android-on导航抽屉项目单击从数据库获取数据并填充另一个列表

Question

我的应用程序包含一个导航抽屉，通过单击从数据库中检索数据的帮助并在主要活动中填充我的listview，我将其列表项用作菜单。我想基于该项向数据库函数发送查询单击抽屉列表，然后根据查询结果填充主活动中的另一个列表视图。

基本上，到目前为止，我一直在尝试根据项目单击分配一个不同的字符串来保存查询，并将其作为参数传递给String类型变量“ query”，并将其作为参数传递给数据库函数，该函数将其作为rawQuery处理并检索结果。由于某些错误，无法检测到它引发了Null Pointer Exception ...

以下是MainActivity.java的代码

public class MainActivity extends ActionBarActivity
implements NavigationDrawerFragment.NavigationDrawerCallbacks {

private NavigationDrawerFragment mNavigationDrawerFragment;
private CharSequence mTitle;

private Cursor c;
public MyDatabaseHandler dbOpen;
private String query; 

@Override
protected void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    setContentView(R.layout.activity_main);


    mNavigationDrawerFragment = 
    (NavigationDrawFragment) 
     getSupportFragmentManager().findFragmentById(R.id.navigation_drawer);
    mTitle = getTitle();


    mNavigationDrawerFragment.setUp(
            R.id.navigation_drawer,
            (DrawerLayout) findViewById(R.id.drawer_layout));


    dbOpen = new MyDatabaseHandler(this);
    c = dbOpen.getListContent(query);
    ListView lv = (ListView) findViewById(R.id.list);
    lv.setAdapter(new SimpleCursorAdapter(this,R.layout.topic_list_each_row,
    c,new String[]{MyDatabaseHandler.KEY_UTOPICNAME,MyDatabaseHandler.KEY_UNAME },
    new int[] {R.id.topicname,R.id.unitname} ,0));
    }

@Override
public void onNavigationDrawerItemSelected(int position) {

    FragmentManager fragmentManager = getSupportFragmentManager();
    fragmentManager.beginTransaction()
            .replace(R.id.container, PlaceholderFragment.newInstance(position + 1))
            .commit();
}
//here where i have thought of passing string into query variable
public void onSectionAttached(int number) {
    switch (number) {
        case 1:
            query = "SELECT * FROM cosmtable ORDER BY utopicname";
             break;
             ...............
             ..............
                }}

public void restoreActionBar() {
    ActionBar actionBar = getSupportActionBar();
    actionBar.setNavigationMode(ActionBar.NAVIGATION_MODE_STANDARD);
    actionBar.setDisplayShowTitleEnabled(true);
    actionBar.setTitle(mTitle);
}


@Override
public boolean onCreateOptionsMenu(Menu menu) {
    if (!mNavigationDrawerFragment.isDrawerOpen()) {
        // Only show items in the action bar relevant to this screen
        // if the drawer is not showing. Otherwise, let the drawer
        // decide what to show in the action bar.
        getMenuInflater().inflate(R.menu.main, menu);
        restoreActionBar();
        return true;
    }
    return super.onCreateOptionsMenu(menu);
}

@Override
public boolean onOptionsItemSelected(MenuItem item) {
    // Handle action bar item clicks here. The action bar will
    // automatically handle clicks on the Home/Up button, so long
    // as you specify a parent activity in AndroidManifest.xml.
    int id = item.getItemId();
    if (id == R.id.action_settings) {
        return true;
    }
    return super.onOptionsItemSelected(item);
}

/**
 * A placeholder fragment containing a simple view.
 */
public static class PlaceholderFragment extends Fragment {
    /**
     * The fragment argument representing the section number for this
     * fragment.
     */
    private static final String ARG_SECTION_NUMBER = "section_number";

    /**
     * Returns a new instance of this fragment for the given section
     * number.
     */
    public static PlaceholderFragment newInstance(int sectionNumber) {
        PlaceholderFragment fragment = new PlaceholderFragment();
        Bundle args = new Bundle();
        args.putInt(ARG_SECTION_NUMBER, sectionNumber);
        fragment.setArguments(args);
        return fragment;
    }

    public PlaceholderFragment() {
    }

    @Override
    public View onCreateView(LayoutInflater inflater, ViewGroup container,
            Bundle savedInstanceState) {
        View rootView = inflater.inflate(R.layout.fragment_main, container, false);
        return rootView;
    }

