android - 处理 Fragment 和 listView 和 JSON

Question

我正在尝试使用包含 4 个 fragment 的 MenuDrawer 制作一个应用程序。
这就是我的应用程序展示的方式。
当它启动时，它应该从网站检索数据。检索到的数据应该在第一个 fragment 的 ListView 中。
第二个 fragment 应该包含一个 webview 来查看我在 android 设备中的页面。
第三是显示一些文本细节。
当第一个 fragment 从网站获取详细信息时，我的应用程序崩溃。这是我的代码:
我的 fragment 布局如下:

我的第一个 fragment 包含以下内容:

<ListView
    android:layout_marginTop="10dp"
    android:layout_below="@+id/txtLabel"
    android:layout_width="match_parent"
    android:layout_height="300dp"
    android:background = "@android:color/transparent"
    android:padding="3dp"
    android:dividerHeight="1dp"
    android:id="@+id/mlv"></ListView>

第一个 fragment 在应用程序启动时被调用。
我有一个列表适配器和一个运行良好的列表模型。

我的 Main 类包含一个获取数据的内部类。
我已经制作了模型类和适配器类:
这是我的主要代码:

  public class MainActivity extends Activity {

// declare properties
private String[] mNavigationDrawerItemTitles;
private DrawerLayout mDrawerLayout;
private ListView mDrawerList;
private ListView mListView;
private ListViewNewsAdapter listViewNewsAdapter;
private ArrayList<ListViewNewsItem> listViewNewsItems;

private JSONParser jsonParser = new JSONParser();

private String READNEWS_URL =
        "xyz-myweb-xyz.com";


private ActionBarDrawerToggle mDrawerToggle;

// nav drawer title
private CharSequence mDrawerTitle;

// used to store app title
private CharSequence mTitle;

@Override
protected void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    setContentView(R.layout.activity_main);
    mListView = (ListView) findViewById(R.id.mlv);



    // for proper titles
    mTitle = mDrawerTitle = getTitle();

    // initialize properties
    mNavigationDrawerItemTitles = getResources().getStringArray(R.array.navigation_drawer_items_array);
    mDrawerLayout = (DrawerLayout) findViewById(R.id.drawer_layout);
    mDrawerList = (ListView) findViewById(R.id.left_drawer);

    // list the drawer items
    ObjectDrawerItem[] drawerItem = new ObjectDrawerItem[4];

    drawerItem[0] = new ObjectDrawerItem(R.drawable.home_icon, "Home");
    drawerItem[1] = new ObjectDrawerItem(R.drawable.website_icon, "Website");
    drawerItem[2] = new ObjectDrawerItem(R.drawable.ic_action_share, "Help");

    // Pass the folderData to our ListView adapter
    DrawerItemCustomAdapter adapter = new DrawerItemCustomAdapter(this, R.layout.listview_item_row, drawerItem);

    // Set the adapter for the list view
    mDrawerList.setAdapter(adapter);

    // set the item click listener
    mDrawerList.setOnItemClickListener(new DrawerItemClickListener());





    // for app icon control for nav drawer
    mDrawerLayout = (DrawerLayout) findViewById(R.id.drawer_layout);
    mDrawerToggle = new ActionBarDrawerToggle(
            this,                  /* host Activity */
            mDrawerLayout,         /* DrawerLayout object */
            R.drawable.ic_drawer,  /* nav drawer icon to replace 'Up' caret */
            R.string.drawer_open,  /* "open drawer" description */
            R.string.drawer_close  /* "close drawer" description */
            ) {

        /** Called when a drawer has settled in a completely closed state. */
        public void onDrawerClosed(View view) {
            super.onDrawerClosed(view);
            getActionBar().setTitle(mTitle);
        }

        /** Called when a drawer has settled in a completely open state. */
        public void onDrawerOpened(View drawerView) {
            super.onDrawerOpened(drawerView);
            getActionBar().setTitle(mDrawerTitle);
        }
    };

    // Set the drawer toggle as the DrawerListener
    mDrawerLayout.setDrawerListener(mDrawerToggle);

