果 (lv!= null) {

lv.setAdapter(适配器)

}