    @Override
    public void onAttach(Activity activity) {
        super.onAttach(activity);
        ((MainActivity) activity).onSectionAttached(
                getArguments().getInt(ARG_SECTION_NUMBER));}}}

activity_main文件是-

<android.support.v4.widget.DrawerLayout   

xmlns:android="http://schemas.android.com/apk/res/android"
xmlns:tools="http://schemas.android.com/tools"
android:id="@+id/drawer_layout"
android:layout_width="match_parent"
android:layout_height="match_parent"
tools:context="com.example.pockettutsforcosm.MainActivity" >

<FrameLayout
    android:id="@+id/container"
    android:layout_width="match_parent"
    android:layout_height="match_parent" />
<LinearLayout xmlns:android="http://schemas.android.com/apk/res/android"
android:orientation="vertical"
android:layout_width="fill_parent"
android:layout_height="fill_parent">
<ListView 
android:id="@+id/list"
android:layout_width="fill_parent"
android:layout_height="fill_parent"
android:dividerHeight="10dp">
</ListView>
</LinearLayout>
<fragment
    android:id="@+id/navigation_drawer"
    android:name="com.example.pockettutsforcosm.NavigationDrawerFragment"
    android:layout_width="@dimen/navigation_drawer_width"
    android:layout_height="match_parent"
    android:layout_gravity="start"
    tools:layout="@layout/fragment_navigation_drawer" />
</android.support.v4.widget.DrawerLayout>

请帮助我找到实现结果的方法。.如果需要更多详细信息，请告诉我。.谢谢

更新我想到的一件事是我试图在错误的位置编写查询，即，不应在onAttach事件中为查询变量分配值...

所以我更改了主要活动中的代码，如下所示：

public void onNavigationDrawerItemSelected(int position) {
    // update the main content by replacing fragments
    //FragmentManager fragmentManager = getSupportFragmentManager();
    //fragmentManager.beginTransaction()
      //      .replace(R.id.container, PlaceholderFragment.newInstance(position))
        //    .commit();

    try
    {
        switch(position)
        {
        case 0:
            query = "SELECT * FROM cosmtable ORDER BY topic_name";
            c = dbOpen.getListContent(query);
            ListView lv = (ListView) findViewById(R.id.list);
            lv.setAdapter(new SimpleCursorAdapter(this,R.layout.topic_list_each_row,c,
            new String[]{MyDatabasehandler.KEY_UTOPICNAME,MyDatabaseHandler.KEY_UNAME},
            new int[] {R.id.topicname,R.id.unitname} ,0));
            break;
             .............................................

        }
    }

    catch (Exception e) {
        Log.e("error on fragment", "creating the mainScreen");
    }
    finally {

        FragmentManager fragmentManager = getSupportFragmentManager();
        fragmentManager.beginTransaction()
        .replace(R.id.container, PlaceholderFragment.newInstance(position + 1 ))
                .commit();}}

这样做之后，第一次启动应用程序时就不会出现要从导航抽屉的第一项加载的内容。...仅在单击该项目时才会显示……我希望它出现在应用程序启动时。 ..

我是否需要在数据库处理程序类中创建单独的方法以针对每种开关情况返回单独的查询结果？

Answer 1

基本上，我所做的是创建了一个单独的函数，该函数可以相应地操纵查询字符串在导航抽屉中每个元素的位置，然后将其传递以获取数据库处理程序中的游标数据函数，然后将查询结果返回给简单对象。 MainActivity中的游标适配器。

另一件事是为启动应用程序时出现黑屏的问题而做的，我称之为调用数据库查询第一个元素的相同函数...

这给了我想要的输出。.如果仍然有其他好的方法可以使用，那么我也很愿意知道这种方法。 到目前为止，我将接受这个答案。