    // enable ActionBar app icon to behave as action to toggle nav drawer
    getActionBar().setDisplayHomeAsUpEnabled(true);
    getActionBar().setHomeButtonEnabled(true);



    if (savedInstanceState == null) {
        // on first time display view for first nav item
        selectItem(0);
    }
}

处理 fragment 的方法，它在 myMainActivity 里面:

     private void selectItem(int position) {

    // update the main content by replacing fragments

    Fragment fragment = null;

    switch (position) {
    case 0:
        fragment = new CreateFragment();
        new GetNewsTask().execute();
        break;
    case 1:

        fragment = new ReadFragment();
        break;
    case 2:
        fragment = new HelpFragment();
        break;

    default:
        break;
    }

    if (fragment != null) {
        FragmentManager fragmentManager = getFragmentManager();
        fragmentManager.beginTransaction().replace(R.id.content_frame, fragment).commit();
        // update selected item and title, then close the drawer
        mDrawerList.setItemChecked(position, true);
        mDrawerList.setSelection(position);
        setTitle(mNavigationDrawerItemTitles[position]);
        mDrawerLayout.closeDrawer(mDrawerList);

    } else {
        // error in creating fragment
        Log.e("MainAct10ity", "Error in creating fragment");
    }

} 

我使用了一个从网络检索数据的内部类:

  private class GetNewsTask extends AsyncTask<Void, Void, Boolean>
{
    private ProgressDialog mProgressDialog;

    private JSONObject jsonObjectResult = null;

    private String error;

    @Override
    protected void onPreExecute()
    {
        super.onPreExecute();
        listViewNewsItems = new ArrayList<ListViewNewsItem>();
        mProgressDialog = ProgressDialog.show(MainActivity.this,
                "Processing...", "Get last news", false, false);
    }

    @Override
    protected Boolean doInBackground(Void... params)
    {
        jsonObjectResult = jsonParser.makeHttpRequest(READNEWS_URL, null);

        if (jsonObjectResult == null)
        {
            error = "Error in the connection";
            return false;
        }

        try
        {
            if (jsonObjectResult.getInt("success") == 1)
            {
                JSONArray jsonArray = jsonObjectResult.getJSONArray("posts");
                for (int i = 0; i < jsonArray.length(); i++)
                {
                    JSONObject news = jsonArray.getJSONObject(i);

                    ListViewNewsItem listViewNewsItem = new ListViewNewsItem
                            (
                                    news.getString("title"),
                                    news.getString("type")
                            );
                    listViewNewsItems.add(listViewNewsItem);
                }
                return true;
            }
            else
                error = jsonObjectResult.getString("message");

        }
        catch (Exception ex)
        {

        }

        return false;
    }

    @Override
    protected void onPostExecute(Boolean aBoolean)
    {
        super.onPostExecute(aBoolean);
        mProgressDialog.dismiss();
        if (aBoolean)
        {
            listViewNewsAdapter = new ListViewNewsAdapter(getApplicationContext(),
                    listViewNewsItems);
            mListView.setAdapter(listViewNewsAdapter);
        }
        else
            Toast.makeText(getApplicationContext(), error, Toast.LENGTH_LONG).show();
    }
}

我必须在哪里使用

new GetNewsTask().execute();

我试图在 onCreate 中使用它，我的应用程序崩溃了。我试图在我的应用程序崩溃的开关中使用它。
你知道如何解决吗？

Answer 1

我通过以下方式修复了它:
将以下内容移至 fragment :

 private ListView mListView;
private ListViewNewsAdapter listViewNewsAdapter;
private ArrayList<ListViewNewsItem> listViewNewsItems;

private JSONParser jsonParser = new JSONParser();

private String READNEWS_URL =
        "xxxxxxxx.xxxxx.xxxxx";


public CreateFragment() {
    // Required empty public constructor
}

然后，在 onCreateView 内部
我创建 :

mListView = (ListView) rootView.findViewById(R.id.mlv);

    new GetNewsTask().execute();

之后，我将内部类写入 fragment 。好吧，我不得不更改一些参数以使我的代码运行良好。
然后，我运行它，它现在是工作属性。
谢谢